XIII. HUMAN POPULATION IN THE FUTURE

XIII. HUMAN POPULATION IN THE FUTURE

§ 371. Any further evolution in the most highly-evolved of terrestrial beings, Man, must be of the same nature as evolution in general. Structurally considered, it may consist in greater integration, or greater differentiation, or both--augmented bulk, or increased heterogeneity and definiteness, or a combination of the two. Functionally considered, it may consist in a larger sum of actions, or more multiplied varieties of actions, or both--a larger amount of sensible and insensible motion generated, or motions more numerous in their kinds and more intricate and exact in their co-ordinations, or motions that are greater alike in quantity, complexity, and precision.

Expressing the change in terms of that more special evolution displayed by organisms; we may say that it must be one which further adapts the moving equilibrium of organic actions. As was pointed out in First Principles, § 173, “the maintenance of such a moving equilibrium, requires the habitual genesis of internal forces corresponding in number, directions, and amounts to the external incident forces--as many inner functions, single or combined, as there are single or combined outer actions to be met.” And it was also pointed out that “the structural complexity accompanying functional equilibration, is definable as one in which there are as many specialized parts as are capable, separately and jointly, of counteracting the separate and joint forces amid which the organism exists.” Clearly, then, since all incompletenesses in Man as now constituted, are failures to meet certain of the outer actions (mostly involved, remote, irregular), to which he is exposed; every advance implies additional co-ordinations of actions and accompanying complexities of organization.

Or, to specialize still further this conception of future progress, we may consider it as an advance towards completion of that continuous adjustment of internal to external relations, which Life shows us. In Part I. of this work, where it was shown that the correspondence between inner and outer actions which under its phenomenal aspect, we call Life, is a particular kind of what, in terms of Evolution, we called a moving equilibrium; it was shown that the degree of life varies as the degree of correspondence. Greater evolution or higher life implies, then, such modifications of human nature as shall make more exact the existing correspondences, or shall establish additional correspondences, or both. Connexions of phenomena of a rare, distant, unobtrusive, or intricate kind, which we either suffer from or do not take advantage of, have to be responded to by new connexions of ideas, and acts properly combined and proportioned: there must be increase of knowledge, or skill, or power, or of all these. And to effect this more extensive, more varied, and more accurate, co-ordination of actions, there must be organization of still greater heterogeneity and definiteness.

§ 372. Let us, before proceeding, consider in what particular ways this further evolution, this higher life, this greater co-ordination of actions, may be expected to show itself.

Will it be in strength? Probably not to any considerable degree. Mechanical appliances are fast supplanting brute force, and doubtless will continue doing this. Though at present civilized nations largely depend for self-preservation on vigour of limb, and are likely to do so while wars continue; yet that progressive adaptation to the social state which must at last bring wars to an end, will leave the amount of muscular power to adjust itself to the requirements of a peaceful regime. Though, taking all things into account, the muscular power then required may not be less than now, there seems no reason why more should be required.

Will it be swiftness or agility? Probably not. In savages these are important elements of the ability to maintain life; but in civilized men they aid self-preservation in quite minor degrees, and there seems no circumstance likely to necessitate an increase of them. By games and gymnastic competitions, such attributes may indeed be artificially increased; but no artificial increase which does not bring a proportionate advantage can be permanent; since, other things equal, individuals and societies that devote the same amounts of energy in ways which subserve life more effectually, must by and by predominate.

Will it be in mechanical skill, that is, in the better-co-ordination of complex movements? Most likely in some degree. Awkwardness is continually entailing injuries and deaths. Moreover the complicated tools which civilization brings into use, are constantly requiring greater delicacy of manipulation. All the arts, industrial and æsthetic, as they develop, imply a corresponding development of perceptive and executive faculties in men: the two act and react.

Will it be in intelligence? Largely, no doubt. There is ample room for advance in this direction, and ample demand for it. Our lives are universally shortened by our ignorance. In attaining complete knowledge of our own natures and of the natures of surrounding things--in ascertaining the conditions of existence to which we must conform, and in discovering means of conforming to them under all variations of seasons and circumstances; we have abundant scope for intellectual progress.

Will it be in morality, that is, in greater power of self-regulation? Largely also: perhaps most largely. Right conduct is usually come short of more from defect of will than defect of knowledge. For the right co-ordination of those complex actions which constitute human life in its civilized form, there goes not only the pre-requisite--recognition of the proper course; but the further pre-requisite--a due impulse to pursue that course. On calling to mind our daily failures to fulfil often-repeated resolutions, we shall perceive that lack of the needful desire, rather than lack of the needful insight, is the chief cause of faulty action. A further endowment of those feelings which civilization is developing in us--sentiments responding to the requirements of the social state--emotive faculties that find their gratifications in the duties devolving on us--must be acquired before the crimes, excesses, diseases, improvidences, dishonesties, and cruelties, that now so greatly diminish the duration of life, can cease.

Thus, looking at the several possibilities, and asking what direction this further evolution, this more complete moving equilibrium, this better adjustment of inner to outer relations, this more perfect co-ordination of actions, is likely to take; we conclude that it must take mainly the direction of a higher intellectual and emotional development.

§ 373. This conclusion we shall find equally forced on us if we inquire for the causes which are to bring about such results. No more in the case of Man than in the case of any other being, can we presume that evolution has taken place, or will hereafter take place, spontaneously. In the past, at present, and in the future, all modifications, functional and organic, have been, are, and must be, immediately or remotely consequent on surrounding conditions. What, then, are those changes in the environment to which, by direct or indirect equilibration, the human organism has been adjusting itself, is adjusting itself now, and will continue to adjust itself? And how do they necessitate a higher evolution of the organism?

Civilization, everywhere having for its antecedent the increase of population, and everywhere having for one of its consequences a decrease of certain race-destroying forces, has for a further consequence an increase of certain other race-destroying forces. Danger of death from predatory animals lessens as men grow more numerous. Though, as they spread over the Earth and divide into tribes, men become wild beasts to one another, yet the danger of death from this cause also diminishes as tribes coalesce into nations. But the danger of death which does not diminish, is that produced by augmentation of numbers itself--the danger from deficiency of food. Supposing human nature to remain unchanged, the mortality hence resulting would, on the average, rise as human beings multiplied. If mortality, under such conditions, does not rise, it must be because the supply of food also augments; and this implies some change in human habits wrought by stress of human needs. Here, then, is the permanent cause of modification to which civilized men are exposed. Though the intensity of its action is ever being mitigated in one direction by greater production of food, it is, in the other direction, ever being added to by the greater production of individuals. Manifestly, the wants of their redundant numbers constitute the only stimulus mankind have to obtain more necessaries of life. Were not the demand beyond the supply, there would be no motive to increase the supply. And manifestly, this excess of demand over supply is perennial: this pressure of population, of which it is the index, cannot be eluded. Though by the emigration that takes place when the pressure arrives at a certain intensity, temporary relief is from time to time obtained; yet as, by this process, all habitable countries must become peopled, it follows that in the end the pressure, whatever it may then be, must be borne in full.

This constant increase of people beyond the means of subsistence causes, then, a never-ceasing requirement for skill, intelligence, and self-control--involves, therefore, a constant exercise of these and gradual growth of them. Every industrial improvement is at once the product of a higher form of humanity, and demands that higher form of humanity to carry it into practice. The application of science to the arts, is the bringing to bear greater intelligence for satisfying our wants, and implies continued progress of that intelligence. To get more produce from the acre, the farmer must study chemistry, must adopt new mechanical appliances, and must, by the multiplication of processes, cultivate both his own powers and the powers of his labourers. To meet the requirements of the market, the manufacturer is perpetually improving his old machines and inventing new ones; and by the premium of high wages incites artizans to acquire greater skill. The daily-widening ramifications of commerce entail on the merchant a need for more knowledge and more complex calculations; while the lessening profits of the ship-owner force him to build more scientifically, to get captains of higher intelligence and better crews. In all cases pressure of population is the original cause. Were it not for the competition this entails, more thought and energy would not daily be spent on the business of life; and growth of mental power would not take place. Difficulty in getting a living is alike the incentive to a higher education of children, and to a more intense and long-continued application in adults. In the mother it prompts foresight, economy, and skilful house-keeping; in the father, laborious days and constant self-denial. Nothing but necessity could make men submit to this discipline; and nothing but this discipline could produce a continued progression.

In this case, as in many others, Nature secures each step in advance by a succession of trials; which are perpetually repeated, and cannot fail to be repeated, until success is achieved. All mankind in turn subject themselves more or less to the discipline described; they either may or may not advance under it; but, in the nature of things, only those who do advance under it eventually survive. For, necessarily, families and races whom this increasing difficulty of getting a living which excess of fertility entails, does not stimulate to improvements in production--that is, to greater mental activity--are on the high road to extinction; and must ultimately be supplanted by those whom the pressure does so stimulate. This truth we have recently seen exemplified in Ireland. And here, indeed, without further illustration, it will be seen that premature death, under all its forms and from all its causes, cannot fail to work in the same direction. For as those prematurely carried off must, in the average of cases, be those in whom the power of self-preservation is the least, it unavoidably follows that those left behind to continue the race, must be those in whom the power of self-preservation is the greatest--must be the select of their generation. So that, whether the dangers to existence be of the kind produced by excess of fertility, or of any other kind, it is clear that by the ceaseless exercise of the faculties needed to contend with them, and by the death of all men who fail to contend with them successfully, there is ensured a constant progress towards a higher degree of skill, intelligence, and self-regulation--a better co-ordination of actions--a more complete life.[67]

§ 374. The proposition at which we have thus arrived is, then, that excess of fertility, through the changes it is ever working in Man’s environment, is itself the cause of Man’s further evolution; and the obvious corollary here to be drawn is, that Man’s further evolution so brought about, itself necessitates a decline in his fertility.

All future progress in civilization which the never-ceasing pressure of population must produce, will be accompanied by an enhanced cost of Individuation, both in structure and function; and more especially in nervous structure and function. The peaceful struggle for existence in societies ever growing more crowded and more complicated, must have for its concomitant an increase of the great nervous centres in mass, in complexity, in activity. That larger body of emotion needed as a fountain of energy for men who have to hold their places and rear their families under the intensifying competition of social life, is, other things equal, the correlative of larger brain. Those higher feelings presupposed by the better self-regulation which, in a better society, can alone enable the individual to leave a persistent posterity, are, other things equal, the correlatives of a more complex brain; as are also those more numerous, more varied, more general, and more abstract ideas, which must also become increasingly requisite for successful life as society advances. And the genesis of this larger quantity of feeling and thought, in a brain thus augmented in size and developed in structure, is, other things equal, the correlative of a greater wear of nervous tissue and greater consumption of materials to repair it. So that both in original cost of construction and in subsequent cost of working, the nervous system must become a heavier tax on the organism. Already the brain of the civilized man is larger by nearly thirty per cent. than the brain of the savage. Already, too, it presents an increased heterogeneity--especially in the distribution of its convolutions. And further changes like these which have taken place under the discipline of civilized life, we infer will continue to take place. But everywhere and always, evolution is antagonistic to procreative dissolution. Whether it be in greater growth of the organs which subserve self-maintenance, whether it be in their added complexity of structure, or whether it be in their higher activity, the abstraction of the required materials implies a diminished reserve of materials for race-maintenance. And we have seen reason to believe that this antagonism between Individuation and Genesis, becomes unusually marked where the nervous system is concerned, because of the costliness of nervous structure and function. In § 346 was pointed out the apparent connexion between high cerebral development and prolonged delay of sexual maturity; and in §§ 366, 367, the evidence went to show that where exceptional fertility exists there is sluggishness of mind, and that where there has been during education excessive expenditure in mental action, there frequently follows a complete or partial infertility. Hence the particular kind of further evolution which Man is hereafter to undergo, is one which, more than any other, may be expected to cause a decline in his power of reproduction.

The higher nervous development and greater expenditure in nervous action, here described as indirectly brought about by increase of numbers, and as thereafter becoming a check on the increase of numbers, must not be taken to imply an intenser strain--a mentally-laborious life. The greater emotional and intellectual power and activity above contemplated, must be understood as becoming, by small increments, organic, spontaneous, and pleasurable. As, even when relieved from the pressure of necessity, large-brained Europeans voluntarily enter on enterprises and activities which the savage could not keep up even to satisfy urgent wants; so, their still larger-brained descendants will, in a still higher degree, find their gratifications in careers entailing still greater mental expenditures. This enhanced demand for materials to establish and carry on the psychical functions, will be a constitutional demand. We must conceive the type gradually so modified, that the more-developed nervous system irresistibly draws off, for its normal and unforced activities, a larger proportion of the common stock of nutriment; and while so increasing the intensity, completeness, and length of the individual life, necessarily diminishing the reserve applicable to the setting up of new lives--no longer required to be so numerous.

Though the working of this process will doubtless be interfered with and modified in the future, as it has been in the past, by the facilitations of living which civilization brings; yet nothing beyond temporary interruptions can so be caused. However much the industrial arts may be improved, there must be a limit to the improvement; while, with a rate of multiplication in excess of the rate of mortality, population must continually tread on the heels of production. So that though, during the earlier stages of civilization, an increased amount of food may accrue from a given amount of labour, there must come a time when this relation will be reversed, and when every additional increment of food will be obtained by a more than proportionate labour: the disproportion growing ever higher, and the diminution of the reproductive power becoming greater.

§ 375. There now remains but to inquire towards what limit this progress tends. So long as the fertility of the race is more than sufficient to balance the diminution by deaths, population must continue to increase. So long as population continues to increase, there must be pressure on the means of subsistence. And so long as there is pressure on the means of subsistence, further mental development must go on, and further diminution of fertility must result; provided that the actions and reactions which have been described are not artificially interfered with. I append this qualifying clause advisedly, and especially emphasize it, because these actions and reactions have been hitherto, and are now, greatly interfered with by governments, and the continuance of the interferences may retard, if not stop, that further evolution which would else go on.

I refer to those hindrances to the survival of the fittest which in earlier times resulted from the undiscriminating charities of monasteries and in later times from the operation of Poor Laws. Of course if the competition which increasing pressure of population entails, is prevented from acting on a considerable part of the community, such part, saved from the needed intellectual and moral stress, will not undergo any further mental development; and must ever tend to leave a posterity, and an increasing posterity, in which none of that higher individuation which checks genesis takes place. Such State-meddlings with the natural play of actions and reactions produce a further evil equally great or greater. For those who are not self-maintained, or but partially self-maintained, are supplied with the means they lack by the better members of the community; and these better members have thus not only to support themselves and their offspring, but also to support or aid the inferior members and their offspring. The under-working of one part is accompanied by the over-working of the other part--by a working which at each stage of progress exceeds that which the normal conditions necessitate, and results sometimes in illness, premature age, or death, or in lessened number of children, or in imperfect rearing of children: the bad are fostered and the good are repressed.

It does not follow that the struggle for life and the survival of the fittest must be left to work out their effects without mitigation. It is contended only that there shall not be a forcible burdening of the superior for the support of the inferior. Such aid to the inferior as the superior voluntarily yield, kept as it will be within moderate limits, may be given with benefit to both--relief to the one, moral culture to the other. And aid willingly given (little to the least worthy and more to the most worthy) will usually be so given as not to further the increase of the unworthy. For in proportion as the emotional nature becomes more evolved, and there grows up a higher sense of parental responsibility, the begetting of children that cannot be properly reared will be universally held intolerable. If, as we see, public opinion in many places and times becomes coercive enough to force men to fight duels, we can scarcely doubt that at a higher stage of evolution it may become so coercive as to prevent men from marrying improvidently. If the frowns of their fellows can make men commit immoral acts, surely they may make men refrain from immoral acts--especially when the actors themselves feel that the threatened frowns would be justified. Hence with a higher moral nature will come a restriction on the multiplication of the inferior.

In brief, the sole requirement is that there shall be no extensive suspension of that natural relation between merit and benefit which constitutes justice. Holding, then, that this all-essential condition will itself come to be recognized and enforced by a more evolved humanity, let us consider what is the goal towards which the restraint on genesis by individuation progresses.

§ 375_a_. Supposing the Sun’s light and heat, on which all terrestrial life depends, to continue abundant for a period long enough to allow the entire evolution we are contemplating; there are still certain changes which must prevent such complete adjustment of human nature to surrounding conditions, as would permit the rate of multiplication to become equal to the rate of mortality. As before pointed out (§ 148), during an epoch of 21,000 years each hemisphere goes through a cycle of temperate seasons and seasons extreme in their heat and cold--variations which are themselves alternately exaggerated and mitigated in the course of far longer cycles; and we saw that these cause perpetual ebbings and flowings of species over different parts of the Earth’s surface. Further, by slow but inevitable geologic changes, especially those of elevation and subsidence, the climate and physical characters of every habitat are modified; while old habitats are destroyed and new are formed. This, too, we noted as a constant cause of migrations and of resulting alterations of environment. Now though the human race differs from other races in having a power of artificially counteracting external changes, yet there are limits to this power; and, even were there no limits, the changes could not fail to work their effects indirectly, if not directly. If, as is thought probable, these astronomic cycles entail recurrent glacial periods in each hemisphere, then parts of the Earth which are at one time thickly peopled, will at another time be almost deserted, and vice versâ. The geologically-caused alterations of climate and surface, must produce further slow re-distributions of population; and other currents of people, to and from different regions, will be necessitated by the rise of successive centres of higher civilization. Consequently, mankind cannot but continue to undergo changes of environment, physical and moral, analogous to those which they have thus far been undergoing. Such changes may eventually become slower and less marked; but they can never cease. And if they can never cease there can never arise a perfect adaptation of human nature to its conditions of existence. To establish that complete correspondence between inner and outer actions which constitutes the highest life and greatest power of self-preservation, there must be a prolonged converse between the organism and circumstances which remain the same. If the external relations are being altered while the internal relations are being adjusted to them, the adjustment can never become exact. And in the absence of exact adjustment, there cannot exist that theoretically-highest power of self-preservation with which there would co-exist the theoretically-lowest power of race-production.

Hence though the number of premature deaths may ultimately become very small, it can never become so small as to allow the average number of offspring from each pair to fall so low as two. Some average number between two and three may be inferred as the limit--a number, however, which is not likely to be quite constant, but may be expected at one time to increase somewhat and afterwards to decrease somewhat, according as variations in physical and social conditions lower or raise the cost of self-preservation.

To this qualification must be added a further qualification. The foregoing argument tacitly assumes that the causes described will continuously operate on all mankind; whereas a survey of the facts makes it clear that some parts only of the Earth’s surface are capable of bearing high types of civilization, and consequently high types of Man. There must remain hereafter, as there are now, considerable parts of its surface which can support only groups of nomads, or other groups obliged by their habitats to lead simple and inferior kinds of life. Only by subjection to the discipline we have been contemplating can there be produced the fully-developed Man; and evidently in many parts of the world this discipline will continue to be eluded. Not only must we conclude that the varieties of our race now living in desert regions and arctic climates will continue hereafter to do so, but we may conclude that always, as now, a certain proportion of men who are born in civilized societies, impatient of the stress which pressure of population puts on them, will escape into unoccupied or sparsely-peopled regions, where they can lead unrestrained lives though lives of hardship. Recognizing as we must the probability that in common with all other things, humanity will continue to differentiate and produce a more heterogeneous assemblage of types, we must infer that only in some of the highest of these will the antagonism of individuation and genesis have the anticipated effects.

Restricting ourselves to these, then, we may conclude that in the end, pressure of population and its accompanying evils will almost disappear; and will leave a state of things requiring from each individual little more than a normal and pleasurable activity. Cessation in the decrease of fertility implies cessation in the development of the nervous system; and this implies a nervous system which has become equal to all that is demanded of it--has not to do more than is natural to it. But that exercise of faculties which does not exceed what is natural, constitutes gratification.

The necessary antagonism of Individuation and Genesis, not only, then, fulfils the à priori law of maintenance of race, from the monad up to Man, but ensures final attainment of the highest form of this maintenance--a form in which the amount of life shall be the greatest possible and the births and deaths the fewest possible. From the beginning pressure of population has been the proximate cause of progress. It produced the original diffusion of the race. It compelled men to abandon predatory habits and take to agriculture. It led to the clearing of the Earth’s surface. It forced men into the social state; made social organization inevitable; and has developed the social sentiments. It has stimulated to progressive improvements in production, and to increased skill and intelligence. It is daily thrusting us into closer contact and more mutually-dependent relationships. And after having caused, as it ultimately must, the due peopling of the globe, and the raising of its habitable parts into the highest state of culture--after having perfected all processes for the satisfaction of human wants--after having, at the same time, developed the intellect into competence for its work, and the feelings into fitness for social life--after having done all this, the pressure of population must gradually approach to an end--an end, however, which for the reasons given it cannot absolutely reach.

§ 377. In closing the argument let us not overlook the self-sufficingness of those universal processes by which the results reached thus far have been wrought out, and which may be expected to work out these future results.

Evolution under all its aspects, general and special, is an advance towards equilibrium. We have seen that the theoretical limit towards which the integration and differentiation of every aggregate advances, is a state of balance between all the forces to which its parts are subject, and the forces which its parts oppose to them (First Prin. § 170). And we have seen that organic evolution is a progress towards a moving equilibrium completely adjusted to environing actions.

It has been also pointed out that, in civilized Man, there is going on a new class of equilibrations--those between his actions and the actions of the societies he forms (First Prin. § 175). Social restraints and requirements are ever altering his activities and by consequence his nature; and as fast as his nature is altered, social restraints and requirements undergo more or less re-adjustment. Here the organism and the conditions are both modifiable; and by successive conciliations of the two, there is effected a progress towards equilibrium.

More recently we have seen that in every species, there establishes itself an equilibrium of an involved kind between the total race-destroying forces and the total race-preserving forces--an equilibrium which implies that where the ability to maintain individual life is small, the ability ta propagate must be great, and vice versâ. Whence it follows that the evolution of a race more in equilibrium with the environment, is also the evolution of a race in which there is a correlative approach towards equilibrium between the number of new individuals produced and the number which survive and propagate.

The final result to be observed is that in Man, all these equilibrations between constitution and conditions, between the structure of society and the nature of its members, between fertility and mortality, advance simultaneously towards a common climax. In approaching an equilibrium between his nature and the ever-varying circumstances of his inorganic environment, and in approaching an equilibrium between his nature and all the requirements of the social state, Man is at the same time approaching that lowest limit of fertility at which the equilibrium of population is maintained by the addition of as many infants as there are subtractions by death in old age. But in a universe of which all parts are in motion and every part is consequently subject to change of conditions, neither this equilibrium nor any other equilibrium can become complete.

THE END.

APPENDICES.

APPENDIX A.

SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS.

I append here the evidences referred to in § 190. The most numerous and striking I have met with among the Umbelliferæ. Monstrosities having the alleged implication, are frequent in the common Cow-Parsnep--so frequent that they must be familiar to botanists; and wild Angelica supplies many over-developments of like meaning. Omitting numerous cases of more or less significance, I will limit myself to two.

One of them is that of a terminal umbel, in which nine of the outer umbellules are variously transformed--here a single flower being made monstrous by the development of some of its members into buds; there several such malformed flowers being associated with rays that bear imperfect umbellules; and elsewhere, flowers being replaced by umbellules: some of which are perfect, and others imperfect only in the shortness of the flower-stalks. The annexed Fig. 69, representing in a somewhat conventionalized way, a part of the dried specimen, will give an idea of this Angelica. At a is shown a single flower partially changed; in the umbellule marked b, one of the rays bears a secondary umbellule; and there may be seen at c and d, several such over-developments.

But the most conclusive instance is that of a Cow-Parsnep, in which a single terminal umbel, besides the transformations already mentioned, exhibits higher degrees of such transformations.[68] The components of this complex growth are;--three central umbellules, abnormal only in minor points; one umbellule, external to these, which is partially changed into an umbel; one rather more out of the centre, which is so far metamorphosed as to be more an umbel than an umbellule: nine peripheral clusters formed by the development of umbellules into umbels, some of which are partially compounded still further. Examined in detail, these structures present the following facts:--1. The innermost umbellule is normal, save in having a peripheral flower of which one member (apparently a petal) is transformed into a flower-bud. 2. The next umbellule, not quite so central, has one of its peripheral flowers made monstrous by the growth of a bud from the base of the calyx. 3. The third of the central umbellules has two abnormal outer flowers. One of them carries a flower-bud on its edge, in place of a foliar member. The other is half flower and half umbellule: being composed of three petals, three stamens, and five flower-buds growing where the other petals and stamens should grow. 4. Outside of these umbellules comes one of the mixed clusters. Its five central flowers are normal. Surrounding these are several flowers transformed in different degrees: one having a stamen partially changed into a flower bud. And then, at the periphery of this mixed cluster, come three complete umbellules and an incomplete one in which some petals and stamens of the original flower remain. 5. A mixed cluster, in which the umbel-structure predominates, stands next. Its three central flowers are normal. Surrounding them are five flowers over-developed in various ways, like those already described. And on its periphery are seven complete umbellules in place of flowers; besides an incomplete umbellule that contains traces of the original flower, one of them being a petal imperfectly twisted up into a bud. 6. Of the nine external clusters, in which the development of simple into compound umbels is most decided, nearly all present anomalies. Three of them have each a central flower untransformed; and in others, the central umbellule is composed of two, three, or four flowers. 7. But the most remarkable fact is, that in sundry of these peripheral clusters, resulting from the metamorphosis of simple umbels into compound umbels, the like metamorphosis is carried a stage higher. Some of the component rays, are themselves the bearers of compound umbels instead of simple umbels. In Fig. 70, a portion of the dried specimen is represented. Two of the central umbellules are marked a and b; those marked c and d are mixed clusters; at e and f are compound umbels replacing simple ones; and g shows one of the rays on which the over-development goes still further.

Does not this evidence, enforced as it is by much more of like kind, go far to prove that foliar organs may be developed into axial organs? Even were not the transitional forms traceable, there would still, I think, be no other legitimate interpretation of the facts last detailed. The only way of eluding the conclusion here drawn, is by assuming that where a cluster of flowers replaces a single flower, it is because the axillary buds which hypothetically belong to the several foliar organs of the flower, become developed into axes; and assuming this, is basing an hypothesis on another hypothesis that is directly at variance with facts. The foliar organs of flowers do not bear buds in their axils; and it would never have been supposed that such buds are typically present, had it not been for that mistaken conception of “type” which has led to many other errors in Biology. Goethe writes: “Now as we cannot realize the idea of a leaf apart from the node out of which it springs, or of a node without a bud, we may venture to infer,” &c. See here an example of a method of philosophizing not uncommon among the Germans. The method is this--Survey a portion of the facts, and draw from them a general conception; project this general conception back into the objective world, as a mould in which Nature casts her products; expect to find it everywhere fulfilled; and allege potential fulfilment where no actual fulfilment is visible.

If instead of imposing our ideal forms on Nature, we are content to generalize the facts as Nature presents them, we shall find no warrant for the morphological doctrine above enunciated. The only conception of type justified by the logic of science, is--that correlation of parts which remains constant under all modifications of the structure to be defined. To ascertain this, we must compare all these modifications, and note what traits are common to them. On doing so with the successive segments of a phænogamic axis, we are brought to a conclusion widely different from that of Goethe. Axillary buds are almost universally absent from the cotyledons; they are habitually present in the axils of fully-developed leaves higher up the axis; they are often absent from leaves that are close to the flower; they are nearly always absent from the bracts; absent from the sepals; absent from the petals; absent from the stamens; absent from the carpels. Thus, out of eight leading forms which folia assume, one has the axillary bud and seven are without it. With these facts before us, it seems to me not difficult to “realize the idea” “of a node without a bud.” If we are not possessed by a foregone conclusion, the evidence will lead us to infer, that each node bears a foliar appendage and may bear an axillary bud.

Even, however, were it granted that the typical segment of a Phænogam includes an axillary bud, which must be regarded as always potentially present, no legitimate counter-interpretation of the monstrosities above described could thence be drawn. If when an umbellule is developed in place of a flower, the explanation is, that its component rays are axillary to the foliar organs of the flower superseded; we may fairly require that these foliar organs to which they are axillary, shall be shown. But there are none. In the last specimen figured, the inner rays of each such umbellule are without them; most of the outer rays are also without them; and in one cluster, only a single ray has a bract at its point of origin. There is a rejoinder ready, however: the foliar organs are said to be suppressed. Though Goethe could not “realize the idea” “of a node without a bud,” those who accept his typical form appear to find no difficulty in realizing the idea of an axillary bud without anything to which it is axillary. But letting this pass, suppose we ask what is the warrant for this assumed suppression. Axillary buds normally occur where the nutrition is high enough to produce fully-developed leaves; and when axillary buds are demonstrably present in flowers, they accompany foliar organs that are more leaf-like than usual--always greener if not always larger. That is to say, the normal and the abnormal axillary buds, are alike the concomitants of foliar organs coloured by that chlorophyll which habitually favours foliar development. How, then, can it be supposed that when, out of a flower there is developed a cluster of flower-bearing rays, the implied excess of nutrition causes the foliar organs to abort? It is true that very generally in a branched inflorescence, the bracts of the several flower-branches are very small (their smallness being probably due to that defective supply of certain chlorophyll-forming matters, which is the proximate cause of flowering); and it is true that, under these conditions, a flowering axis of considerable size, for the development of which chlorophyll is less needful, grows from the axil of a dwarfed leaf. But the inference that the foliar organ may therefore be entirely suppressed, seems to me irreconcilable with the fact, that the foliar organ is always developed to some extent before the axillary bud appears. Until it has been shown that in some cases a lateral bud first appears, and a foliar organ afterwards grows out beneath it, to form its axil, the conception of an axillary bud of which the foliar organ is suppressed, will remain at variance with the established truths of development.

* * * * *

The above originally formed a portion of § 190. I have transferred it to the Appendix, partly because it contains too much detail to render it fit for the general argument, and partly because the interpretations being open to some question, it seemed undesirable to risk compromising that argument by including them. The criticisms passed upon these interpretations have not, however, sufficed to convince me of their incorrectness. Unfortunately, I have since had no opportunity of verifying the above statements by microscopic examinations, as I had intended.

Though unable to enforce the inference drawn by further facts more minutely looked into, I may add some arguments based on facts that are well known. One of these is the fact that the so-called axillary bud is not universally axillary--is not universally seated in the angle made by the axis and an appended foliar organ. In certain plants the axillary bud is placed far above the node, half-way between it and the succeeding node. So that not only may a segment of a phæenogamic axis be without the axillary bud, but the axillary bud, when present, may be removed from that place in which, according to Goethe, it necessarily exists. Another fact not congruous with the current doctrine, is the common occurrence of “adventitious” buds--the buds that are put out from roots and from old stems or branches bare of leaves. The name under which they are thus classed, is meant to imply that they may be left out of consideration. Those, however, who have not got a theory to save by putting anomalies out of sight, may be inclined to think that the occurrence of buds where they are avowedly unconnected with nodes, and are axillary to nothing, tells very much against the assumption that every bud implies a node and a corresponding foliar organ. And they may also see that the development of these adventitious buds at places where there is excess of nutritive materials, favours the view above set forth. For if a bud thus arises at a place where it is not morphologically accounted for, simply because there happens to be at that place an abundance of unorganized protoplasm; then, clearly, it is likely that if the mass of protoplasm from which a leaf would usually arise, is greatly increased in mass by excess of nutrition, it may develop into an axis instead of a leaf.

* * * * *

Many years after this work was published, I discovered among my papers a memorandum which unfortunately I had overlooked, containing further evidence in support of the foregoing conclusion. With the omission of an error concerning the species of plant, I reproduce this memorandum just as it stood:--

“I found at Dieppe, July 1, 1860, in a garden near the sea a sample of cultivated wild flower (I thought it was grown as an ornamental flower) in which some of the single flowers of the umbel were developed into groups of flowers thus:--

“In the case where the transformation was fully effected the umbellule had six flowers, answering to the six petals of the original flowers. In other cases the transformation was incomplete. There were instances where but two of the petals were developed into flowers; and the other petals remained unchanged. Others in which three were developed; and others where four were developed. In some cases, too, the development of a petal into a flower was imperfect, in the absence of the flower-stalk--the flowers were sessile in the place where the petals would have been. In one case there was an imperfect flower sessile; another imperfect flower on a short stalk; and three perfect flowers on long stalks.

“I was in some doubt whether the petals or the stamens were developed. In cases of imperfect transformation the petals at the base of the umbellule seemed to stand in the position of calyx or involucrum, giving the idea that the stamens were developed into flowers. But in the case where there were six flowers developed there were no petals at the base.

“That it was a matter of extra nutrition was shown by this:--

“1. That they were cultivated as garden flowers.

“2. That where there was one perfectly developed umbellule, it was the only one in the umbel.

“3. That where there were three umbellules they were all imperfect.

“4. That in this imperfect umbellule the perfect flowers were on long stalks and the imperfect ones sessile.

“5. That the umbellules were on stalks both longer and thicker than those of single flowers.”

* * * * *

[Concerning the foregoing argument at large an expert writes:--“The abnormalities you describe certainly show that an axis may arise abnormally in the place of a normal leaf-structure, and every modern botanist would be in agreement with you in your criticism of the older form of the doctrine of axillary buds. I think we are largely emancipated from the dextrous juggling with the arrangements and relations of organs which used to pass current as morphology.

“You have quoted sufficient evidence in the text (§ 190) to establish the conclusion that no sharp line can be drawn between axes and leaf-structure; and a very great deal more could be added in the same sense. Petioles for instance, exist which the most highly trained histological observer could not distinguish from stems.

“But I must demur to the suggestion that the replacement of one by the other is primarily a question of nutrition. We are as ignorant as ever of the proximate cause of the production of a leaf or a shoot at a certain spot in meristematic tissue.”

To this last remark I had at first made only the reply that the plants exhibiting the abnormalities were in all cases excessively luxuriant in their growths; but to this I am now able to add a more definite reply. The expert from whom I have just quoted, had read this appendix before there had been made to it the above addition describing the flower from Dieppe; and I was not myself aware, until I came to read over this addition, what clear evidence it contains that extra nutrition was the cause of these transformations of foliar structures into axial structures; but the above paragraphs 1, 2, 3, 4, 5, contain different evidences conspiring to prove this.]

APPENDIX B.

A CRITICISM ON PROF. OWEN’S THEORY OF THE VERTEBRATE SKELETON.

[From the BRITISH & FOREIGN MEDICO-CHIRURGICAL REVIEW FOR OCT., 1858.]

I. On the Archetype and Homologies of the Vertebrate Skeleton. By RICHARD OWEN, F.R.S.--London, 1848. pp. 172.

II. Principes d’Ostéologie Comparée, ou Recherches sur l’Archétype et les Homologies du Squelette Vertébré. Par RICHARD OWEN.--Paris.

Principles of Comparative Osteology; or, Researches on the Archetype and the Homologies of the Vertebrate Skeleton. By RICHARD OWEN.

III. _On the Nature of Limbs. A Discourse delivered on Friday, February 9, at an Evening Meeting of the Royal Institution of Great Britain._ By RICHARD OWEN, F.R.S.--London, 1849. pp. 119.

Judging whether another proves his position is a widely different thing from proving your own. To establish a general law requires an extensive knowledge of the phenomena to be generalized; but to decide whether an alleged general law is established by the evidence assigned, requires merely an adequate reasoning faculty. Especially is such a decision easy where the premises do not warrant the conclusion. It may be dangerous for one who has but little previous acquaintance with the facts, to say that a generalization is demonstrated; seeing that the argument may be one-sided: there may be many facts unknown to him which disprove it. But it is not dangerous to give a negative verdict when the alleged demonstration is manifestly insufficient. If the data put before him do not bear out the inference, it is competent for every logical reader to say so.

From this standpoint, then, we venture to criticize some of Professor Owen’s osteological theories. For his knowledge of comparative osteology we have the highest respect. We believe that no living man has so wide and detailed an acquaintance with the bony structure of the Vertebrata. Indeed, there probably has never been any one whose information on the subject was so nearly exhaustive. Moreover, we confess that nearly all we know of this department of biology has been learnt from his lectures and writings. We pretend to no independent investigations, but merely to such knowledge of the phenomena as he has furnished us with. Our position, then, is such that, had Professor Owen simply enunciated his generalizations, we should have accepted them on his authority. But he has brought forward evidence to prove them. By so doing he has tacitly appealed to the judgments of his readers and hearers--has practically said, “Here are the facts; do they not warrant these conclusions?” And all we propose to do, is to consider whether the conclusions are warranted by the facts brought forward.

Let us first limit the scope of our criticisms. On that division of comparative osteology which deals with what Professor Owen distinguishes as “special homologies,” we do not propose to enter. That the wing of a bird is framed upon bones essentially parallel to those of a mammal’s fore-limb; that the cannon-bone of a horse’s leg answers to the middle metacarpal of the human hand; that various bones in the skull of a fish are homologous with bones in the skull of a man--these and countless similar facts, we take to be well established. It may be, indeed, that the doctrine of special homologies is at present carried too far. It may be that, just as the sweeping generalization at one time favoured, that the embryonic phases of the higher animals represent the adult forms of lower ones, has been found untrue in a literal sense, and is acceptable only in a qualified sense; so the sweeping generalization that the skeletons of all vertebrate animals consist of homologous parts, will have to undergo some modification. But that this generalization is substantially true, all comparative anatomists agree.

The doctrine which we are here to consider, is quite a separate one--that of “general homologies.” The truth or falsity of this may be decided on quite apart from that of the other. Whether certain bones in one vertebrate animal’s skeleton correspond with certain bones in another’s, or in every other’s, is one question; and whether the skeleton of every vertebrate animal is divisible into a series of segments, each of which is modelled after the same type, is another question. While the first is answered in the affirmative, the last may be answered in the negative; and we propose to give reasons why it should be answered in the negative.

* * * * *

In so far as his theory of the skeleton is concerned, Professor Owen is an avowed disciple of Plato. At the conclusion of his Archetype and Homologies of the Vertebrate Skeleton, he quotes approvingly the Platonic hypothesis of ἰδέαι, “a sort of models, or moulds in which matter is cast, and which regularly produce the same number and diversity of species.” The vertebrate form in general (see diagram of the Archetypus), or else the form of each kind of vertebrate animal (see p. 172, where this seems implied), Professor Owen conceives to exist as an “idea”--an “archetypal exemplar on which it has pleased the Creator to frame certain of his living creatures.” Whether Professor Owen holds that the typical vertebra also exists as an “idea,” is not so certain. From the title given to his figure of the “ideal typical vertebra,” it would seem that he does; and at p. 40 of his Nature of Limbs, and indeed throughout his general argument, this supposition is implied. But on the last two pages of the Archetype and Homologies, it is distinctly alleged that “the repetition of similar segments in a vertebral column, and of similar elements in a vertebral segment, is analogous to the repetition of similar crystals as the result of polarizing force in the growth of an inorganic body;” it is pointed out that, “as we descend the scale of animal life, the forms of the repeated parts of the skeleton approach more and more to geometrical figures;” and it is inferred that “the Platonic ἰδέα or specific organizing principle or force, would seem to be in antagonism with the general polarizing force, and to subdue and mould it in subserviency to the exigencies of the resulting specific form.” If Professor Owen’s doctrine is to be understood as expressed in these closing paragraphs of his Archetype and Homologies--if he considers that “the ἰδέα” “which produces the diversity of form belonging to living bodies of the same materials,” is met by the “counter-operation” of “the polarizing force pervading all space,” which produces “the similarity of forms, the repetition of parts, the signs of unity of organization,” and which is “subdued” as we ascend “in the scale of being;” then we may pass on with the remark that the hypothesis is too cumbrous and involved to have much vraisemblance. If, on the other hand, Professor Owen holds, as every reader would suppose from the general tenor of his reasonings, that not only does there exist an archetypal or ideal vertebrate skeleton, but that there also exists an archetypal or ideal vertebra; then he carries the Platonic hypothesis much further than Plato does. Plato’s argument, that before any species of object was created it must have existed as an idea of the Creative Intelligence, and that hence all objects of such species must be copies of this original idea, is tenable enough from the anthropomorphic point of view. But while those who, with Plato, think fit to base their theory of creation upon the analogy of a carpenter designing and making a table, must yield assent to Plato’s inference, they are by no means committed to Professor Owen’s expansion of it. To say that before creating a vertebrate animal, God must have had the conception of one, does not involve saying that God gratuitously bound himself to make a vertebrate animal out of segments all moulded after one pattern. As there is no conceivable advantage in this alleged adhesion to a fundamental pattern--as, for the fulfilment of the intended ends, it is not only needless, but often, as Professor Owen argues, less appropriate than some other construction would be (see Nature of Limbs, pp. 39, 40), to suppose the creative processes thus regulated, is not a little startling. Even those whose conceptions are so anthropomorphic as to think they honour the Creator by calling him “the Great Artificer,” will scarcely ascribe to him a proceeding which, in a human artificer, they would consider a not very worthy exercise of ingenuity.

But whichever of these alternatives Professor Owen contends for--whether the typical vertebra is that more or less crystalline figure which osseous matter ever tends to assume in spite of “the ἰδέα or organizing principle,” or whether the typical vertebra is itself an “ἰδέα or organizing principle”--there is alike implied the belief that the typical vertebra has an abstract existence apart from actual vertebræ. It is a form which, in every endo-skeleton, strives to embody itself in matter--a form which is potentially present in each vertebra; which is manifested in each vertebra with more or less clearness; but which, in consequence of antagonizing forces, is nowhere completely realized. Apart from the philosophy of this hypothesis, let us here examine the evidence which is thought to justify it.

* * * * *

And first as to the essential constituents of the “ideal typical vertebra.” Exclusive of “diverging appendages” which it “may also support,” “it consists in its typical completeness of the following elements and parts”:--A centrum round which the rest are arranged in a somewhat radiate manner; above it two neurapophyses--converging as they ascend, and forming with the centrum a trianguloid space containing the neural axis; a neural spine, surmounting the two neurapophyses, and with them completing the neural arch; below the centrum two hæmapophyses and a hæmal spine, forming a hæmal arch similar to the neural arch above, and enclosing the hæmal axis; two pleurapophyses radiating horizontally from the sides of the centrum; and two parapophyses diverging from the centrum below the pleurapophyses. “These,” says Professor Owen, “being usually developed from distinct and independent centres, I have termed ‘autogenous elements.’” The remaining elements, which he classes as “exogenous,” because they “shoot out as continuations from some of the preceding elements,” are the diapophyses diverging from the upper part of the centrum as the parapophyses do below, and the zygapophyses which grow out of the distal ends of the neurapophyses and hæmapophyses.

If, now, these are the constituents of the vertebrate segment “in its typical completeness;” and if the vertebrate skeleton consists of a succession of such segments; we ought to have in these constituents, representatives of all the elements of the vertebrate skeleton--at any rate, all its essential elements. Are we then to conclude that the “diverging appendages,” which Professor Owen regards as rudimental limbs, and from certain of which he considers actual limbs to be developed, are typically less important than some of the above-specified exogenous parts--say the zygapophyses?

That the meaning of this question may be understood, it will be needful briefly to state Professor Owen’s theory of The Nature of Limbs; and such criticisms as we have to make on it must be included in the parenthesis. In the first place, he aims to show that the scapular and pelvic arches, giving insertion to the fore and hind limbs respectively, are displaced and modified hæmal arches, originally belonging in the one case to the occipital vertebra, and in the other case to some trunk-vertebra not specified. In support of this assumption of displacement, carried in some cases to the extent of twenty-seven vertebræ, Professor Owen cites certain acknowledged displacements which occur in the human skeleton to the extent of half a vertebra--a somewhat slender justification. But for proof that such a displacement has taken place in the scapular arch, he chiefly relies on the fact that in fishes, the pectoral fins, which are the homologues of the fore-limbs, are directly articulated to certain bones at the back of the head, which he alleges are parts of the occipital vertebra. This appeal to the class of fishes is avowedly made on the principle that these lowest of the Vertebrata approach closest to archetypal regularity, and may therefore be expected to show the original relations of the bones more nearly. Simply noting the facts that Professor Owen does not give us any transitional forms between the alleged normal position of the scapular arch in fishes, and its extraordinary displacement in the higher Vertebrata; and that he makes no reference to the embryonic phases of the higher Vertebrata, which might be expected to exhibit the progressive displacement; we go on to remark that, in the case of the pelvic arch, he abandons his principle of appealing to the lowest vertebrate forms for proof of the typical structure. In fishes, the rudimentary pelvis, widely removed from the spinal column, shows no signs of having belonged to any vertebra; and here Professor Owen instances the perennibranchiate Batrachia as exhibiting the typical structure: remarking that “mammals, birds, and reptiles show the rule of connexion, and fishes the exception.” Thus in the case of the scapular arch, the evidence afforded by fishes is held of great weight, because of their archetypal regularity; while in the case of the pelvic arch, their evidence is rejected as exceptional. But now, having, as he considers, shown that these bony frames to which the limbs are articulated are modified hæmal arches, Professor Owen points out that the hæmal arches habitually bear certain “diverging appendages;” and he aims to show that the “diverging appendages” of the scapular and pelvic arches respectively, are developed into the fore and hind limbs. There are several indirect ways in which we may test the probability of this conclusion. If these diverging appendages are “rudimental limbs”--“future possible or potential arms, legs, wings, or feet,” we may fairly expect them always to bear to the hæmal arches a relation such as the limbs do. But they by no means do this. “As the vertebræ approach the tail, these appendages are often transferred gradually from the pleurapophysis to the parapophysis, or even to the centrum and neural arch.” (Arch. and Hom., p. 93.) Again, it might naturally be assumed that in the lowest vertebrate forms, where the limbs are but little developed, they would most clearly display their alliance with the appendages, or “rudimental limbs,” by the similarity of their attachments. Instead of this, however, Professor Owen’s drawings show that whereas the appendages are habitually attached to the pleurapophyses, the limbs, in their earliest and lowest phase, alike in fishes and in the Lepidosiren, are articulated to the hæmapophyses. Most anomalous of all, however, is the process of development. When we speak of one thing as being developed out of another, we imply that the parts next to the germ are the first to appear, and the most constant. In the evolution of a tree out of a seed, there come at the outset the stem and the radicle; afterwards the branches and divergent roots; and still later the branchlets and rootlets; the remotest parts being the latest and most inconstant. If, then, a limb is developed out of a “diverging appendage” of the hæmal arch, the earliest and most constant bones should be the humerus and femur; next in order of time and constancy should come the coupled bones based on these; while the terminal groups of bones should be the last to make their appearance, and the most liable to be absent. Yet, as Professor Owen himself shows, the actual mode of development is the very reverse of this. At p. 16 of the Archetype and Homologies, he says:--

“The earlier stages in the development of all locomotive extremities are permanently retained or represented in the paired fins of fishes. First the essential part of the member, the hand or foot, appears: then the fore-arm or leg, both much shortened, flattened, and expanded, as in all fins and all embryonic rudiments of limbs: finally come the humeral and femoral segments; but this stage I have not found attained in any fish.”

That is to say, alike in ascending through the Vertebrata, generally, and in tracing up the successive phases of a mammalian embryo, the last-developed and least constant division of the limb, is that basic one by which it articulates with the hæmal arch. It seems to us that, so far from proving his hypothesis, Professor Owen’s own facts tend to show that limbs do not belong to the vertebræ at all: that they make their first appearance peripherally; that their development is centripetal; and that they become fixed to such parts of the vertebrate axis as the requirements of the case determine.

But now, ending here this digressive exposition and criticism, and granting the position that limbs “are developments of costal appendages,” let us return to the question above put--Why are not these appendages included as elements of the “ideal typical vertebra?” It cannot be because of their comparative inconstancy; for judging from the illustrative figures, they seem to be as constant as the hæmal spine, which is one of the so-called autogenous elements: in the diagram of the Archetypus, the appendage is represented as attached to every vertebrate segment of the head and trunk, which the hæmal spine is not. It cannot be from their comparative unimportance; seeing that as potential limbs they are essential parts of nearly all the Vertebrata--much more obviously so than the diapophyses are. If, as Professor Owen argues, “the divine mind which planned the archetype also foreknew all its modifications;” and if, among these modifications, the development of limbs out of diverging appendages was one intended to characterize all the higher Vertebrata; then, surely, these diverging appendages must have been parts of the “ideal typical vertebra.” Or, if the “ideal typical vertebra” is to be understood as a crystalline form in antagonism with the organizing principle; then why should not the appendages be included among its various offshoots? We do not ask this question because of its intrinsic importance. We ask it for the purpose of ascertaining Professor Owen’s method of determining what are true vertebral constituents. He presents us with a diagram of the typical vertebra, in which are included certain bones, and from which are excluded certain others. If relative constancy is the criterion, then there arises the question--What degree of constancy entitles a bone to be included? If relative importance is the criterion, there comes not only the question--What degree of importance suffices? but the further question--How is importance to be measured? If neither of these is the criterion, then what is it? And if there is no criterion, does it not follow that the selection is arbitrary?

* * * * *

This question serves to introduce a much wider one:--Has the “ideal typical vertebra” any essential constituents at all? It might naturally be supposed that though some bones are so rarely developed as not to seem worth including, and though some that are included are very apt to be absent, yet that certain others are invariable: forming, as it were, the basis of the ideal type. Let us see whether the facts bear out this supposition. In his “summary of modifications of corporal vertebræ” (p. 96), Professor Owen says--“The hæmal spine is much less constant as to its existence, and is subject to a much greater range of variety, when present, than its vertical homotype above, which completes the neural arch.” Again he says--“The hæmapophyses, as osseous elements of a vertebra, are less constant than the pleurapophyses.” And again--“The pleurapophyses are less constant elements than the neurapophyses.” And again--“Amongst air-breathing vertebrates the pleurapophyses of the trunk segments are present only in those species in which the septum of the heart’s ventricle is complete and imperforate, and here they are exogenous and confined to the cervical and anterior thoracic vertebræ.” And once more, both the neurapophyses and the neural spine “are absent under both histological conditions, at the end of the tail in most air-breathing vertebrates, where the segments are reduced to their central elements.” That is to say, of all the peripheral elements of the “ideal typical vertebra,” there is not one which is always present. It will be expected, however, that at any rate the centrum is constant: the bone which “forms the axis of the vertebral column, and commonly the central bond of union of the peripheral elements of the vertebrate (p. 97), is of course an invariable element. No: not even this is essential.

“The centrums do not pass beyond the primitive stage of the notochord (undivided column) in the existing lepidosiren, and they retained the like rudimental state in every fish whose remains have been found in strata earlier than the permian æra in Geology, though the number of vertebræ is frequently indicated in Devonian and Silurian ichthyolites by the fossilized neur-and hæmapophyses and their spines” (p. 96).

Indeed, Professor Owen himself remarks that “the neurapophyses are more constant as osseous or cartilaginous elements of the vertebræ than the centrums” (p. 97). Thus, then, it appears that the several elements included in the “ideal typical vertebra” have various degrees of constancy, and that no one of them is essential. There is no one part of a vertebra which invariably answers to its exemplar in the pattern-group. How does this fact consist with the hypothesis? If the Creator saw fit to make the vertebrate skeleton out of a series of segments, all formed on essentially the same model--if, for the maintenance of the type, one of these bony segments is in many cases formed out of a coalesced group of pieces, where, as Professor Owen argues, a single piece would have served as well or better; then we ought to find this typical repetition of parts uniformly manifested. Without any change of shape, it would obviously have been quite possible for every actual vertebra to have contained all the parts of the ideal one--rudimentally where they were not wanted. Even one of the terminal bones of a mammal’s tail might have been formed out of the nine autogenous pieces, united by suture but admitting of identification. As, however, there is no such uniform typical repetition of parts, it seems to us that to account for the typical repetition which does occur, by supposing the Creator to have fixed on a pattern-vertebra, is to ascribe to him the inconsistency of forming a plan and then abandoning it.

If, on the other hand, Professor Owen means that the “ideal typical vertebra” is a crystalline form in antagonism with “the idea or organizing principle;” then we might fairly expect to find it most clearly displaying its crystalline character, and its full complement of parts, in those places where the organizing principle may be presumed to have “subdued” it to the smallest extent. Yet in the Vertebrata generally, and even in Professor Owen’s Archetypus, the vertebræ of the tail, which must be considered as, if anything, less under the influence of the organizing principle than those of the trunk, do not manifest the ideal form more completely. On the contrary, as we approach the end of the tail, the successive segments not only lose their remaining typical elements, but become as uncrystalline-looking as can be conceived.

* * * * *

Supposing, however, that the assumption of suppressed or undeveloped elements be granted--supposing it to be consistent with the hypothesis of an “ideal typical vertebra,” that the constituent parts may severally be absent in greater or less number, sometimes leaving only a single bone to represent them all; may it not be that such parts as are present, show their respective typical natures by some constant character: say their mode of ossification?

To this question some parts of the Archetype and Homologies seem to reply, “Yes;” while others clearly answer, “No.” Criticising the opinions of Geoffrey St. Hilaire and Cuvier, who agreed in thinking that ossification from a separate centre was the test of a separate bone, and that thus there were as many elementary bones in the skeleton as there were centres of ossification, Professor Owen points out that, according to this test, the human femur, which is ossified from four centres, must be regarded as four bones; while the femur in birds and reptiles, which is ossified from a single centre, must be regarded as a single bone. Yet, on the other hand, he attaches weight to the fact that the skull of the human fœtus presents “the same ossific centres” as do those of the embryo kangaroo and the young bird. (Nature of Limbs, p. 40.) And at p. 104 of the Homologies, after giving a number of instances, he says--

“These and the like correspondences between the points of ossification of the human fœtal skeleton, and the separate bones of the adult skeletons of inferior animals, are pregnant with interest, and rank among the most striking illustrations of unity of plan in the vertebrate organization.”

It is true that on the following page he seeks to explain this seeming contradiction by distinguishing

“between those centres of ossification that have homological relations, and those that have teleological ones--i.e., between the separate points of ossification of a human bone which typify vertebral elements, often permanently distinct bones in the lower animals; and the separate points which, without such signification, facilitate the progress of osteogeny, and have for their obvious final cause the well-being of the growing animal.”

But if there are thus centres of ossification which have homological meanings, and others which have not, there arises the question--How are they always to be distinguished? Evidently independent ossification ceases to be a homological test, if there are independent ossifications that have nothing to do with the homologies. And this becomes the more evident when we learn that there are cases where neither a homological nor a teleological meaning can be given. Among various modes of ossification of the centrum, Professor Owen points out that “the body of the human atlas is sometimes ossified from two, rarely from three, distinct centres placed side by side” (p. 89); while at p. 87 he says:--“In osseous fishes I find that the centrum is usually ossified from six points.” It is clear that this mode of ossification has here no homological signification; and it would be difficult to give any teleological reason why the small centrum of a fish should have more centres of ossification than the large centrum of a mammal. The truth is, that as a criterion of the identity or individuality of a bone, mode of ossification is quite untrustworthy. Though, in his “ideal typical vertebra,” Professor Owen delineates and classifies as separate “autogenous” elements, those parts which are “usually developed from distinct and independent centres;” and though by doing so he erects this characteristic into some sort of criterion; yet his own facts show it to be no criterion. The parapophyses are classed among the autogenous elements; yet they are autogenous in fishes alone, and in these only in the trunk vertebræ, while in all air-breathing vertebrates they are, when present at all, exogenous. The neurapophyses, again, “lose their primitive individuality by various kinds and degrees of confluence:” in the tails of the higher Vertebrata they, in common with the neural spine, become exogenous. Nay, even the centrum may lose its autogenous character. Describing how, in some batrachians, “the ossification of the centrum is completed by an extension of bone from the bases of the neurapophyses, which effects also the coalescence of these with the centrum,” Professor Owen adds:--“In Pelobates fuscus and Pelobates cultripes, Müller found the entire centrum ossified from this source, without any independent points of ossification” (p. 88). That is to say, the centrum is in these cases an exogenous process of the neurapophyses. We see, then, that these so-called typical elements of vertebræ have no constant developmental character by which they can be identified. Not only are they undistinguishable by any specific test from other bones not included as vertebral elements; not only do they fail to show their typical characters by their constant presence; but, when present, they exhibit no persistent marks of individuality. The central element may be ossified from six, four, three, or two points; or it may have no separate point of ossification at all: and similarly with various of the peripheral elements. The whole group of bones forming the “ideal typical vertebra” may severally have their one or more ossific centres; or they may, as in a mammal’s tail, lose their individualities in a single bone ossified from one or two points.

* * * * *

Another fact which seems very difficult to reconcile with the hypothesis of an “ideal typical vertebra,” is the not infrequent presence of some of the typical elements in duplicate. Not only, as we have seen, may they severally be absent, but they may severally be present in greater number than they should be. When we see, in the ideal diagram, one centrum, two neurapophyses, two pleurapophyses, two hæmapophyses, one neural spine, and one hæmal spine, we naturally expect to find them always bearing to each other these numerical relations. Though we may not be greatly surprised by the absence of some of them, we are hardly prepared to find others multiplied. Yet such cases are common. Thus the neural spine “is double in the anterior vertebræ of some fishes” (p. 98). Again, in the abdominal region of extinct saurians, and in crocodiles, “the freely-suspended hæmapophyses are compounded of two or more overlapping bony pieces” (p. 100). Yet again, at p. 99, we read--“I have observed some of the expanded pleurapophyses in the great Testudo elephantopus ossified from two centres, and the resulting divisions continuing distinct, but united by suture.” Once more “the neurapophyses, which do not advance beyond the cartilaginous stage in the sturgeon, consist in that fish of two distinct pieces of cartilage; and the anterior pleurapophyses also consist of two or more cartilages, set end on end” (p. 91). And elsewhere referring to this structure, he says:--

“Vegetative repetition of perivertebral parts not only manifests itself in the composite neurapophyses and pleurapophyses, but in a small accessory (interneural) cartilage, at the fore and back part of the base of the neurapophysis; and by a similar (interhæmal) one at the fore and back part of most of the parapophyses” (p. 87).

Thus the neural and hæmal spines, the neurapophyses, the pleurapophyses, the hæmapophyses, may severally consist of two or more pieces. This is not all: the like is true even of the centrums.

“In Heptanchus (Squalus cinereus) the vertebral centres are feebly and vegetatively marked out by numerous slender rings of hard cartilage in the notochordal capsule, the number of vertebræ being more definitely indicated by the neurapophyses and parapophyses.... In the piked dog-fish (Acanthias) and the spotted dog-fish (Scyllium) the vertebral centres coincide in number with the neural arches” (p. 87).

Is it not strange that the pattern-vertebra should be so little adhered to, that each of its single typical pieces may be transformed into two or three?

But there are still more startling departures from the alleged type. The numerical relations of the elements vary not only in this way, but in the opposite way. A given part may be present not only in greater number than it should be, but also in less. In the tails of homocercal fishes, the centrums “are rendered by centripetal shortening and bony confluence fewer in number than the persistent, neural, and hæmal arches of that part”--that is, there is only a fraction of a centrum to each vertebra. Nay, even this is not the most heteroclite structure. Paradoxical as it may seem, there are cases in which the same vertebral element is, considered under different aspects, at once in excess and defect. Speaking of the hæmal spine, Professor Owen says:--

“The horizontal extension of this vertebral element is sometimes accompanied by a median division, or in other words, it is ossified from two lateral centres; this is seen in the development of parts of the human sternum; the same vegetative character is constant in the broader thoracic hæmal spines of birds; though, sometimes, as e.g., in the struthionidæ, ossification extends from the same lateral centre lengthwise--i.e., forwards and backwards, calcifying the connate cartilaginous homologues of halves of four or five hæmal spines, before these finally coalesce with their fellows at the median line” (p. 101).

So that the sternum of the ostrich, which according to the hypothesis, should, in its cartilaginous stage, have consisted of four or five transverse pieces, answering to the vertebral segments, and should have been ossified from four or five centres, one to each cartilaginous piece, shows not a trace of this structure; but instead, consists of two longitudinal pieces of cartilage, each ossified from one centre, and finally coalescing on the median line. These four or five hæmal spines have at the same time doubled their individualities transversely, and entirely lost them longitudinally!

* * * * *

There still remains to be considered the test of relative position. It might be held that, spite of all the foregoing anomalies, if the typical parts of the vertebræ always stood towards each other in the same relations--always preserved the same connexions, something like a case would be made out. Doubtless, relative position is an important point; and it is one on which Professor Owen manifestly places great dependence. In his discussion of “moot cases of special homology,” it is the general test to which he appeals. The typical natures of the alisphenoid, the mastoid, the orbito-sphenoid, the prefrontal, the malar, the squamosal, &c. he determines almost wholly by reference to the adjacent nerve-perforations and the articulations with neighbouring bones (see pp. 19 to 72): the general form of the argument being--This bone is to be classed as such or such, because it is connected thus and thus with these others, which are so and so. Moreover, by putting forth an “ideal typical vertebra,” consisting of a number of elements standing towards each other in certain definite arrangement, this persistency of relative position is manifestly alleged. The essential attribute of this group of bones, considered as a typical group, is the constancy in the connexions of its parts: change the connexions, and the type is changed. But the constancy of relative position thus tacitly asserted, and appealed to as a conclusive test in “moot cases of special homology,” is clearly negatived by Professor Owen’s own facts. For instance, in the “ideal typical vertebra,” the hæmal arch is represented as formed by the two hæmapophyses and the hæmal spine; but at p. 91 we are told that

“The contracted hæmal arch in the caudal region of the body may be formed by different elements of the typical vertebra: e.g., by the parapophyses (fishes generally); by the pleurapophyses (lepidosiren); by both parapophyses and pleurapophyses (Sudis, Lepidosteus), and by hæmapophyses, shortened and directly articulated with the centrums (reptiles and mammals).”

And further, in the thorax of reptiles, birds, and mammals, “the hæmapophyses are removed from the centrum, and are articulated to the distal ends of the pleurapophyses; the bony hoop being completed by the intercalation of the hæmal spine” (p. 82). So that there are five different ways in which the hæmal arch may be formed--four modes of attachment of the parts different from that shown in the typical diagram! Nor is this all. The pleurapophyses “may be quite detached from their proper segment, and suspended to the hæmal arch of another vertebra;” as we have already seen, the entire hæmal arch may be detached and removed to a distance, sometimes reaching the length of twenty-seven vertebræ; and, even more remarkable, the ventral fins of some fishes, which theoretically belong to the pelvic arch, are so much advanced forward as to be articulated to the scapular arch--“the ischium elongating to join the coracoid.” With these admissions it seems to us that relative position and connexions cannot be appealed to as tests of homology, nor as evidence of any original type of vertebra.

In no class of facts, then, do we find a good foundation for the hypothesis of an “ideal typical vertebra.” There is no one conceivable attribute of this archetypal form which is habitually realised by actual vertebræ. The alleged group of true vertebral elements is not distinguished in any specified way from bones not included in it. Its members have various degrees of inconstancy; are rarely all present together; and no one of them is essential. They are severally developed in no uniform way: each of them may arise either out of a separate piece of cartilage, or out of a piece continuous with that of some other element; and each may be ossified from many independent points, from one, or from none. Not only may their respective individualities be lost by absence, or by confluence with others; but they may be doubled, or tripled, or halved, or may be multiplied in one direction and lost in another. The entire group of typical elements may coalesce into one simple bone representing the whole vertebra; and even, as in the terminal piece of a bird’s tail, half-a-dozen vertebræ, with all their many elements, may become entirely lost in a single mass. Lastly, the respective elements, when present, have no fixity of relative position: sundry of them are found articulated to various others than those with which they are typically connected; they are frequently displaced and attached to neighbouring vertebræ; and they are even removed to quite remote parts of the skeleton. It seems to us that if this want of congruity with the facts does not disprove the hypothesis, no such hypothesis admits of disproof.

* * * * *

Unsatisfactory as is the evidence in the case of the trunk and tail vertebræ, to which we have hitherto confined ourselves, it is far worse in the case of the alleged cranial vertebræ. The mere fact that those who have contended for the vertebrate structure of the skull, have differed so astonishingly in their special interpretations of it, is enough to warrant great doubt as to the general truth of their theory. From Professor Owen’s history of the doctrine of general homology, we gather that Duméril wrote upon “la tête considérée comme une vertèbre;” that Kielmeyer, “instead of calling the skull a vertebra, said each vertebra might be called a skull;” that Oken recognized in the skull three vertebræ and a rudiment; that Professor Owen himself makes out four vertebræ; that Goethe’s idea, adopted and developed by Carus, was, that the skull is composed of six vertebræ; and that Geoffrey St. Hilaire divided it into seven. Does not the fact that different comparative anatomists have arranged the same group of bones into one, three, four, six, and seven vertebral segments, show that the mode of determination is arbitrary, and the conclusions arrived at fanciful? May we not properly entertain great doubts as to any one scheme being more valid than the others? And if out of these conflicting schemes we are asked to accept one, ought we not to accept it only on the production of some thoroughly conclusive proof--some rigorous test showing irrefragably that the others must be wrong and this alone right? Evidently where such contradictory opinions have been formed by so many competent judges, we ought, before deciding in favour of one of them, to have a clearness of demonstration much exceeding that required in any ordinary case. Let us see whether Professor Owen supplies us with any such clearness of demonstration.

To bring the first or occipital segment of the skull into correspondence with the “ideal typical vertebra,” Professor Owen argues, in the case of the fish, that the parapophyses are displaced, and wedged between the neurapophyses and the neural spine--removed from the hæmal arch and built into the upper part of the neural arch. Further, he considers that the pleurapophyses are teleologically compound. And then, in all the higher vertebrata, he alleges that the hæmal arch is separated from its centrum, taken to a distance, and transformed into the scapular arch. Add to which, he says that in mammals the displaced parapophyses are mere processes of the neurapophyses (p. 133): these vertebral elements, typically belonging to the lower part of the centrum, and in nearly all cases confluent with it, are not only removed to the far ends of elements placed above the centrum, but have become exogenous parts of them!

Conformity of the second or parietal segment of the cranium with the pattern-vertebra, is produced thus:--The petrosals are excluded as being partially-ossified sense-capsules, not forming parts of the true vertebral system, but belonging to the “splanchno-skeleton.” A centrum is artificially obtained by sawing in two the bone which serves in common as centrum to this and the preceding segment; and this though it is admitted that in fishes, where their individualities ought to be best seen, these two hypothetical centrums are not simply coalescent, but connate. Next, a similar arbitrary bisection is made of certain elements of the hæmal arches. And then, “the principle of vegetative repetition is still more manifest in this arch than in the occipital one:” each pleurapophysis is double; each hæmapophysis is double; and the hæmal spine consists of six pieces!

The interpretation of the third and fourth segments being of the same general character, need not be detailed. The only point calling for remark being, that in addition to the above various modes of getting over anomalies, we find certain bones referred to the dermo-skeleton.

Now it seems to us, that even supposing no antagonist interpretations had been given, an hypothesis reconcilable with the facts only by the aid of so many questionable devices, could not be considered satisfactory; and that when, as in this case, various comparative anatomists have contended for other interpretations, the character of this one is certainly not of a kind to warrant the rejection of the others in its favour; but rather of a kind to make us doubt the possibility of all such interpretations. The question which naturally arises is, whether by proceeding after this fashion, groups of bones might not be arranged into endless typical forms. If, when a given element was not in its place, we were at liberty to consider it as suppressed, or connate with some neighbouring element, or removed to some more or less distant position;--if, on finding a bone in excess, we might consider it, now as part of the dermo-skeleton, now as part of the splanchno-skeleton, now as transplanted from its typical position, now as resulting from vegetative repetition, and now as a bone teleologically compound (for these last two are intrinsically different, though often used by Professor Owen as equivalents);--if, in other cases, a bone might be regarded as spurious (p. 91), or again as having usurped the place of another;--if, we say, these various liberties were allowed us, we should not despair of reconciling the facts with various diagrammatic types besides that adopted by Professor Owen.

When, in 1851, we attended a course of Professor Owen’s lectures on Comparative Osteology, beginning though we did in the attitude of discipleship, our scepticism grew as we listened, and reached its climax when we came to the skull; the reduction of which to the vertebrate structure, reminded us very much of the interpretation of prophecy. The delivery, at the Royal Society, of the Croonian Lecture for 1858, in which Professor Huxley, confirming the statements of several German anatomists, has shown that the facts of embryology do not countenance Professor Owen’s views respecting the formation of the cranium, has induced us to reconsider the vertebral theory as a whole. Closer examination of Professor Owen’s doctrines, as set forth in his works, has certainly not removed the scepticism generated years ago by his lectures. On the contrary, that scepticism has deepened into disbelief. And we venture to think that the evidence above cited shows this disbelief to be warranted.

* * * * *

There remains the question--What general views are we to take respecting the vertebrate structure? If the hypothesis of an “ideal typical vertebra” is not justified by the facts, how are we to understand that degree of similarity which vertebræ display?

We believe the explanation is not far to seek. All that our space will here allow, is a brief indication of what seems to us the natural view of the matter.

Professor Owen, in common with other comparative anatomists, regards the divergences of individual vertebræ from the average form, as due to adaptive modifications. If here one vertebral element is largely developed, while elsewhere it is small--if now the form, now the position, now the degree of coalescence, of a given part varies; it is that the local requirements have involved this change. The entire teaching of comparative osteology implies that differences in the conditions of the respective vertebræ necessitate differences in their structures.

Now, it seems to us that the first step towards a right conception of the phenomena, is to recognize this general law in its converse application. If vertebræ are unlike in proportion to the unlikeness of their circumstances, then, by implication, they will be like in proportion to the likeness of their circumstances. While successive segments of the same skeleton, and of different skeletons, are all in some respects more or less differently acted on by incident forces, and are therefore required to be more or less different; they are all, in other respects, similarly acted on by incident forces, and are therefore required to be more or less similar. It is impossible to deny that if differences in the mechanical functions of the vertebræ involve differences in their forms; then, community in their mechanical functions, must involve community in their forms. And as we know that throughout the Vertebrata generally, and in each vertebrate animal, the vertebræ, amid all their varying circumstances, have a certain community of function, it follows necessarily that they will have a certain general resemblance--there will recur that average shape which has suggested the notion of a pattern-vertebra.

A glance at the facts at once shows their harmony with this conclusion. In an eel or a snake, where the bodily actions are such as to involve great homogeneity in the mechanical conditions of the vertebræ, the series of them is comparatively homogeneous. On the contrary, in a mammal or a bird, where there is considerable heterogeneity in their circumstances, their similarity is no longer so great. And if, instead of comparing the vertebral columns of different animals, we compare the successive vertebræ of any one animal, we recognize the same law. In the segments of an individual spine, where is there the greatest divergence from the common mechanical conditions? and where may we therefore expect to find the widest departure from the average form? Obviously at the two extremities. And accordingly it is at the two extremities that the ordinary structure is lost.

Still clearer becomes the truth of this view, when we consider the genesis of the vertebral column as displayed throughout the ascending grades of the Vertebrata. In its first embryonic stage, the spine is an undivided column of flexible substance. In the early fishes, while some of the peripheral elements of the vertebræ were marked out, the central axis was still a continuous unossified cord. And thus we have good reason for thinking that in the primitive vertebrate animal, as in the existing Amphioxus, the notochord was persistent. The production of a higher, more powerful, more active creature of the same type, by whatever method it is conceived to have taken place, involved a change in the notochordal structure. Greater muscular endowments presupposed a firmer internal fulcrum --a less yielding central axis. On the other hand, for the central axis to have become firmer while remaining continuous, would have entailed a stiffness incompatible with the creature’s movements. Hence, increasing density of the central axis necessarily went hand in hand with its segmentation: for strength, ossification was required; for flexibility, division into parts. The production of vertebræ resulting thus, there obviously would arise among them a general likeness, due to the similarity in their mechanical conditions, and more especially the muscular forces bearing on them. And then observe, lastly, that where, as in the head, the terminal position and the less space for development of muscles, entailed smaller lateral bendings, the segmentation would naturally be less decided, less regular, and would be lost as we approached the front of the head.

But, it may be replied, this hypothesis does not explain all the facts. It does not tell us why a bone whose function in a given animal requires it to be solid, is formed not of a single piece, but by the coalescence of several pieces, which in other creatures are separate; it does not account for the frequent manifestations of unity of plan in defiance of teleological requirements. This is quite true. But it is not true, as Professor Owen argues respecting such cases, that “if the principle of special adaptation fails to explain them, and we reject the idea that these correspondences are manifestations of some archetypal exemplar, on which it has pleased the Creator to frame certain of his living creatures, there remains only the alternative that the organic atoms have concurred fortuitously to produce such harmony.” This is not the only alternative: there is another, which Professor Owen has overlooked. It is a perfectly tenable supposition that all higher vertebrate forms have arisen by the superposing of adaptations upon adaptations. Either of the two antagonist cosmogonies consists with this supposition. If, on the one hand, we conceive species to have resulted from acts of special creation; then it is quite a fair assumption that to produce a higher vertebrate animal, the Creator did not begin afresh, but took a lower vertebrate animal, and so far modified its pre-existing parts as to fit them for the new requirements; in which case the original structure would show itself through the superposed modifications. If, on the other hand, we conceive species to have resulted by gradual differentiations under the influence of changed conditions; then, it would manifestly follow that the higher, heterogeneous forms, would bear traces of the lower and more homogeneous forms from which they were evolved.

Thus, besides finding that the hypothesis of an “ideal typical vertebra” is irreconcilable with the facts, we find that the facts are interpretable without gratuitous assumptions. The average community of form which vertebræ display, is explicable as resulting from natural causes. And those typical similarities which are traceable under adaptive modifications, must obviously exist if, throughout creation in general, there has gone on that continuous superposing of modifications upon modifications which goes on in every unfolding organism.

* * * * *

[I might with propriety have added to the foregoing criticisms, the remark that Professor Owen has indirectly conferred a great benefit by the elaborate investigations he has made with the view of establishing his hypothesis. He has himself very conclusively proved that the teleological interpretation is quite irreconcilable with the facts. In gathering together evidence in support of his own conception of archetypal forms, he has disclosed adverse evidence which I think shows his conception to be untenable. The result is that the field is left clear for the hypothesis of Evolution as the only tenable one.]

APPENDIX C.

[From the TRANSACTIONS OF THE LINNEAN SOCIETY, VOL. XXV.]

XV. On Circulation and the Formation of Wood in Plants. By HERBERT SPENCER, Esq. Communicated by GEORGE BUSK, _Esq., F.R.S., Sec. L.S._

Read March 1st, 1866.

Opinions respecting the functions of the vascular tissues in plants appear to make but little progress towards agreement. The supposition that these vessels and strings of partially-united cells, lined with spiral, annular, reticulated, or other frameworks, are carriers of the plant-juices, is objected to on the ground that they often contain air: as the presence of air arrests the movement of blood through arteries and veins, its presence in the ducts of stems and petioles is assumed to unfit them as channels for sap. On the other hand, that these structures have a respiratory office, as some have thought, is certainly not more tenable, since, if the presence of air in them negatives the belief that their function is to distribute liquid, the presence of liquid in them equally negatives the belief that their function is to distribute air. Nor can any better defence be made for the hypothesis which I find propounded, that these parts serve “to give strength to the parenchyma.” Tubes with fenestrated and reticulated internal skeletons have, indeed, some power of supporting the tissue through which they pass; but tubes lined with spiral threads can yield extremely little support, while tubes lined with annuli, or spirals alternating with annuli, can yield no support whatever. Though all these types of internal framework are more or less efficient for preventing closure by lateral pressure, they are some of them quite useless for holding up the mass through which the vessels pass; and the best of them are for this purpose mechanically inferior to the simple cylinder. The same quantity of matter made into a continuous tube would be more effective in giving stiffness to the cellular tissue around it.

In the absence of any feasible alternative, the hypothesis that these vessels are distributors of sap claims reconsideration. The objections are not, I think, so serious as they seem. The habitual presence of air in the ducts that traverse wood, can scarcely be held anomalous if when the wood is formed their function ceases. The canals which ramify through a Stag’s horn, contain air after the Stag’s horn is fully developed; but it is not thereby rendered doubtful whether it is the function of arteries to convey blood. Again, that air should frequently be found even in the vessels of petioles and leaves, will not appear remarkable when we call to mind the conditions to which a leaf is subject. Evaporation is going on from it. The thinner liquids pass by osmose out of the vessels into the tissues containing the liquids thickened by evaporation. And as the vessels are thus continually drained, a draught is made upon the liquid contained in the stem and roots. Suppose that this draught is unusually great, or suppose that around the roots there exists no adequate supply of moisture. A state of capillary tension must result--a tendency of the liquid to pass into the leaves resisted below by liquid cohesion. Now, had the vessels impermeable coats, only their upper extremities would under these conditions be slowly emptied. But their coats, in common with all the surrounding tissues, are permeable by air. Hence, under this state of capillary tension, air will enter; and as the upper ends of the tubes, being both smaller in diameter and less porous than the lower, will retain the liquids with greater tenacity, the air will enter the wider and more porous tubes below--the ducts of the stem and branches. Thus the entrance of air no more proves that these ducts are not sap-carriers, than does the emptiness of tropical river-beds in the dry season prove that they are not channels for water. There is, however, a difficulty which seems more serious. It is said that air, when present in these minute canals, must be a great obstacle to the movement of sap through them. The investigations of Jamin have shown that bubbles in a capillary tube resist the passage of liquid, and that their resistance becomes very great when the bubbles are numerous--reaching, in some experiments, as much as three atmospheres. Nevertheless the inference that any such resistance is offered by the air-bubbles in the vessels of a plant, is, I think, an erroneous one. What happens in a capillary tube having impervious sides, with which these experiments were made, will by no means happen in a capillary tube having pervious sides. Any pressure brought to bear on the column of liquid contained in the porous duct of a plant, must quickly cause the expulsion of a contained air-bubble through the minute openings in the coats of the duct. The greater molecular mobility of gases than liquids, implies that air will pass out far more readily than sap. Whilst, therefore, a slight tension on the column of sap will cause it to part and the air to enter, a slight pressure upon it will force out the air and reunite the divided parts of the column.

To obtain data for an opinion on this vexed question, I have lately been experimenting on the absorption of dyes by plants. So far as I can learn, experiments of this kind have most, if not all of them, been made on stems, and, as it would seem from the results, on stems so far developed as to contain all their characteristic structures. The first experiments I made myself were on such parts, and yielded evidence that served but little to elucidate matters. It was only after trying like experiments with leaves of different ages and different characters, and with undeveloped axes, as well as with axes of special kinds, that comprehensible results were reached; and it then became manifest that the appearances presented by ordinary stems when thus tested, are in a great degree misleading. Let me briefly indicate the differences.

If an adult shoot of a tree or shrub be cut off, and have its lower end placed in an alumed decoction of logwood or a dilute solution of magenta,[69] the dye will, in the course of a few hours, ascend to a distance varying according to the rate of evaporation from the leaves. On making longitudinal sections of the part traversed by it, the dye is found to have penetrated extensive tracts of the woody tissue; and on making transverse sections, the openings of the ducts appear as empty spaces in the midst of a deeply-coloured prosenchyma. It would thus seem that the liquid is carried up the denser parts of the vascular bundles; neglecting the cambium layer, neglecting the central pith, and neglecting the spiral vessels of the medullary sheath. Apparently the substance of the wood has afforded the readiest channel. When, however, we examine these appearances critically, we find reasons for doubting this conclusion. If a transverse section of the lower part, into which the dye passed first and has remained longest, be compared with a transverse section of the part which the dye has but just reached, a marked difference is visible. In the one case the whole of the dense tissue is stained; in the other case it is not. This uneven distribution of stain in the part which the dye has incompletely permeated is not at random; it admits of definite description. A tolerably regular continuous ring of colour distinguishes the outer part of the wood from the inner mass, implying a passage of liquid up the elongated cells next the cambium layer. And the inner mass is coloured more round the mouths of the pitted ducts than elsewhere: the dense tissue is darkest close to the edges of these ducts; the colour fades away gradually on receding from their edges; there is most colour where there are several ducts together; and the dense tissue which is fully dyed for some space, is that which lies between two or more ducts. These are indications that while the layer of pitted cells next the cambium has served as a channel for part of the liquid, the rest has ascended the pitted ducts, and oozed out of these into the prosenchyma around. And this conclusion is confirmed by the contrast between the appearances of the lowest part of a shoot under different conditions. For if, instead of allowing the dye time for oozing through the prosenchyma, the end of the shoot be just dipped into the dye and taken out again, we find, on making transverse sections of the part into which the dye has been rapidly taken up, that, though it has diffused to some distance round the ducts, it has left tracts of wood between the ducts uncoloured--a difference which would not exist had the ascent been through the substance of the wood. Even still stronger is the confirmation obtained by using one dye after another. If a shoot that has absorbed magenta for an hour be placed for five minutes in the logwood decoction, transverse sections of it taken at a short distance from its end show the mouths of the ducts surrounded by dark stains in the midst of the much wider red stains.

Based on these comparisons only, the inference pointed out has little weight; but its weight is increased by the results of experiments on quite young shoots, and shoots that develope very little wood. The behaviour of these corresponds perfectly with the expectation that a liquid will ascend capillary tubes in preference to simple cellular tissue or tissue not differentiated into continuous canals. The vascular bundles of the medullary sheath are here the only channels which the coloured liquid takes. In sections of the parts up to which the dye has but just reached, the spiral, fenestrated, scalariform, or other vessels contained in these bundles are alone coloured, and lower down it is only after some hours that such an exudation of dye takes place as suffices partially to colour the other substances of the bundle. Further, it is to be noted that at the terminations of shoots, where the vessels are but incompletely formed out of irregularly-joined fibrous cells which still retain their original shapes, the dye runs up the incipient vessels and does not colour in the smallest degree the surrounding tissue.

Experiments with leaves bring out parallel facts. On placing in a dye a petiole of an adult leaf of a tree, and putting it before the fire to accelerate evaporation, the dye will be found to ascend the mid-rib and veins at various rates, up even to a foot per hour. At first it is confined to the vessels; but by the time it has reached the point of the leaf, it will commonly be seen that at the lower part it has diffused itself into the sheaths of the vessels. In a quite young leaf from the same shoot, we find a much more rigorous restriction of the dye to the vessels. On making oblique sections of its petiole, mid-rib, and veins, the vessels have the appearance of groups of sharply defined coloured rods imbedded in the green prosenchyma; and this marked contrast continues with scarcely an appreciable change after plenty of time has been allowed for exudation.

The facts thus grouped and thus contrasted seem, at first sight, to imply that while they are young the coats of these ramifying canals lined with spiral or allied structures are not readily permeable, but that, becoming porous as they grow old, they allow the liquids they carry to escape with increasing facility; and hence a possible interpretation of the fact that, in the older parts, the staining of the tissue around the vessels is so rapid as to suggest that the dye has ascended directly through this tissue, whereas in the younger parts the reverse appearance necessitates the reverse conclusion. But now, is this difference determined by difference of age, or is it otherwise determined? The evidence as presented in ordinary stems and leaves shows us that the parts of the vascular system at which there is a rapid escape of dye are not simply older parts, but are parts where a deposit of woody matter is taking place. Is it, then, that the increasing permeability of the ducts, instead of being directly associated with their increasing age, is directly associated with the increasing deposit of dense substance around them?

To get proof that this last connexion is the true one, we have but to take a class of cases in which wood is formed only to a small extent. In such cases experiments show us a far more general and continued limitation of the dye to the vessels. Ordinary herbs and vegetables, when contrasted with shrubs and trees, illustrate this; as instance the petioles of Celery, or of the common Dock, and the leaves of Cabbages or Turnips. And then in very succulent plants, such as Bryophyllum calycinum, Kalanchoë rotundifolia, the various species of Crassula, Cotyledon, Kleinia, and others of like habit, the ducts of old and young leaves alike retain the dye very persistently: the concomitant in these cases being the small amount of prosenchyma around the ducts, or the small amount of deposit in it, or both. More conclusive yet is the evidence which meets us when we turn from very succulent leaves to very succulent axes. The tender young shoots of Kleinia ante-euphorbium, or Euphorbia Mauritanica, which for many inches of their lengths have scarcely any ligneous fibres, show us scarcely any escape of the coloured liquid from the vessels of the medullary sheath. So, too, is it with Stapelia Buffonia, a plant of another order, having soft swollen axes. And then we have a repetition of the like connexion of facts throughout the Cactaceæ: the most succulent showing us the smallest permeability of the vessels. In two species of Rhipsalis, in two species of Cereus, and in two species of Mammillaria, which I have tried, I have found this so. Mammillaria gracilis may be named as exemplifying the relation under its extreme form. Into one of these small spheroidal masses, the dye ascends through the large bundles of spiral or annular ducts, or cells partially united into such ducts, colouring them deeply, and leaving the feebly-marked sheath of prosenchyma, together with the surrounding watery cellular tissue, perfectly uncoloured.

The most conclusive evidence, however, is furnished by those Cactaceæ in which the transition from succulent to dense tissue takes place variably, according as local circumstances determine. Opuntia yields good examples. If a piece of it including one of the joints at which wood is beginning to form, be allowed to absorb a coloured liquid, the liquid, running up the irregular bundles of vessels and into many of their minute ramifications, is restricted to these where they pass through the parenchyma forming the mass of the stem; but near the joints the hardened tissue around the vessels is coloured. In one of these fleshy growths we get clear evidence that the escape of the dye has no immediate dependence on the age of the vessels, since, in parts of the stem that are alike in age, some of the vessels retain their contents while others do not. Nay, we even find that the younger vessels are more pervious than the older ones, if round the younger ones there is a formation of wood.

Thus, then, is confirmed the inference before drawn, that in ordinary stems the staining of the wood by an ascending coloured liquid is due, not to the passage of the coloured liquid up the substance of the wood, but to the permeability of its ducts and such of its pitted cells as are united into irregular canals. And the facts showing this, at the same time indicate with tolerable clearness the process by which wood is formed. What in these cases is seen to take place with a dye, may be fairly presumed to take place with sap. Where the dye exudes but slowly, we may infer that the sap exudes but slowly; and it is a fair inference that where the dye leaks rapidly out of the vessels, the sap does the same. Inferring, thus, that where-ever there is a considerable formation of wood there is a considerable escape of the sap, we see in the one the result of the other. The thickening of the prosenchyma is proportionate to the quantity of nutritive liquid passing into it; and this nutritive liquid passes into it from the vessels, ducts, and irregular canals it surrounds.

But an objection is made to such experiments as the foregoing, and to all the inferences drawn from them. It is said that portions of plants cut off and thus treated, have their physiological actions arrested, or so changed as may render the results misleading; and it is said that when detached shoots and leaves have their cut ends placed in solutions, the open mouths of their vessels and ducts are directly presented with the liquids to be absorbed, which does not happen in their natural states. Further, making these objections look serious, it is alleged that when solutions are absorbed through the roots, quite different results are obtained: the absorbed matters are found in the tissues and not in the vessels. Clearly, were the experiments yielding these adverse results conducted in unobjectionable ways, the conclusion implied by them would negative the conclusions above drawn. But these experiments are no less objectionable than those to which they are opposed. Such mineral matters as salts of iron, solutions of which have in some cases been supplied to the roots for their absorption, are obviously so unlike the matters ordinarily absorbed, that they are likely to interfere fatally with the physiological actions. If experiments of this kind are made by immersing the roots in a dye, there is, besides the difficulty that the mineral mordant contained by the dye is injurious to the plant, the further difficulty that the colouring matter, being seized by the substances for which it has an affinity, is left behind in the first layers of root-tissues passed through, and that the decolorized water passing up into the plant is not traceable. To be conclusive, then, an experiment on absorption through roots must be made with some solution which will not seriously interfere with the plant’s vital processes, and which will not have its distinctive element left behind. To fulfil these requirements I adopted the following method. Having imbedded a well-soaked broad-bean in moist sand, contained in an inverted cone of cardboard with its apex cut off for the radicle to come through--having placed this in a wide-mouthed dwarf bottle, partly filled with water, so that the protruding radicle dipped into the water--and having waited until the young bean had a shoot some three or more inches high, and a cluster of secondary rootlets from an inch to an inch and a-half long--I supplied for its absorption a simple decoction of logwood, which, being a vegetal matter, was not likely to do it much harm, and which, being without a mordant, would not leave its suspended colour in the first tissues passed through. To avoid any possible injury, I did not remove the plant from the bottle, but slightly raising the cone out of its neck, I poured away the water through the crevice and then poured in the logwood decoction; so that there could have been no broken end or abraded surface of a rootlet through which the decoction might enter. Being prepared with some chloride of tin as a mordant, I cut off, after some three hours, one of the lowest leaves, expecting that the application of the mordant to the cut surface would bring out the characteristic colour if the logwood decoction had risen to that height. I got no reaction, however. But after eight hours I found, on cutting off another leaf, that the vessels of its petiole were made visible as dark streaks by the colour with which they were charged--a colour differing, as was to be expected, from that of the logwood decoction, which spontaneously changes even by simple exposure. It was then too late in the day to pursue the observations; but next morning the vessels of the whole plant, as far as the petioles of its highest unfolded leaves, were full of the colouring matter; and on applying chloride of tin to the cut surfaces, the vessels assumed that purplish red which this mordant produces when directly mixed with the logwood decoction. Subsequently, when one of the cotyledons was cut open by Prof. Oliver, to whom, in company with Dr. Hooker, I showed the specimen, we found that the whole of its vascular system was filled with the decoction, which everywhere gave the characteristic reaction. And it became manifest that the liquid absorbed through the rootlets, in the central vessels of which it was similarly traceable, had part of it passed directly up the vessels of the axis, while part of it had passed through other vessels into the cotyledon, out of which, no doubt, the liquid ordinarily so carried returns charged with a supply of the stored nutriment. I have since obtained a verification by varying the method. Digging up some young plants (Marigolds happened to afford the best choice) with large masses of soil round them, placing them in water, so as gradually to detach the soil without injuring the rootlets, planting them afresh in a flower-pot full of washed sand, and then, after a few days, watering them with a logwood decoction, I found, as before, that in less than twenty-four hours the colouring matter had run up into the vessels of the leaves. Though the reaction produced by the mordant was not so strong as before, it was marked enough to be quite unquestionable.

As these experiments were so conducted that there was no access to the vessels except through the natural channels, and as the vital actions of the plants were so little interfered with that at the end of twenty-four hours they showed no traces of disturbance, I think the results must be held conclusive.

Taking it, then, as a fact that in plants possessing them the vessels and ducts are the channels through which sap is distributed, we come now to the further question--What determines the varying permeability of the walls of the vessels and ducts, and the consequent varying formation of wood? To this question I believe the true reply is--The exposure of the parts to intermittent mechanical strains, actual or potential, or both. By actual strains I of course mean those which the plant experiences in the course of its individual life. By potential strains I mean those which the form, attitude, and circumstances common to its kind involve, and which its inherited structure is adapted to meet. In plants with stems, petioles, and leaves, having tolerably constant attitudes, the increasing porosity of the tubes and consequent deposit of dense tissue takes place in anticipation of the strains to which the parts of the individual are liable, but takes place at parts which have been habitually subject to such strains in ancestral individuals. But though in such plants the tendency to repeat that distribution of dense tissue caused by mechanical actions on past generations, goes on irrespective of the mechanical actions to which the developing individual is subject, these direct actions, while they greatly aid the assumption of the typical structure, are the sole causes of those deviations in the relative thickenings of parts which distinguish the individual from others of its kind. And then, in certain irregularly growing plants, such as Cactuses and Euphorbias, where the strains fall on parts that do not correspond in successive individuals, we distinctly trace a direct relation between the degrees of strain and the rates of these changes which result in dense tissue. I will not occupy space in detailing the evidence of this relation, which is conspicuous in the orders named, but will pass to the question--What are the physical processes by which intermittent mechanical strains produce this deposit of resistant substance at places where it is needed to meet the strains? We have not to seek far for an answer. If a trunk, a bough, a shoot, or a petiole, is bent by a gust of wind, the substance of its convex side is subject to longitudinal tension: the substance of its concave side being at the same time compressed. This is the primary mechanical effect. There is, however, a secondary mechanical effect, which here chiefly concerns us. That bend by which the tissues of the convex side are stretched, also produces lateral compression of them. Buttoning on a tight glove and then closing the hand, will make this necessity clear: the leather, while it is strained along the backs of the fingers, presses with considerable force on the knuckles. It is demonstrable that the tensions of the outer layer of a mass made convex by bending, must, by composition of forces, produce at every point a resultant at right angles to the layer beneath it; that, similarly, the joint tensions of these two layers must throw a pressure on the next deeper layer; and so on. Hence, if at some little distance beneath the surface of a stem, twig, or leaf-stalk, there exist longitudinal tubes, these tubes must be squeezed each time the side of the branch they are placed on becomes convex. Modifying the illustration just drawn from the clenched hand will make this clear. When, on forcibly grasping something, the skin is drawn tightly over the back of the hand, the whitening of the knuckles shows how the blood is expelled from the vessels below the surface by the pressure of the tightened skin. If, then, the sap-vessels must be thus compressed, what will happen to the liquid they contain? It will move away along the lines of least resistance. Part, and probably the greater part, will escape lengthways from the place of greatest pressure: some of it being expelled downwards, and some of it upwards. But, at the same time, part of it will be likely to ooze through the walls of the tubes. If these walls are so perfect as to permit the passage of liquid only by osmose, it may still be inferred that the osmose will increase under pressure; and probably, under recurrent pressure, the places at which the osmotic current passes most readily will become more and more permeable, until they eventually form pores. At any rate it is manifest that where pores and slits exist, whether thus formed or formed in any other way, the escape of sap into the adjacent tissue at each bend will become easy and rapid. What further must happen? When the branch or shoot recoils, the vessels on the side that was convex, being relieved from pressure, will tend to resume their previous diameters; and will be helped to do this by the elasticity of the surrounding tissue, as well as by those spiral, annular, and allied structures which they contain. But this resumption of their previous diameters must cause an immediate rush of sap back into them. Whence will it come? Not to any considerable extent from the surrounding tissues into which part of it has been squeezed, seeing that the resistance to the return of liquid through small pores will be greater than the resistance to its return along the vessels themselves. Manifestly the sap which was thrust up and down the vessels from the place of compression will return--the quantities returning from above and from below varying, as we shall hereafter see, according to circumstances. But this is not all. From some side a greater quantity must come back than was sent away; for the amount that has escaped out of the tube into the prosenchyma has to be replaced. Thus during the time when the side of the branch or twig becomes concave, more sap returns from above or below than was expelled upwards or downwards during the previous compression. The refilled vessels, when the next bend renders their side convex, again have part of their contents forced through their parietes, and are again refilled in the same way. There is thus set up a draught of sap to the place where these intermittent strains are going on, an exudation proportionate to the frequency and intensity of the strains, and a proportionate nutrition or thickening of the wood-cells, fitting them to resist the strains. A rude idea of this action may be obtained by grasping in one hand a damp sponge, having its lower end in water, while holding a piece of blotting-paper in contact with its upper end, and then giving the sponge repeated squeezes. At each squeeze some of the water will be sent into the blotting-paper; at each relaxation the sponge will refill from below, to give another portion of its contents to the blotting-paper when again squeezed.

But how does this explanation apply to roots? If the formation of wood is due to intermittent transverse strains, such as are produced in the aërial parts of upright plants by the wind, how does it happen that woody matter is deposited in roots, where there are no lateral oscillations, no transverse strains? The answer is, that longitudinal strains also are capable of causing the effects described. It is true that perfectly straight fibres united into a bundle and pulled lengthways would not exert on one another any lateral pressure, and would not laterally compress any similarly-straight canals running along with them. But if the fibres united into a bundle are variously bent or twisted, they cannot be longitudinally strained without compressing one another and structures imbedded in them. It needs but to watch a wet rope drawn tight by a capstan, to see that an action like that which squeezes the water out of its strands, will squeeze the sap out of the vessels of a root into the surrounding tissue, as often as the root is pulled by the swaying of the plant it belongs to. Here, too, as before, the vessels will refill when the pull intermits; and so, in the roots as in the branches, this rude pumping process will produce a growth of hard tissue proportionate to the stress to be borne.

These conclusions are supported by the evidence which exceptional cases supply. If intermittent mechanical strains thus cause the formation of wood where wood is found, then where it is not found, there should be an absence of intermittent mechanical strains. There is such an absence. Vascular plants characterized by little or no deposit of dense substance, are those having vessels so conditioned that no considerable pressures are borne by them. The more succulent a petiole or leaf becomes, the more do the effects of transverse strains fall on its outer layers of cells. Its mechanical support is chiefly derived from the ability of these minute vesicles, full of liquid, to resist bursting and tearing under the compressions and tensions they are exposed to. And just as fast as this change from a thin leaf or foot-stalk to a thick one entails increasing stress on the superficial tissue, so fast does it diminish the stress on the internally-seated vascular tissue. The succulent leaf cannot be swayed about by the wind as much as an ordinary leaf; and such small bends as can be given to it and its foot-stalk are prevented from affecting in any considerable degree the tubes running through its interior. Hence the retentiveness of the vessels in these fleshy leaves, as shown by the small exudation of dye; and hence the small thickening of their surrounding prosenchyma by woody deposit. Still more conspicuously is this connexion of facts shown when, from the soft thick leaves before named and such others as those of Echeveria, Rochea, Pereskia, we turn to the thick leaves that have strong exo-skeletons. Gasteria serves as an illustration. The leathery or horny skin here evidently bears the entire weight of the leaf, and is so stiff as to prevent any oscillation. Here, then, the vessels running inside are protected from all mechanical stress; and accordingly we find that the cells surrounding them are not appreciably thickened.

Equally clear, and more striking because more obviously exceptional, is the evidence given by succulent stems which are leafless. Stapelia Buffonia, having soft procumbent axes not liable to be bent backwards and forwards in any considerable degree by the wind, has, ramifying through its tissue, vessels that allow but an extremely slow escape of dye and have unthickened sheaths. Such of the Euphorbias as have acquired the fleshy character while retaining the arborescent growth, like Euphorbia Canariensis, teach us the same truth in another way. In them the formation of wood around the vessels is inconspicuous where the intermittent strains are but slight; but it is conspicuous at those joints on which lateral oscillations of the attached branches throw great extensions and compressions of tissue. Throughout the Cactaceæ we find varied examples of the alleged relation. Mammillaria furnishes a very marked one. The substance of one of these globular masses, resting on the ground, admits of no bending from side to side; and accordingly its large bundles of spiral and annular vessels, or partially-united cells, have very feebly-marked sheaths not at all thickened. In such types as Cereus and Opuntia we see, as in the Euphorbias, that where little stress falls on the vessels, little deposit takes place around them; while there is much deposit where there is much stress. Here let me add a confirmation obtained since writing the above. After observing among the Cactuses the very manifest relation between strain and the formation of wood, I inquired of Mr. Croucher, the intelligent foreman of the Cactus-house at Kew, whether he found this relation a constant one. He replied that he did, and that he had frequently tested it by artificially subjecting parts of them to strains. Neglecting at the time to inquire how he had done this, it afterwards occurred to me that if he had so done it as to cause constant strains, the observed result would not tell in favour of the foregoing interpretation. Subsequently, however, I learned that he had produced the strains by placing the plants in inclined attitudes--a method which, by permitting oscillations of the strained joints, allowed the strains to intermit. And then, making the proof conclusive, Mr. Croucher volunteered the statement that where he had produced constant strains by tying, no formation of wood took place.

Aberrant growths of another class display the same relations of phenomena. Take first the underground stems, such as the Potato and the Artichoke. The vessels which run through these, slowly take up the dye without letting it pass to any considerable extent into the surrounding tissues.[70] Only after an interval of many hours does the prosenchyma become stained in some places. Here, as before, an absence of rapid exudation accompanies an absence of woody deposit; and both these go along with the absence of intermittent strains. Take again the fleshy roots. The Turnip, the Carrot, and the Beetroot, have vessels that retain very persistently the coloured liquids they take up. And differing in this, as these roots do, from ordinary roots, we see that they also differ from them in not being woody, and in not being appreciably subject to the usual mechanical actions. In these cases, as in the others, parts that ordinarily become dense, deviate from this typical character when they are not exposed to those forces which produce dense tissue by increasing the extravasation of sap.

To complete the proof that such a relation exists, let me add the results of some experiments on equal and similarly-developed parts, kept respectively at rest and in motion. I have tested the effects on large petioles, on herbaceous shoots, and on woody shoots. If two such petioles as those of Rhubarb, with their leaves attached, have their cut ends inserted in bottles of dye, and the one be bent backwards and forwards while the other remains motionless, there arises, after the lapse of an hour, scarcely any difference in the states of their vessels: a certain proportion of these are in both cases charged with the dye, and little exudation has been produced by the motion. Here, however, it is to be observed that the causes of exudation are scarcely operative; the vascular bundles are distributed all through the mass of the petiole, which is formed of soft watery tissue; and they are, therefore, not so circumstanced as to be effectually compressed by the bends. In herbaceous stems, such as those of the Jerusalem Artichoke and of the Foxglove, an effect scarcely more decided is produced; and here, too, when we seek a reason, we find it in the non-fulfilment of the mechanical conditions; for the vascular bundles are not so seated between a tough layer of bark and a solid core as to be compressed at each bend. When, however, we come to experiment upon woody shoots, we meet with conspicuous effects, though by no means uniformly. In some cases oscillations produce immense amounts of exudation--parallel transverse sections of the compared shoots showing that where, in the one that has been at rest, there are spots of colour round but a few pitted ducts, in the one that has been kept in motion the substance of the wood is soaked almost uniformly through with dye. In other cases, especially where there is much undifferentiated tissue remaining, the exudation is not very marked. The difference appears to depend on the quantity of liquid contained in the shoot. If its substance is relatively dry, the exudation is great; but it is comparatively small if all the tissues are fully charged with sap. This contrast of results is one which contemplation of the mechanical actions will lead us to expect.

And now, with these facts to aid our interpretation, let us return to ordinary stems. If the upper end of a growing shoot, the prosenchyma of which is but little thickened, be allowed to imbibe the dye, the vessels of its medullary sheath alone become charged; and from them there takes place but a slow oozing. If a like experiment be tried with a lower part of the shoot, where the wood in course of formation has its inner boundary marked but not its outer boundary, we find that the pitted ducts, and more especially the inner ones, come into play. And then lower still, where the wood has its periphery defined and its histological characters decided, the appearances show that the tissue forming its outer surface begins to take a leading part in the transmission of liquid. What now is the explanation of these changes, mechanically considered? In the young soft part of the shoot, as in all normal and abnormal growths that have not formed wood, the channels for the passage of sap are the spiral, annular, fenestrated, or reticulated vessels. These vessels, here included in the bundles of the medullary sheath, are, in common with the tissues around them, subject, by the bendings of the shoot, to slight intermittent compressions, and, especially the outermost of them, are thus forced to give the prosenchyma an extra supply of nutritive liquid. The thickening of the prosenchyma, spreading laterally as well as outwards from each bundle of the medullary sheath, goes on until it meets the thickenings that spread from the other bundles; and there is so formed an irregular cylinder of hardened tissue, surrounding the medulla and the vascular bundles of its sheath. As soon as this happens, these vascular bundles become, to a considerable extent, shielded from the effects of transverse strains, since the tensions and compressions chiefly fall on the developing wood outside of them. Clearly, too, the greatest stress must be felt by the outer layer of the developing wood: being further removed from the neutral axis, it must be subject to severer strains at each bend; and lying between the bark and the layer of wood first formed, it must be most exposed to lateral compressions. Among the elongated cells of this outer layer, some unite to form the pitted ducts. Being, as we see, better circumstanced mechanically, they become greater carriers of sap than the original vessels, and, in consequence of this, as well as in consequence of their relative proximity, become the sources of nutrition to the still more external layers of wood-cells. The same causes and the same effects hold with each new indurated coat deposited round the previously indurated coats.

This description may be thought to go far towards justifying the current views respecting the course taken by the sap. But the justification is more apparent than real. In the first place, the implication here is that the sap-carrying function is at first discharged entirely by the vessels of the medullary sheath, and that they cease to discharge this function only as fast as they are relatively incapacitated by their mechanical circumstances. And the second implication is, that it is not the wood itself, but the more or less continuous canals formed in it, which are the subsequent sap-distributors. This, though readily made clear by microscopic examination of the large pitted ducts in a partially lignified shoot that has absorbed the dye, is less manifestly true of the peripheral layer of sap-carrying tissue finally formed. But it is really true here. For this layer, though nominally a layer of wood, is practically a layer of inosculating vessels. It is formed out of irregular lines and networks of elongated pitted cells, obliquely united by their ends. Examination of them after absorption of a dye, shows that it is only along the continuous channels they unite to form that the current has passed. But the essentially vascular character of this outer and latest-formed layer of the alburnum is best seen in the fact that the vascular systems of new axes take their rise from it, and form with it continuous canals. If a shoot of last year in which growth is recommencing, be cut lengthways after it has imbibed a dye, clear proof is obtained that the passage of the dye into a lateral bud takes place from this outermost layer of pitted cells, and that the channels taken by the dye through the new tissue are composed of cells that pass through modified forms into the spiral vessels of the new medullary sheath. This transition may be still more clearly traced in a terminal bud that continues the line of last year’s shoot. A longitudinal section of this shows that the vessels of the new medullary sheath do not obtain their sap from the vessels of last year’s sheath (which, as shown by the non-absorption of dye, have become inactive), but that their supplies are obtained from those inosculating canals formed out of last year’s outermost layer of prosenchyma, and that between the component cells of this and those of the new vascular system there are all gradations of structure.[71]

It is not the aim of the foregoing reasoning to show that mechanical actions are the sole causes of the formation of dense tissue in plants. Dense tissue is in many cases formed where no such causes have come into play--as, for example, in thorns and in the shells of nuts. Here the natural selection of variations can alone have operated. It is manifest, too, that even those supporting structures the building up of which is above ascribed to intermittent strains, may, in the individual plant of a species that ordinarily has them, be developed to a great extent when intermittent strains are prevented. We see this in trees that are artificially supported by nailing to walls; and we also see a kindred fact in natural climbers. Though in these cases the formation of wood is obviously less than it would be were the stem and branches habitually moved about by the wind, it nevertheless goes on. Clearly the tendency of the plant to repeat the structure of its type (in the one case the structure of its species; and in the other case that of the order from which it has diverged in becoming a climber) is here almost the sole cause of wood formation. But though in plants so circumstanced intermittent mechanical strains have little or no direct share, it may still be true, and I believe is true, that intermittent mechanical strains are the original cause; for, as before hinted, the typical structure which the individual thus repeats irrespective of its own conditions, is interpretable as a typical structure that is itself the product of these actions and reactions between the plant and its environment. Grant the inheritance of functionally-produced modifications; grant that natural selection will always co-operate in such way as to favour those individuals and families in which functionally-produced modifications have progressed most advantageously; and it will follow that this mechanically-caused formation of dense substance, accumulating from generation to generation by the survival of the fittest, will result in an organic habit of forming dense tissue at the required places. The deposit arising from exudation at the places of greatest strain, recurring from generation to generation at the same places, will come to be reproduced in anticipation of strain, and will continue to be reproduced for a long time after a changed habit of the species prevents the strain--eventually, however, decreasing, both through functional inactivity and natural selection, to the point at which it is in equilibrium with the requirement.

Another side of the general question may now be considered. We have seen how, by intermittent pressures on capillary vessels and ducts and inosculating canals, there must be produced a draught of sap towards the point of compression to replace the sap squeezed out. But we have still to inquire what will be the effect on the distribution of sap throughout the plant as a whole. It was concluded that out of the compressed vessels the greater part of the liquid would escape longitudinally--the longitudinal resistance to movement being least. In every case the probabilities are infinity to one against the resistances being equal upwards and downwards. Always, then, more sap will be expelled in one direction than in the other. But in whichever direction least sap is expelled, from that same direction most sap will return when the vessels are relieved from pressure--the force which is powerful in arresting the back current in that direction being the same force which is powerful in producing a forward current. Ordinarily, the more abundant supply of liquid being from below, there will result an upward current. At each bend a portion of the contents will be squeezed out through the sides of the vessels--a portion will be squeezed downwards, reversing the current ascending from the roots, but soon stopped by its resistance; while a larger portion will be squeezed upwards towards the extremities of the vessels, where consumption and loss are most rapid. At each recoil the vessels will be replenished, chiefly by the repressed upward current; and at the next bend more of it will be thrust onwards than backwards. Hence we have everywhere in action a kind of rude force-pump, worked by the wind; and we see how sap may thus be raised to a height far beyond that to which it could be raised by capillary action, aided by osmose and evaporation.

Thus far, however, the argument proceeds on the assumption that there is liquid enough to replenish every time the vessels subject to this process. But suppose the supply fails--suppose the roots have exhausted the surrounding stock of moisture. Evidently the vessels thus repeatedly having their contents squeezed out into the surrounding tissue, cannot go on refilling themselves from other vessels without tending to empty the vascular system. On the one hand, evaporation from the leaves causing a draught on the capillary tubes that end in them, continually generates a capillary tension upwards; while, on the other hand, the vessels below, expanding after their sap has been squeezed out, produce a tension both upwards and downwards towards the point of loss. Were the limiting membranes of the vessels impermeable, the movement of sap would, under these conditions, soon be arrested. But these membranes are permeable; and the surrounding tissues readily permit the passage of air. This state of tension, then, will cause an entrance of air into the tubes; the columns of liquid they contain will be interrupted by bubbles. It seems, indeed, not improbable that this entrance of air may take place even when there is a good supply of liquid, if the mechanical strains are so violent and the exudation so rapid that the currents cannot refill the half-emptied vessels with sufficient rapidity. And in this case the intruding air may possibly play the same part as that contained in the air-chamber of a force-pump--tending, by moderating the violence of the jets, and by equalizing the strains, to prevent rupture of the apparatus. Of course when the supply of liquid becomes adequate, and the strains not too violent, these bubbles will be expelled as readily as they entered.

Here, as before, let me add the conclusive proof furnished by a direct experiment. To ascertain the amount of this propulsive action, I took from the same tree, a Laurel, two equal shoots, and placing them in the same dye, subjected them to conditions that were alike in all respects save that of motion: while one remained at rest, the other was bent backwards and forwards, now by switching and now by straining with the fingers. After the lapse of an hour, I found that the dye had ascended the oscillating shoot three times as far as it had ascended the stationary shoot--this result being an average from several trials. Similar trials brought out similar effects in other structures. The various petioles and herbaceous shoots experimented upon for the purpose of ascertaining the amount of exudation produced by transverse strains, showed also the amount of longitudinal movement. It was observable that the height ascended by the dye was in all cases greater where there had been oscillation than where there had been rest--the difference, however, being much less marked in succulent structures than in woody ones.

It need scarcely be said that this mechanical action is not here assigned as the sole cause of circulation, but as a cause co-operating with others, and helping others to produce effects that could not otherwise be produced. Trees growing in conservatories afford us abundant proof that sap is raised to considerable heights by other forces. Though it is notorious that trees so circumstanced do not thrive unless, through open sashes, they are frequently subject to breezes sufficient to make their parts oscillate, yet there is evidently a circulation that goes on without mechanical aid. The causes of circulation are those actions only which disturb the liquid equilibrium in a plant, by permanently abstracting water or sap from some part of it; and of these the first is the absorption of materials for the formation of new tissue in growing parts; the second is the loss by evaporation, mainly through adult leaves; and the third is the loss by extravasation, through compressed vessels. Only so far as it produces this last, can mechanical strain be regarded as truly a cause of circulation. All the other actions concerned must be classed as aids to circulation--as facilitating that re-distribution of liquid that continually restores the equilibrium continually disturbed; and of these capillary action may be named as the first, osmose as the second, and the propulsive effect of mechanical strains as the third. The first two of these aids are doubtless capable by themselves of producing a large part of the observed result--more of the observed result than is at first sight manifest; for there is an important indirect effect of osmotic action which appears to be overlooked. Osmose does not aid circulation only by setting up, within the plant, exchange currents between the more dense and the less dense solutions in different parts of it; but it aids circulation much more by producing distention of the plant as a whole. In consequence of the average contrast in density between the water outside of the plant and the sap inside of it, the constant tendency is for the plant to absorb a quantity in excess of its capacity, and so to produce distention and erection of its tissues. It is because of this that the drooping plant raises itself when watered; for capillary action alone could only refill its tissues without changing their attitudes. And it is because of this that juicy plants with collapsible structures bleed so rapidly when cut, not only from the cut surface of the rooted part, but from the cut surface of the detached part--the elastic tissues tending to press out the liquid which distends them. And manifestly if osmose serves thus to maintain a state of distention throughout a plant, it indirectly furthers circulation; since immediately evaporation or growth at any part, by abstracting liquid from the neighbouring tissues, begins to diminish the liquid pressure within such tissues, the distended structures throughout the rest of the plant thrust their liquid contents towards the place of diminished pressure. This, indeed, may very possibly be the most efficient of the agencies at work. Remembering how great is the distention producible by osmotic absorption--great enough to burst a bladder--it is clear that the force with which the distended tissues of a plant urge forward the sap to places of consumption, is probably very great. We must therefore regard the aid which mechanical strains give as being one of several. Oscillations help directly to restore any disturbed liquid equilibrium; and they also help indirectly, by facilitating the re-distribution caused by capillary action and the process just described; but in the absence of oscillations the equilibrium may still be restored, though less rapidly and within narrower limits of distance.

One half of the problem of the circulation, however, has been left out of sight. Thus far our inquiry has been, how the ascending current of sap is produced. There remains the rationale of the descending current. What forces cause it, and through what tissues it takes place, are questions to which no satisfactory answers have been given. That the descent is due to gravitation, as some allege, is difficult to conceive, since, as gravitation acts equally on all liquid columns contained in the stem, it is not easy to see why it should produce downward movements in some while permitting upward movements in others--unless, indeed, there existed descending tubes too wide to admit of much capillary action, which there do not. Moreover, gravitation is clearly inadequate to cause currents towards the roots out of branches that droop to the ground. Here the gravitation of the contained liquid columns must nearly balance that of the connected columns in the stem, leaving no appreciable force to cause motion. Nor does there seem much probability in the assumption that the route of the descending sap is through the cambium layer, since experiments on the absorption of dyes prove that simple cellular tissue is a very bad conductor of liquids: their movement through it does not take place with one-fiftieth of the rapidity with which it takes place through vessels.[72]

Of course the defence for these hypotheses is, that there must be a downward current, which must have a course and a cause; and the very natural assumption has been that the course and the cause must be other than those which produce the ascending current. Nevertheless there is an alternative supposition to which the foregoing considerations introduce us. It is quite possible for the same vascular system to serve as a channel for movement in opposite directions at different times. We have among animals well-known cases in which the blood-vessels carry a current first in one direction and then, after a brief pause, in the reverse direction. And there seems an à priori probability that, lowly-organized as they are, plants are more likely to have distributing appliances of this imperfect kind than to have two sets of channels for two simultaneous currents. If, led by this suspicion, we inquire whether among the forces which unite to produce movements of sap, there are any variations or intermissions capable of determining the currents in different directions, we quickly discover that there are such, and that the hypothesis of an alternating motion of the sap, now centrifugal and now centripetal, through the same vessels, has good warrant. What are the several forces at work? First may be set down that tendency existing in every part of a plant to expand into its typical form, and to absorb nutritive liquids in doing this. The resulting competition for sap will, other things being equal, cause currents towards the most rapidly-growing parts--towards unfolding shoots and leaves, but not towards adult leaves. Next we have evaporation, acting more on the adult leaves than on those which are in the bud, or but partially developed. This evaporation is both regularly and irregularly intermittent. Depending chiefly on the action of the sun, it is, in fine weather, greatly checked or wholly arrested every evening; and in cloudy weather must be much retarded during the day. Further, every hygrometric variation, as well as every variation in the movement of the air, must vary the evaporation. This chief action, therefore, which, by continually emptying the ends of the capillary tubes, makes upward currents possible, is one which intermits every night, and every day is strong or feeble as circumstances determine. Then, in the third place, we have this rude pumping process above described, going on with greater vigour when the wind is violent, and with less vigour when it is gentle--drawing liquid towards different parts according to their degrees of oscillation, and from different parts according as they can most readily furnish it. And now let us ask what must result under changing conditions from these variously-conflicting and conspiring forces. When a warm sunshine, causing rapid evaporation, is emptying the vessels of the leaves, the osmotic and capillary actions that refill them will be continually aided by the pumping action of the swaying petioles, twigs, and branches, provided their oscillations are moderate. Under these conditions the current of sap, moving in the direction of least resistance, will set towards the leaves. But what will happen when the sun sets? There is now nothing to determine currents either upwards or downwards, except the relative rates of growth in the parts and the relative demands set up by the oscillations; and the oscillations acting alone, will draw sap to the oscillating parts as much from above as from below. If the resistance to be overcome by a current setting back from the leaves is less than the resistance to be overcome by a current setting up from the roots, then a current will set back from the leaves. Now it is, I think, tolerably manifest that in the swaying twigs and minor branches, less force will be required to overcome the inertia of the short columns of liquid between them and the leaves than to overcome the inertia of the long columns between them and the roots. Hence during the night, as also at other times when evaporation is not going on, the sap will be drawn out of the leaves into the adjacent supporting parts; and their nutrition will be increased. If the wind is strong enough to produce a swaying of the thicker branches, the back current will extend to them also; and a further strengthening will result from their absorption of the elaborated sap. And when the great branches and the stem are bent backwards and forwards by a gale, they too will share in the nutrition. It may at first sight seem that these parts, being nearer to the roots than to the leaves, will draw their supplies from the roots only. But the quantity which the roots can furnish is insufficient to meet so great a demand. Under the conditions described, the exudation of sap from the vessels will be very great, and the draught of liquid required to refill them, not satisfied by that which the root-fibres can take in, will extend to the leaves. Thus sap will flow to the several parts according to their respective degrees of activity--to the leaves while light and heat enable them to discharge their functions, and back to the twigs, branches, stem, and roots when these become active and the leaves inactive, or when their activity dominates over that of the leaves. And this distribution of nutriment, varying with the varying activities of the parts, is just such a distribution as we know must be required to keep up the organic balance.

To this explanation it may be objected that it does not account for the downward current of sap in plants that are sheltered. The stem and roots of a drawing-room Geranium display a thickening which implies that nutritive matters have descended from the leaves, although there are none of those oscillations by which the sap is said to be drawn downwards as well as upwards. The reply is, that the stem and roots tend to repeat their typical structures, and that the absorption of sap for the formation of their respective dense tissues, is here the force which determines the descent. Indeed it must be borne in mind that the mechanical strains and the pumping process which they keep up, as well as the distention caused by osmose, do not in themselves produce a current either upwards or downwards: they simply help to move the sap towards that place where there is the most rapid abstraction of it--the place towards which its motion is least resisted. Whether there is oscillation or whether there is not, the physiological demands of the different parts of the plant determine the direction of the current; and all which the oscillations and the distention do is to facilitate the supply of these demands. Just as much, therefore, in a plant at rest as in a plant in motion, the current will set downwards when the function of the leaves is arrested, and when there is nothing to resist that abstraction of sap caused by the tendency of the stem- and root-tissues to assume their typical structures. To which admission, however, it must be added that since this typical structure assumed, though imperfectly assumed, by the hothouse plant, is itself interpretable as the inherited effect of external mechanical actions on its ancestors, we may still consider the current set up by the assumption of the typical structure to be indirectly due to such actions.

Interesting evidence of another order here demands notice. In the course of experiments on the absorption of dyes by leaves, it happened that in making sections parallel to the plane of a leaf, with the view of separating its middle layer containing the vessels, I came upon some structures that were new to me. These structures, where they are present, form the terminations of the vascular system. They are masses of irregular and imperfectly united fibrous cells, such as those out of which vessels are developed; and they are sometimes slender, sometimes bulky--usually, however, being more or less club-shaped. In transverse sections of leaves their distinctive characters are not shown: they are taken for the smaller veins. It is only by carefully slicing away the surface of a leaf until we come down to that part which contains them, that we get any idea of their nature. Fig. 1 represents a specimen taken from a leaf of Euphorbia neriifolia. Occupying one of the interspaces of the ultimate venous network, it consists of a spirally-lined duct or set of ducts, which connects with the neighbouring vein a cluster of half-reticulated, half-scalariform cells. These cells have projections, many of them tapering, that insert themselves into the adjacent intercellular spaces, thus producing an extensive surface of contact between the organ and the imbedding tissues. A further trait is, that the ensheathing prosenchyma is either but little developed or wholly absent; and consequently this expanded vascular structure, especially at its end, comes immediately in contact with the tissues concerned in assimilation. The leaf of Euphorbia neriifolia is a very fleshy one; and in it these organs are distributed through a compact, though watery, cellular mass. But in any leaf of the ordinary type which possesses them, they lie in the network-parenchyma composing its lower layer; and wherever they occur in this layer its cells unite to enclose them. This arrangement is shown in fig. 2, representing a sample from the Caoutchouc-leaf, as seen with the upper part of its envelope removed; and it is shown still more clearly in a sample from the leaf of Panax Lessonii, fig. 3. Figures 4 and 5 represent, without their sheaths, other such organs from the leaves of Panax Lessonii and Clusia flava. Some relation seems to exist between their forms and the thicknesses of the layers in which they lie. Certain very thick leaves, such as those of Clusia flava, have them less abundantly distributed than is usual, but more massive. Where the parenchyma is developed not to so great an extreme, though still largely, as in the leaves of Holly, Aucuba, Camellia, they are not so bulky; and in thinner leaves, like those of Privet, Elder, &c., they become longer and less conspicuously club-shaped. Some adaptations to their respective positions seem implied by these modifications; and we may naturally expect that in many thin leaves these free ends, becoming still narrower, lose the distinctive and suggestive characters possessed by those shown in the diagrams. Relations of this kind are not regular, however. In various other genera, members of which I have examined, as Rhus, Viburnum, Griselinia, Brexia, Botryodendron, Pereskia, the variations in the bulk and form of these structures are not directly determined by the spaces which the leaves allow: obviously there are other modifying causes. It should be added that while these expanded free extremities graduate into tapering free extremities, not differing from ordinary vessels, they also pass insensibly into the ordinary inosculations. Occasionally, along with numerous free endings, there occur loops; and from such loops there are transitions to the ultimate meshes of the veins.

These organs are by no means common to all leaves. In many that afford ample spaces for them they are not to be found. So far as I have observed, they are absent from the thick leaves of plants which form very little wood. In Sempervivum, in Echeveria, in Bryophyllum, they do not appear to exist; and I have been unable to discover them in Kalanchoë rotundifolia, in Kleinia ante-euphorbium and ficoides, in the several species of Crassula, and in other succulent plants. It may be added that they are not absolutely confined to leaves, but occur in stems that have assumed the functions of leaves. At least I have found, in the green parenchyma of Opuntia, organs that are analogous though much more rudely and irregularly formed. In other parts, too, that have usurped the leaf-function, they occur, as in the phyllodes of the Australian Acacias. These have them abundantly developed; and it is interesting to observe that here, where the two vertically-placed surfaces of the flattened-out petiole are equally adapted to the assimilative function, there exist two layers of these expanded vascular terminations, one applied to the inner surface of each layer of parenchyma.

Considering the structures and positions of these organs, as well as the natures of the plants possessing them, may we not form a shrewd suspicion respecting their function? Is it not probable that they facilitate absorption of the juices carried back from the leaf for the nutrition of the stem and roots? They are admirably adapted for performing this office. Their component fibrous cells, having angles insinuated between the cells of the parenchyma, are shaped just as they should be for taking up its contents; and the absence of sheathing tissue between them and the parenchyma facilitates the passage of the elaborated liquids. Moreover there is the fact that they are allied to organs which obviously have absorbent functions. I am indebted to Dr. Hooker for pointing out the figures of two such organs in the “Icones Anatomicæ” of Link. One of them is from the end of a dicotyledonous root-fibre, and the other is from the prothallus of a young Fern. In each case a cluster of fibrous cells, seated at a place from which liquid has to be drawn, is connected by vessels with the parts to which liquid has to be carried. There can scarcely be a doubt, then, that in both cases absorption is effected through them. I have met with another such organ, more elaborately constructed, but evidently adapted to the same office, in the common Turnip-root. As shown by the end view and longitudinal section in figs. 6 and 7, this organ consists of rings of fenestrated cells, arranged with varying degrees of regularity into a funnel, ordinarily having its apex directed towards the central mass of the Turnip, with which it has, in some cases at least, a traceable connexion by a canal. Presenting as it does an external porous surface terminating one of the branches of the vascular system, each of these organs is well fitted for taking up with rapidity the nutriment laid by in the Turnip-root, and used by the plant when it sends up its flower-stalk. Nor does even this exhaust the analogies. The cotyledons of the young bean, experimented upon as before described, furnished other examples of such structures, exactly in the places where, if they are absorbents, we might expect to find them. Amid the branchings and inosculations of the vascular layer running through the mass of nutriment deposited in each cotyledon, there are conspicuous free terminations that are club-shaped, and prove to be composed, like those in leaves, of irregularly formed and clustered fibrous cells; and some of them, diverging from the plane of the vascular layer, dip down into the mass of starch and albumen which the young plant has to utilize, and which these structures can have no other function but to take up.

Besides being so well fitted for absorption, and besides being similar to organs which we cannot doubt are absorbents, these vascular terminations in leaves afford us yet another evidence of their functions. They are seated in a tissue so arranged as specially to facilitate the abstraction of liquid. The centripetal movement of the sap must be set up by a force that is comparatively feeble, since, the parietes of the ducts being porous, air will enter if the tension on the contained columns becomes considerable. Hence it is needful that the exit of sap from the leaves should meet with very little resistance. Now were it not for an adjustment presently to be described, it would meet with great resistance, notwithstanding the peculiar fitness of these organs to take it in. Liquid cannot be drawn out of any closed cavity without producing a collapse of the cavity’s sides; and if its sides are not readily collapsible, there must be a corresponding resistance to the abstraction of liquid from it. Clearly the like must happen if the liquid is to be drawn out of a tissue which cannot either diminish in bulk bodily or allow its components individually to diminish in bulk. In an ordinary leaf, the upper layer of parenchyma, formed as it is of closely-packed cells that are without interspaces, and are everywhere held fast within their framework of veins, can neither contract easily as a mass, nor allow its separate cells to do so. Quite otherwise is it with the network-parenchyma below. The long cells of this, united merely by their ends and having their flexible sides surrounded by air, may severally have their contents considerably increased and decreased without offering appreciable resistances: and the network-tissue which they form will, at the same time, be capable of undergoing slight expansions and contractions of its thickness. In this layer occur these organs that are so obviously fitted for absorption. Here we find them in direct communication with its system of collapsible cells. The probability appears to be, that when the current sets into the leaf, it passes through the vessels and their sheaths chiefly into the upper layer of cells (this upper layer having a larger surface of contact with the veins than the lower layer, and being the seat of more active processes); and that the juices of the upper layer, enriched by the assimilated matters, pass into the network-parenchyma, which serves as a reservoir from which they are from time to time drawn for the nutrition of the rest of the plant, when the actions determine the downward current. Should it be asked what happens where the absorbents, instead of being inserted in a network-parenchyma, are, as in the leaves of Euphorbia neriifolia, inserted in a solid parenchyma, the reply is, that such a parenchyma, though not furnished with systematically arranged air-chambers, nevertheless contains air in its intercellular spaces; and that when there occurs a draught upon its contents, the expansion of this air and the entrance of more from without, quickly supply the place of the abstracted liquid.

If then, returning to the general argument, we conclude that these expanded terminations of the vascular system in leaves are absorbent organs, we find a further confirmation of the views set forth respecting the alternating movement of the sap along the same channels. These spongioles of the leaves, like the spongioles of the roots, being appliances by which liquid is taken up to be carried into the mass of the plant, we are obliged to regard the vessels that end in these spongioles of the leaves as being the channels of the down current whenever it is produced. If the elaborated sap is abstracted from the leaves by these absorbents, then we have no alternative but to suppose that, having entered the vascular system, the elaborated sap descends through it. And seeing how, by the help of these special terminations, it becomes possible for the same vessels to carry back a quality of sap unlike that which they bring up, we are enabled to understand tolerably well how this rhythmical movement produces a downward transfer of materials for growth.

* * * * *

The several lines of argument may now be brought together; and along with them may be woven up such evidences as remain. Let me first point out the variety of questions to which the hypothesis supplies answers.

It is required to account for the ascent of sap to a height beyond that to which capillary action can raise it. This ascent is accounted for by the propulsive action of transverse strains, joined with that of osmotic distention. A cause has to be assigned for that rise of sap which, in the spring, while yet there is no considerable evaporation to aid it, goes on with a power which capillarity does not explain. The co-operation of the same two agencies is assignable for this result also.[73] The circumstance that vessels and ducts here contain sap and there contain air, and at the same place contain at different seasons now air and now sap is a fact calling for explanation. An explanation is furnished by these mechanical actions which involve the entrance or expulsion of air according to the supply of liquid. That vessels and ducts which were originally active sap-carriers go completely out of use, and have their function discharged by other vessels or ducts, is an anomaly that has to be solved. Again, we are supplied with a solution: these deserted vessels and ducts are those which, by the formation of dense tissue outside of them, become so circumstanced that they cannot be compressed as they originally were. A channel has to be found for the downward current of sap, which, on any other hypothesis than the foregoing, must be a channel separate from that taken by the upward current; and yet no good evidence of a separate channel has been pointed out. Here, however, the difficulty disappears, since one channel suffices for the current alternating upwards and downwards according to the conditions. Moreover there has to be found a force producing or facilitating the downward current, capable even of drawing sap out of drooping branches; and no such force is forthcoming. The hypothesis set forth dispenses with this necessity; under the recurring change of conditions, the same distention and oscillation which before raised the sap to the places of consumption, now bring it down to the places of consumption. A physical process has to be pointed out by which the material that forms dense tissue is deposited at the places where it is wanted, rather than at other places. This physical process the hypothesis indicates. It is requisite to find an explanation of the fact that, when plants ordinarily swayed about by the wind are grown indoors, the formation of wood is so much diminished that they become abnormally slender. Of this an explanation is supplied. Yet a further fact to be interpreted is, that in the same individual plant homologous parts, which, according to the type of the plant, should be equally woody, become much thicker one than another if subject to greater mechanical stress. And of this too an interpretation is similarly afforded.

Now the sufficiency of the assigned actions to account for so many phenomena not otherwise explained, would be strong evidence that the rationale is the true one, even were it of a purely hypothetical kind. How strong, then, becomes the reason for believing it the true one when we remember that the actions alleged demonstrably go on in the way asserted. They are ever operating before our eyes; and that they produce the effects in question is a conclusion deducible from mechanical principles, a conclusion established by induction, and a conclusion verified by experiment. These three orders of proof may be briefly summed up as follows.

That plants which have to raise themselves above the earth’s surface, and to withstand the actions of the wind, must have a power of developing supporting structure, is an à priori conclusion which may be safely drawn. It is an equally safe à priori conclusion, that if the supporting structure, either as a whole or in any of its parts, has to adapt itself to the particular strains which the individual plant is subject to by its particular circumstances, there must be at work some process by which the strength of the supporting structure is everywhere brought into equilibrium with the forces it has to bear. Though the typical distribution of supporting structure in each kind of plant may be explained teleologically by those whom teleological explanations satisfy; and though otherwise this typical distribution may be ascribed to natural selection acting apart from any directly adaptive process; yet it is manifest that those departures from the typical distribution which fit the parts of each plant to their special conditions are explicable neither teleologically nor by natural selection. We are, therefore, compelled to admit that, if in each plant there goes on a balancing of the particular strains by the particular strengths, there must be a physical or physico-chemical process by which the adjustments of the two are effected. Meanwhile we are equally compelled to admit, à priori, that the mechanical actions to be resisted, themselves affect the internal tissues in such ways as to further the increase of that dense substance by which they are resisted. It is demonstrable that bending the petioles, shoots, and stems must compress the vessels beneath their surfaces, and increase the exudation of nutritive matters from them, and must do this actively in proportion as the bends are great and frequent; so that while, on the one hand, it is a necessary deduction that, if the parts of each plant are to be severally strengthened according to the several strains, there must be some direct connexion between strains and strengths, it is, on the other hand, a necessary deduction from mechanical principles that the strains do act in such ways as to aid the increase of the strengths. How a like correspondence between two à priori arguments holds in the case of the circulation, needs not to be shown in detail. It will suffice to remind the reader that while the raising of sap to heights beyond the limit of capillarity implies some force to effect it, we have in the osmotic distention and the intermittent compressions caused by transverse strains, forces which, under the conditions, cannot but tend to effect it; and similarly with the requirement for a downward current, and the production of a downward current.

Among the inductive proofs we find a kindred agreement. Different individuals of the same species, and different parts of the same individual, do strengthen in different degrees; and there is a clearly traceable connexion between their strengthenings and the intermittent strains they are exposed to. This evidence, derived from contrasts between growths on the same plant or on plants of the same type, is enforced by evidence derived from contrasts between plants of different types. The deficiency of woody tissue which we see in plants called succulent, is accompanied by a bulkiness of the parts which prevents any considerable oscillations; and this character is also habitually accompanied by a dwarfed growth. When, leaving these relations as displayed externally, we examine them internally, we find the facts uniting to show, by their agreements and differences, that between the compression of the sap-canals and the production of wood there is a direct relation. We have the facts, that in each plant, and in every new part of each plant, the formation of sap-canals precedes the formation of wood; that the deposit of woody matter, when it begins, takes place around these sap-canals, and afterwards around the new sap-canals successively developed; that this formation of wood around the sap-canals takes place where the coats of the canals are demonstrably permeable, and that the amount of wood formation is proportionate to the permeability. And then that the permeability and extravasation of sap occur wherever, in the individual or in the type, there are intermittent compressions, is proved alike by ordinary cases and by exceptional cases. In the one class of cases we see that the deposit of wood round the vessels begins to take place when they come into positions that subject them to intermittent compressions, while it ceases when they become shielded from compressions. And in the other class of cases, where, from the beginning, the vessels are shielded from compression by surrounding fleshy tissue, there is a permanent absence of wood formation.

To which complete agreement between the deductive and inductive inferences has to be added the direct proof supplied by experiments. It is put beyond doubt by experiment that the liquids absorbed by plants are distributed to their different parts through their vessels--at first by the spiral or allied vessels originally developed, and then by the better-placed ducts formed later. By experiment it is demonstrated that the intermittent compressions caused by oscillations urge the sap along the vessels and ducts. And it is also experimentally proved that the same intermittent compressions produce exudation of sap from vessels and ducts into the surrounding tissue.

That the processes here described, acting through all past time, have sufficed of themselves to develope the supporting and distributing structures of plants, is not alleged. What share the natural selection of variations distinguished as spontaneous, has had in establishing them, is a question which remains to be discussed. Whether acting alone natural selection would have sufficed to evolve these vascular and resisting tissues, I do not profess to say. That it has been a co-operating cause, I take to be self-evident: it must all along have furthered the action of any other cause, by preserving the individuals on which such other cause had acted most favourably. Seeing, however, the conclusive proof which we have that another cause has been in action--certainly on individuals, and, in all probability, by inheritance on races--we may most philosophically ascribe the genesis of these internal structures to this cause, and regard natural selection as having here played the part of an accelerator.

EXPLANATION OF PLATE.

Fig. 1. Absorbent organ from the leaf of Euphorbia neriifolia. The cluster of fibrous cells forming one of the terminations of the vascular system is here imbedded in a solid parenchyma.

Fig. 2. A structure of analogous kind from the leaf of Ficus elastica. Here the expanded terminations of the vessels are imbedded in the network-parenchyma, the cells of which unite to form envelopes for them.

Fig. 3. Shows on a larger scale one of these absorbents from the leaf of Panax Lessonii. In this figure is clearly seen the way in which the cells of the network-parenchyma unite into a closely-fitting case for the spiral cells.

Fig. 4. Represents a much more massive absorbent from the same leaf, the surrounding tissues being omitted.

Fig. 5. Similarly represents, without its sheath, an absorbent from the leaf of Clusia flava.

Fig. 6. End view of an absorbent organ from the root of a Turnip. It is taken from the outermost layer of vessels. Its funnel-shaped interior is drawn as it presents itself when looked at from the outside of this layer, its narrow end being directed towards the centre of the Turnip.

Fig. 7. A longitudinal section through the axis of another such organ, showing its annuli of reticulated cells when cut through. The cellular tissue which fills the interior is supposed to be removed.

Fig. 8. A less developed absorbent, showing its approximate connexion with a duct. In their simplest forms, these structures consist of only two fenestrated cells, with their ends bent round so as to meet. Such types occur in the central mass of the Turnip, where the vascular system is relatively imperfect. Besides the comparatively regular forms of these absorbents, there are forms composed of amorphous masses of fenestrated cells. It should be added that both the regular and irregular kinds are very variable in their numbers: in some turnips they are abundant, and in others scarcely to be found. Possibly their presence depends on the age of the Turnip. Judging from the period during which my investigations were made, namely winter and early spring, I suspect that they are developed only in preparation for sending up the flower-stalk.

Let me add that experiments on circulation in plants made during the state of inactivity, when it is to be presumed that the vessels and tissues contain but little gap, are much more successful than those made in the summer. It would seem that when the tissues are fully charged with sap the taking up of dyes is comparatively slow and the above-described effects are not so easily demonstrable.

* * * * *

[An expert writes concerning this essay:--“I have not attempted to annotate critically this paper. There is no doubt that many of your conclusions are perfectly sound, particularly those relating to the passage of crude sap through the cavities of the elements of the wood, though the opinion that the actual passage was through the walls very generally held till about 12 years ago.”]

APPENDIX D.

ON THE ORIGIN OF THE VERTEBRATE TYPE.

[When studying the development of the vertebrate skeleton, there occurred to me the following idea respecting the possible origin of the notochord. I was eventually led to omit the few pages of Appendix in which I had expressed this idea, because it was unsupported by developmental evidence. The developmental evidence recently discovered, however, has led Professor Haeckel and others to analogous views respecting the affiliation of the Vertebrata on the Molluscoida. Having fortunately preserved a proof of the suppressed pages, I am able now to add them. With the omission of a superfluous paragraph, they are reprinted verbatim from this proof, which dates back to the autumn of 1865, at which time the chapter on “The Shapes of Vertebrate Skeletons” was written.--December, 1869.]

The general argument contained in Chap. XVI. of Part IV., I have thought it undesirable to implicate with any conception more speculative than those essential to it; and to avoid so implicating it, I transfer to this place an hypothesis respecting the derivation of the rudimentary vertebrate structure, which appears to me worth considering.

Among those molluscoid animals with which the lowest vertebrate animal has sundry traits in common, it very generally happens that while the adult is stationary the larva is locomotive. The locomotion of the larva is effected by the undulations of a tail. In shape and movement one of these young Ascidians is not altogether unlike a Tadpole. And as the tail of the Tadpole disappears when its function comes to be fulfilled by limbs; so the Ascidian larva’s tail disappears when fixation of the larva renders it useless. This disappearance of the tail, however, is not without exception. The Appendicularia is an Ascidian which retains its tail throughout life; and by its aid continues throughout life to swim about. Now this tail of the Appendicularia has a very suggestive structure. It is long, tapering to a point, and flattened. From end to end there runs a mid-rib, which appears to be an imbedded gelatinous rod, not unlike a notochord. Extending along the two sides of this mid-rib, are bundles of muscular fibres; and its top bears a gangliated nervous thread, giving off, at intervals, branches to the muscular fibres. In the Appendicularia this tail, which is inserted at the lower part of the back, is bent forwards, so as not to be adapted for propelling the body of the animal head foremost; but the homologous tails of the larval Ascidians are directed backwards, so as to produce forward movement. If we suppose a type like the Appendicularia in the structure and insertion of its permanent tail, but resembling the larval forms in the direction of its tail, it is, I think, not difficult to see that functional adaptation joined with natural selection, might readily produce a type approximating to that whose origin we are considering. It is a fair assumption that an habitually-locomotive creature would profit by increased power of locomotion. This granted, it follows that such further development of the tail-structures as might arise from enhanced function, and such better distribution of them as spontaneous variation might from time to time initiate, would be perpetuated. What must be the accompanying changes? The more vigorous action of such an appendage implies a firmer insertion into the body; and this would be effected by the prolongation forwards of the central axis of the tail into the creature’s back. As fast as there progressed this fusion of the increasingly-powerful tail with the body, the body would begin to partake of its oscillations; and at the same time that the resistant axis of the tail advanced along the dorsal region, its accompanying muscular fibres would spread over the sides of the body: gradually taking such modified directions and insertions as their new conditions rendered most advantageous. Without further explanation, those who examine drawings of the structures described, will, I think, see that in such a way a tail homologous with that of the Appendicularia, would be likely, in the course of that development required for its greater efficiency, gradually to encroach on the body, until its mid-rib became the dorsal axis, its gangliated nerve-thread the spinal chord, and its muscular fibres the myocommata. Such a development of an appendage into a dominant part of the organism, though at first sight a startling supposition, is not without plenty of parallels: instance the way in which the cerebral ganglia, originally mere adjuncts of the spinal chord, eventually become the great centres of the nervous system to which the spinal chord is quite subordinate; or instance the way in which the limbs, small and inconspicuous in fishes, become, in Man, masses which, taken together, outweigh the trunk. It may be added that these familiar cases have a further appropriateness; for they exhibit higher degrees of that same increasing dominance of the organs of external relation, which the hypothesis itself implies.

Of course, if the rudimentary vertebrate apparatus thus grew into, and spread over, a molluscoid visceral system, the formation of the notochord under the action of alternating transverse strains, did not take place as suggested in § 255; but it does not therefore follow that its differentiation from surrounding tissues was not mechanically initiated in the way described. For what was said in that section respecting the effects of lateral bendings of the body, equally applies to lateral bendings of the tail; and as fast as the developing tail encroached on the body, the body would become implicated in the transverse strains, and the differentiation would advance forwards under the influences originally alleged. Obviously, too, though the lateral muscular masses would in this case have a different history; yet the segmentation of them would be eventually determined by the assigned causes. For as fast as the strata of contractile fibres, developing somewhat in advance of the dorsal axis, spread along the sides, they would come under the influence of the alternate flexions; and while, by survival of the fittest, their parts became adjusted in direction, their segmentation would, as before, accompany their increasing massiveness. The actions and reactions due to lateral undulations would still, therefore, be the causes of differentiation, with which natural selection would co-operate.

APPENDIX D 2.

THE ANNULOSE TYPE.

The production of a segmental structure by undulatory movements, suggested in Appendix D, as also in B (first published in 1858) as explaining the vertebral column, has been recently suggested by Prof. Korschelt as the cause of that segmentation of the annulose type which gives the name to it. He espouses a--

“view which is based upon the assumption that at first an unsegmented, elongated ancestral form was produced by terminal growth, whereupon the entire body became separated at once into a large number of segments by a re-arrangement of the individual organs. This assumption is supported by the consideration that with the lateral sinuous movement of the body, and with the rigidity of the tissues caused by increasing differentiation, the formation of alternating regions of greater and less motility was of considerable advantage to the individual, and rendered possible a further elongation of the body. The first cause for the appearance of metameric segmentation would then be sought in the manner of locomotion and in mechanical conditions. However, this latter view is not supported in any way by embryology.” (Embryology of Invertebrates, Part I, pp. 349–50.)

I venture to think the confession that this view “is not supported in any way by embryology” should be joined with the confession that it is at variance with that abstract embryology which comprehends the process of development in general. The assumption that there took place “a re-arrangement of the individual organs” of “an unsegmented, elongated ancestral form,” in such wise that the organs, previously single, presently became multiple, so that instead of one organ of each kind there were substituted many organs of each kind, is inconsistent with the general law of evolution, organic and other--implies not integration but disintegration. Everywhere the advance is from many like parts performing like functions to relatively few unlike parts performing unlike functions. The higher forms of the annulose type itself show this. Compare a myriapod and a crab. In the one we have not only a great number of similar segments bearing similar limbs, but we have in each segment a dilatation of the main blood-vessel--a rudimentary heart--a swollen portion of the nerve cord--a small ganglion--and so on; whereas in the other, besides relatively few segments and few limbs (sundry of them extremely unlike the rest) we have a vascular system concentrated into a central heart with arteries and a concentrated nervous system, such that the great ganglia in the integrated carapace immensely subordinate the ganglia of the remaining segments; and similarly with the other organs. Now unless it be denied that these highest decapods have been evolved from low types akin to myriapods in composition, it must be admitted that the progress has been from a string of many like segments with similar sets of organs to a group of relatively-few unlike segments with dissimilar sets of organs. If so we cannot rationally deny that the progress has been of this nature up from the lowest annelid, instead of having been, as Prof. Korschelt’s hypothesis implies, of opposite nature at the beginning.

In a preceding passage a clear recognition of the normal course of development occurs. In opposing the view set forth in §§ 205–7 of this work, Prof. Korschelt says:--

“It seems scarcely favourable to this theory that the degree of independence which the individual segments present is comparatively slight. The most important organs (nervous system, body musculature, blood-vascular system) show themselves to be single fundaments of the entire body, and are also developed as such even though they also exhibit evidences of metamerism. Even the excretory canals may give up their segmental isolation and become united to one another by means of longitudinal canals.” (Ib. p. 348.)

On turning back to § 206, the reader will, I think, demur to the assertion that the independence is “comparatively slight”; seeing that, as in Ctenodrilus, a single segment sometimes becomes separate and reproduces other segments to form a new series. Instead of admitting that “the most important organs” “show themselves to be single fundaments of the entire body,” it may be held, contrariwise, that their original independence in each segment is masked only to the degree involved by their co-operation as parts of a compound organism. But chiefly I remark that when it is said that “the excretory canals may give up their segmental isolation and become united” by “longitudinal canals,” there is a clear confession that the isolation of these organs was original and their union superinduced--an implication that the course of evolution is as I have described it, and at variance with the course of evolution assumed by Prof. Korschelt.

Yet another incongruity is involved in his interpretation. He writes:--

“Just as in the consideration of the tapeworm chain we were induced by the comparison with unsegmented forms to refer the entire chain to an unsegmented individual, and, on the other hand, to see in the proglottis, not a complete individual, but only the abstricted hinder portion of the body of the Cestode, in the same manner, and with much more reason, we adhere to the individuality of the Annelid body.” (P. 349.)

And then on the preceding page, referring to the composition of the Annelid body, he says:--“The most natural comparisons are those with the tapeworm chain and with the strobila of the Scyphomedusæ.” Now since it is here assumed that the tapeworm and the strobila are analogous in composition, it is implied that the detached proglottis and the detached medusa are analogous; and hence if we are to regard the proglottis as “not a complete individual but only the abstricted hinder portion of the body of the Cestode,” then we must similarly regard the medusa as not a complete individual, but only the abstricted hinder portion of the strobila. This commits us to the strange conclusion that whereas individuality is ascribed to the original simple polyp, and by and by to the partially-segmented strobila, though these are without special senses and with only rudiments of muscular and nervous systems, individuality is denied to the detached medusa, which has organs of sense, a distinct nervo-muscular system and a considerable power of locomotion, as well as a generative system: traits which in other cases characterize developed individuals. Here also, then, there seems to be an inversion of the ordinary conception.

This conception of the proglottis and the medusa is, I see, accepted by some as tenable. But if we accept it we must accept also an analogous conception, which will I think be regarded as untenable. It is that supplied by the Aphides. From an egg proceeds a series of sexless and wingless females, and at the end of the series there come winged males and females with resulting gamic reproduction. If instead of forming a discrete series the imperfect females formed a concrete series, the members of which could individually feed without being detached from one another, as the segments of a tapeworm can, the parallelism would be complete; and then, according to the view in question, we should have to regard the perfect males and females eventually arising, not as individuals but as terminal portions of the series, containing generative products and having wings for the dispersion of them--locomotive egg-bearing segments of the chain. Whoever espouses this view must hold either that the first imperfect female of the series was the individual or that the entire string of them constituted the individual (in conformity with a view once propounded by Prof. Huxley). But he must do more than this. Since the Aphides have descended from some winged species of the order Hemiptera, he must hold that among those remote ancestors each particular fly, male or female, was an individual; but that when abundant food and inert life led to the partheno-genetic habit, and to chains of sexless forms, the males and females eventually produced at the end of each chain, though, like their remote ancestors, possessed of procreative organs and wings, are not individuals.

[Some memoranda bearing on the question here discussed, mislaid at the time when the chapter dealing with it was revised, have been discovered in time for utilization in this appendix.]

One of my critics says:--

“You have overstated the case in your favour: the alimentary canal does not, as you suggest, show a segmentation corresponding to that of the other organs in Annelids. Either it is a simple uniform tube, or else its differentiations (pharynx, œsophagus, crop, intestine) are quite independent of the repetition of the somites.”

In presence of statements made in works of authority, this objection greatly surprises me. I meet with the descriptive word “moniliform” applied to the intestine in some Annelids, and then in the Text Book of Claus, translated and edited by Sedgwick, it is said, concerning the alimentary canal in the Annelida:--

“This is followed by the gastric region of the gut, which occupies the greatest portion of the length of the body, and is either regularly constricted in correspondence with the segments, or possesses lateral diverticula.” (P. 365.)

And again on p. 369 it is said:--

“The intestine usually preserves the same structure in its entire length and is divided by regular constrictions into a number of divisions or chambers, which correspond to the segments and dilate again into lateral diverticula and cæca.”

The alimentary canal thus presents the segmental character as clearly as consists with fulfilment of its function. If the successive segments are co-operating units of a compound animal having but one mouth, then, necessarily, the gut cannot be completely cut into parts, each answering to a segment, for there could be, in that case, no passage for the food. If the portion of the intestine belonging to each segment has a conspicuous dilatation, or has a cæcum on each side, it exhibits the segmental character as much as the physical requirements permit. So far from being at variance with the hypothesis, its structure exhibits a verification of it.

The next objection runs as follows:--

“Then, again, the ovaries and testes do not exhibit a corresponding segmentation. When it is allowable to speak of ovary or testis at all as in Lumbricus, we find that in the case of both organs we have at most two pairs.”

It seems to me that the distribution of the generative organs in a comparatively-developed member of the Annelid type, is not the question. We have to ask what it is in undeveloped members of that type. Among them the repetition of generative parts is in some cases just what the theory implies. Thus in Claus I read:--“In the marine Chætopoda, the ova or spermatozoa originate on the body-wall from cells of the peritoneal membrane, either in the anterior segments alone or along the whole length of the body.” So that in these last cases there are, in all the segments, parts from which arise generative products. The fact that these parts are not definite ovaries and testes is irrelevant. Ovaries and testes are developed generative structures, and in the order of evolution are preceded by undeveloped ones; and the fact that these undeveloped ones are found in little-developed members of the type conforms perfectly to the hypothesis. [I may remark in passing that here is a good illustration of that process of evolution which, in the above speculation of Prof. Korschelt, is supposed to be inverted: many dispersed, similar, and indefinite parts, are integrated into a few localized and definite parts.]

In continuation the critic above quoted says:--“My position is that the repetition of segments in an Annelid is a phenomenon of the same nature as the repetition of hairs in a Mammal or of scutes in a Reptile”, and he proceeds to give instances of repetitions of organs in other types, as of the reproductive structures and excretory system in the young Dog-fish or of the ovaries in Amphioxus. These examples do not seem to me relevant. No parallelism exists between the repetition of a particular organ in an animal, and the repetition of an entire cluster of organs constituting a physiological whole. The repetitions of the ovaries in Amphioxus and of the excretory system in a young Dog-fish, occur without threatening to divide into similar parts the entire organism. But the segmental repetitions in an annulose creature implicate the structures at large, and would, if pushed a little further, result in separate creatures. The segment of a low Annelid contains alimentary, vascular, nervous, excretory, reproductive, sensory and locomotive organs--all the organs required for carrying on life, save certain organs of external relation which its position excludes. When there is shown some vertebrate animal, or proto-vertebrate animal, that is divisible into parts each of which is in great measure physiologically independent, I shall feel obliged to abandon my position.

* * * * *

While this appendix is in hand I have received from another expert, whose view is in general agreement with my own, a letter containing the following passage:--

“You will see that Dohrn’s theory was the antithesis of your own view of vertebrate structure, namely that the vertebræ were formed by the segmentation, from mechanical causes of a body originally simple. This view of yours has been confirmed by later researches, which have shown that the most primitive forms allied to the Vertebrates, possessing the essential organs, viz., gill-slits, notochord, and dorsal nerve cord, are not segmented animals, like Annelids and Crustacea, but simple animals, having at most three regions, not exactly corresponding to segments. These primitive unsegmented forms are Ascidian tadpoles, Balanoglossus, and certain other primitive forms. The embryology of Vertebrates also proves that they are originally simple and not segmented animals, especially the fact that there is originally one pronephric duct or primitive kidney.”

Nevertheless there survives a leaning towards the notion of a segmental origin of the Vertebrata. But the repetitions of organs named in support of this notion have, I think, no more relation to the genesis of the vertebrate type than the multiplication of vertebræ in a snake has relation to the genesis of the vertebral column.

APPENDIX E.

THE SHAPES AND ARRANGEMENTS OF FLOWERS.

In Part IV., Chapter X., under the title of “The Shapes of Flowers,” I have, after describing their several kinds of symmetry, as habitually related to their positions, made some remarks by way of interpretation. The truth that flowers exhibit a radial symmetry when they are so placed as to be equally affected all round by incident forces, having been exemplified, and also the truth that they assume a bilateral symmetry when they are so placed that their two sides are conditioned in ways different from the ways in which their upper and lower parts are conditioned; I have gone on to inquire (in § 234) by what causes such modifications of form are produced. I have stated that, originally, I inclined to ascribe them entirely to differences in the relations of the parts to physical forces--light, heat, gravitation, etc.; but that I found sundry facts stood in the way of this interpretation. And I have said that “Mr. Darwin’s investigations into the fertilization of Orchids led me to take into account an unnoticed agency.” Continuing to recognize the physical forces as factors having some influence, I have concluded that the most important factor is the action of insects; which, aiding most the fertilization of those flowers which most facilitate their entrance, produce, in course of generations, a form of flower specially adapted to the special position.

Though still adhering to this interpretation, I have since found reason to think that the original interpretation contains a larger portion of truth than I supposed at the time when I was led thus to revise it. While staying at Mürren, in Switzerland, in 1872, I observed some modifications in a species of Gentian, which proved to me that the action of incident physical forces on flowers is, in some cases, very rapid and decided. The species furnishing this evidence was the Gentiana Asclepiadea; which I found in a copse formed of bushes that were here wide apart and there close together. In some places not near to the bushes, the individuals of the species grew vertically; in other places, partially shaded, their inclined shoots curved in such directions as to get the most light; and in other cases their shoots were led to take directions almost or quite horizontal. That, along with these modifications in the directions of their shoots, there went adjustments in the attitudes of their leaves, was a fact not specially worthy of remark; for plants placed inside the windows of houses habitually show us that leaves quickly bend themselves into attitudes giving them the greatest amounts of light. But the fact which attracted my attention was, that the flowers changed their attitudes in an equally-marked manner. The radial distribution passed into a bilateral distribution with the greatest readiness. Comparison of the annexed figures will show the character of this change.

Figure I. represents part of a vertically-growing shoot. This belonged to an individual growing unimpeded by bushes, and getting light on all sides. Here it is observable that the pairs of leaves, placed alternately in directions transverse to one another--one pair pointing, say, north and south, and the next pair pointing east and west--maintain, taking them in the aggregate, a radial distribution; and it is also observable that the alternate pairs of flowers are similarly arranged.

Figure II. is a sketch from a shoot which leaned towards one side, and of which the higher part, as it bent more and more, got its upper side more and more differently conditioned from its lower side. Here we find that not only the leaves, but also the flowers, have adjusted themselves to the changed conditions. The leaves of the lowest pair hang out in the normal way, on the opposite sides of the axis, so that a plane passing through their surfaces will cut the axis transversely; and their two axillary flower-buds, c and d, are similarly placed on opposite sides of the axis. But at the other part of the shoot, we see both that the leaves have adjusted themselves so that their planes, no longer cutting the axis transversely, keep a fit adjustment with respect to the light; and also that the flowers, no longer on opposite sides of the axis, have bent round to the upper side, as at a and b.

Figure III. shows us this re-arrangement carried still further. The shoot it represents was growing in a direction nearly horizontal, and therefore receiving the light only on one side. And here, besides seeing that the leaves have so adjusted themselves that they all lie in approximately the same plane, which is parallel to the axis instead of transverse to it, we see that the two pairs of flower-buds have both come round to the upper side of the axis. So that in this shoot, the original radial symmetry in the arrangement of leaves and flowers, is completely changed into a bilateral symmetry.

These facts do not, it is true, prove any modification in the forms of the flowers themselves: they only prove modification in the grouping of the flowers. But beyond showing, as they do conclusively, how readily a bilateral arrangement of flowers is producible out of an arrangement that was not bilateral, by the action of light, etc.; they give increased probability to the belief that changes in the shapes of flowers are producible by the same agencies. Doubtless this change in the attitudes of the flower-buds is due to the action of light on their calyces and peduncles more than to its action on their unfolding corollas. But along with an action so decided on the growth of these sheathing and supporting organs containing chlorophyll, it is scarcely probable that there is no action on the growth of the petals, containing other colouring matter; considering that in both cases the development of the colouring matter depends on the action of light, and considering also the effect of light on petals, familiarly shown by their opening and closing. And if even but a small effect is producible on the growth of the corolla, then it is to be expected that light will be an agent in changing the form of the corolla, when the attitude of the flower causes its parts to be differently exposed. For a small effect on the individual flower will become a great effect in the flowers of remote descendants; provided the changed attitudes of the flowers preserve considerable constancy throughout the succession of individuals.

Be this as it may, however, the facts I have here described, which I doubt not other observers have seen paralleled in other plants, are instructive, as showing how quickly certain metamorphoses are produced, and as implying the easy establishment of such metamorphoses as permanent characters in a species, if the modifying conditions become permanent. The changes of arrangement I have pointed out, do not become permanent in this species because its individuals are variously affected by the modifying forces: on some they do not act at all, on some a little, on some much; and even on the same individual the different shoots are quite differently affected. But if the habit of this plant were greatly changed--if, for instance, by spreading into habitats yielding abundant nutriment, the plant became very luxuriant, and, multiplying its branches, grew shrub-like; it is clear that, being shaded by one another, these branches would be habitually circumstanced in a way like that which we here see produces bilateralness in the distribution of the flowers, if not in the flowers themselves; and being thus permanently affected, would become permanently bilateral. Accumulating by inheritance, what is here only an individual peculiarity, would become a peculiarity of the species--a specific character.

APPENDIX F.

PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS.

There has recently come before me a fact which has a significant bearing on the hypothesis of Constitutional units: serving, indeed, to give an apparently conclusive proof of its truth. Before stating it, however, I may with advantage re-state the several evidences already assigned in support of it.

* * * * *

1. First comes the à priori reason. These units in the germ of an organism which cause development into a special structure, cannot be chemical units--cannot be simply molecules of proteid substance in one or other of its forms; since these are not special to any type of creature but common to all creatures. Nor can they be what we may call morphological units--the cells or protoplasts; because in the early stages of development the cells of one organism are indistinguishable from those of others, and because were cells the units of composition there could be no interpretation of what are called unicellular organisms--nothing to account for the innumerable varieties of them. Hence, of necessity, the structural elements of which each organism is built, being neither proteid molecules nor cells, must be something between them: probably some complex combination of different isomeric forms of proteids.

* * * * *

2. That units of such natures are the essential components of each species of organism, is shown by the fact that in low types of creatures, little differentiated into special tissues, any considerable portion of the body will, when separated, begin to assume the structure proper to the species--a truth recently shown afresh by Prof. T. H. Morgan’s experiments on the regeneration of Planaria maculata (already referred to in § 206) showing that various fragments cut out develop into new individuals, and that when, being too small they die before doing this, there is always an abortive attempt to assume the specific structure.

* * * * *

3. This truth that a portion of undifferentiated tissue, if adequate in quantity, assumes the structure of the type, illustrating as it does the proclivity of the constitutional units towards the structure of the species, allies itself with the phenomena of both agamogenesis and gamogenesis. The first of these shows us how a fissiparously-detached portion of the parental tissue takes on the same form as the parent; and the second shows how those small detached portions distinguished as sperm-cell and germ-cell also, when united and supplied with the needful materials, do the same thing.

* * * * *

4. But the set of phenomena following the union of sperm-cell and germ-cell differ in a certain way from those which follow when a gemma or other unfertilized portion of parental tissue is detached. The incomprehensibleness of this difference as otherwise contemplated, and the partial comprehensibleness of it when joined with the hypothesis of physiological units, furnish a further support for the hypothesis.

The familiar truth learnt by the tyro in algebra that an apparent solution which contains the unknown quantity is no solution, is a truth apt to be overlooked in other spheres than the algebraic. An illustration is supplied by the answer once given in Parliament to the question “What is an Archdeacon?”--“One who discharges archidiaconal functions.” But science as well as daily life furnishes examples. When it is said by Engelmann, Hensen, Hertwig, and Maupas that “the essential end of sexuality is rejuvenescence, that is, the restoration of growth-energy,” we have another instance of an explanation which explains nothing. What is the phenomenon to be explained? That unfolding of an organism from a germ which displays growth-energy. And what is the explanation? The giving of fresh growth-energy. The unknown quantity “growth-energy” is contained in the explanation proposed. There exists no conception of “juvenescence” save that derived from observing developing plants and animals; and if “re” be prefixed, no interpretation is thereby given to the unexplained thing “juvenescence.”

Coleridge somewhere comments on a source of fallacy which he calls the “hypostasis of a relation”--the changing of a relation into a thing. The plumber who tells you that water rises in a pump “by suction” supplies an instance. Having assumed suction to be an agent, he thinks that he understands how the piston does its work. Some of the explanations given of fertilization supply further instances. When it is said that sexual union has for its end “to give increased vigour to all the vital processes,” it is tacitly implied that vigour is a something--a something which can be given. But now, in the first place, it is only by the hypostasis of a relation that we are led to think of vigour as a thing. Vigour is a state--that state of a living body which enables it to give out much motion. What enables it to do this? The presence in it of abundant molecules containing much molecular motion which can be transformed into molar motion: the transformation being effected by the falling of these molecules into their simpler and relatively-inert components, which are thereupon excreted. Energy-containing matter is used up, and more energy or vigour can be given only by supplying more such matter. How then can the union of two nuclei--those of the sperm-cell and germ-cell--give vigour? Only an infinitesimal portion of vigour in the sense above explained exists in either, and the union of them leaves it still infinitesimal. And then, even supposing the vigour to be an entity and to be appreciable in quantity, how could it go on producing that immense combination of physiological actions seen in the unfolding of the germ into an organism? and how could it go on producing the physiological actions of an adult organism during a whole century?

May we not then say that these proposed explanations leave the question where it was--are nominal solutions, not real solutions?

* * * * *

5. But the hypothesis of constitutional units furnishes, if not a satisfactory answer yet, something in the nature of an answer--a true cause; that is to say, a cause actually known to us as operating in other cases. In § 92 it was pointed out that in proportion as units are similar, there may be built up from them an aggregate which is relatively stable, and that along with increasing dissimilarity the stability of the aggregate decreases. It was inferred that if a group of constitutional units belonging to one individual which have become moulded into relatively exact congruity with the organism and with one another by long co-operation, are mingled with some belonging to another individual which, differently circumstanced, has become somewhat different in itself and in its units, then the mass formed by the union of the two groups will be relatively unstable--relatively modifiable by incident forces. Whereas in either organism, no longer perpetually changed in the relations of its parts by growth, there is an approach towards equilibrium between the whole and its components, the components contributed by the two to form a germ, being slightly unlike one another, will not form a group in a state of equilibrium. The group they form will be capable of easy change by incident forces; and they will so be rendered free to follow their proclivities towards the typical form of the species. Inferring this we must also infer that so long as these two sets of slightly different units are not exposed to any constant forces tending to coerce them into the same form, there will continue to exist in the nuclei of all descendant cells this same relative instability and consequent plasticity.

Such evidence as we have verifies this interpretation. There is first the universal fact that development of the germ begins when it is exposed to an incident force--heat--the undulations of which, increasing the oscillations of the mixed units, give them greater freedom to arrange themselves in conformity with their type. We see this alike when spring warmth makes a seed germinate and when the warmth of a sitting hen sets up organization in her eggs. Heat frees the molecules of inorganic matter from local restraints and, as we see in molten metal, lets them yield to other forces; and similarly in this organic matter, the units are made free to follow their proclivities. Then, secondly, there comes the evidence from comparisons between the effects of mixing constitutional units differing in various degrees. Let the cluster of mixed units be derived from animals that are ordinally distinct. Nothing happens. The units each contributes tend to arrange themselves after the parental type. Hence a conflict between the tendencies towards two markedly unlike structures, and no structure arises. Suppose the mixed units come from two kindred species--say horse and ass. The structures which they respectively tend to form, being in their main characters alike, there is such co-operation as produces a working organism but an organism in certain respects imperfect--a mule. Suppose, again, the units come from two varieties of the same species. A perfect organism results, and, as shown by Mr. Darwin when detailing the effects of crossing, an unusually vigorous organism. The units being more unlike than those belonging to the same variety, the instability of the germ-plasm is unusually great, and the transformations which constitute development and action become unusually active. When, as in ordinary cases, the units are supplied by members of the same variety who have not been made very much alike by their antecedents, there follows the usual amount of organic vigour. Coming now to the results of breeding in-and-in--breeding between individuals whose constitutions (i.e. constitutional units) have for generations been growing more alike in the absence of crossing with other stirps--we see that diminution of organic vigour is displayed: there is a decrease in the rate of physiological change. Finally, on coming to a closer relationship, as in marriages between cousins, in whom the constitutional units are more than commonly alike, we see there frequently follows either barrenness or the production of feeble offspring.

All these facts, then, are congruous with the hypothesis that the use of fertilization is the mixing of unlike units, and consequent production of plasticity. Leaving out cases in which the unlikenesses are so great as wholly to prevent co-operation among the units, the degree of vigour, that is, the activity of physiological change, is great where the unlikeness is great and diminishes with the approach towards likeness.

* * * * *

6. The existence of constitutional units seems otherwise necessarily implied. I refer to the fact that no organism is a homogeneous mean between its parents but consists of a mixture of parts, some following one parent and some the other. Among illustrations of this the most conspicuous are those yielded by the variously-mixed colours of hair or feathers. Horses, cattle, dogs, cats, hens, pigeons display these mixtures: colours in one place like the mother and in another place like the father. As the internal organs are invisible, and as visible organs have indefinite shapes and graduate indefinitely into adjacent ones, the mixture of traits is elsewhere less conspicuous; but occasional marked cases (especially in malformations) leave no doubt that it pervades the entire organism.

This peculiarity of transmission seems necessarily to imply that there are distinct units derived from the two parents, and that in the course of development there is more or less segregation of them--those of the one origin predominating so far in some places as to give special likeness to one parent, and those derived from the other doing the like in other places. All which interpretation is impossible unless the hypothesis of constitutional units be admitted.

* * * * *

7. I come at length to the special evidence referred to at the outset. It is evidence of the same nature as that just assigned, but carried to a higher stage. It is furnished not by the segregation of traits derived from two parents of the same variety, but is furnished by the segregation of traits derived from parents of different varieties. In articles on “Bud Variations or Sports” (Gardener’s Chronicle, 1891) Dr. Masters gives various examples of the separation or unmixing of ancestral constitutions. Mr. Noble formed a hybrid between Clematis Jackmani and C. patens. One of these varieties flowers in the autumn on new wood, while the other flowers in the spring on old wood; and the result is that flowers of two kinds, quite unlike, are produced at different parts of the year, and that by pruning so as to cut away one or other set of shoots, the plant may be made to produce exclusively for the time being one or other sort of flower.

“Another very interesting case of unmixing, or, if it be preferred, of partial mixture, is afforded by Neubert’s Berberis. This is a hybrid between the evergreen pinnate-leaved Mahonia and the deciduous simple-leaved Berberis vulgaris, and it bears leaves some of which are intermediate in appearance, while others are much like those of one or other of its parents.

“A not uncommon illustration of a similar kind, is the production of a Peach and a Nectarine on the same branch, and we have just learnt from Canon Ellacombe that some of the Berlin Hellebores show evidence of their hybrid nature by occasionally producing foliage [and flowers?] of the two parents separately from the same root-stock.

“In addition to the cases given above, we may here cite a few more which have come under our notice, such as a Chrysanthemum, half the florets of which are of one colour, half of another. A hybrid Calanthe, showing a similar piebald variation, is shown in Fig. 14. A very curious case was that of the Narcissus received from Mr. Walker, and in which flowers of two distinct varieties sprang from the same bulb. Grapes not uncommonly show their crossed origin by presenting a striped appearance, one stripe being of one colour, one of another, as may also be seen in the Orange, Apple, Lemon, and Currant.”

Thus, however the germ-plasm is constituted its essential components cannot be all alike. Before there can be this dissociation of ancestral characters, there must be in the germ-plasm different elements capable of being dissociated. This single fact seems to compel us to assume constitutional units.

APPENDIX G.

THE INHERITANCE OF FUNCTIONALLY-CAUSED MODIFICATIONS.

In Part II, Chapter X^A, I have confessed that the process by which a structure changed by use or disuse affects the sperm-cells or germ-cells whence arise descendants, is unimaginable: without, however, inferring that therefore such a process does not exist. With others it seems different. Some three years ago the following expression of opinion came to me from a zoological expert:--

“Many zoologists--most of us here at Cambridge--are intensely opposed to the doctrine of the inheritability of acquired variations. Even assuming that the developmental power of a germ is determined by its molecular structure (and I for one would question this--Driesch and his school when they find that they can squeeze a developing egg into all sorts of shapes without altering the final result, that one blastomere in an egg which has divided into 8 is still able to reproduce a whole embryo--question it also), we still fail to conceive any means by which, for instance, a change in the development of a muscle or nerve can effect a corresponding change in that part of the germ which is destined to produce a corresponding part in the descendant.”

Here it will be observed that belief in the inheritance of structural effects wrought by use and disuse, is rejected because of inability “to conceive any means” by which the modifications produced in an organ can effect a correlated modification in the germ of a descendant: failure to conceive is the test. The implication is that some alternative hypothesis is accepted because the correlating of a variation in an organ with a corresponding germ-variation is effected by a means which is conceivable. This is the hypothesis of Weismann. Concerning its conceivability I have, in the chapter just named, already written as follows:--

“If we follow Prof. Weismann we are led into an astounding supposition. He admits that every variable part must have a special determinant, and that this results in the assumption of over two hundred thousand for the four wings of a butterfly. Let us ask what must happen in the case of a peacock’s feather. On looking at the eye near its end, we see that the minute processes on the edge of each lateral thread must have been in some way exactly adjusted, in colour and position, so as to fall into line with the processes on adjacent threads: otherwise the symmetrical arrangement of coloured rings would be impossible. Each of these processes, then, being an independent variable, must have had its particular determinant. Now there are about 300 threads on the shaft of a large feather, and each of them bears on the average 1,600 processes, making for the whole feather 480,000 of these processes. For one feather alone there must have been 480,000 determinants, and for the whole tail many millions. And these, along with the determinants for the detailed parts of all the other feathers, and for the variable components of all organs forming the body at large, must have been contained in the microscopic head of a spermatozoon!” [And each of them must, throughout all the complex developmental processes, have preserved the ability to find its way to the exact place where it was wanted!]

If my Cambridge correspondent is able to conceive this process implied by the hypothesis of Weismann, I can only say that he has an enviable power of imagination.

* * * * *

But now comes the strange fact that an impossibility of thought implied by Weismann’s hypothesis does not cause rejection of it, but yet is urged as a reason for rejecting an alternative hypothesis which does not imply it. One objector cannot conceive that “a change in the development of a muscle or nerve can effect a corresponding change in that part of the germ which is destined to produce a corresponding part in the descendant”; and another objector says it is “very hard to believe” that a functionally-changed organ will so affect spermatozoa and ova that “one particular part of them will be so altered that the organisms which grow up from them will be able to present the same modification on the application of a different stimulus.” It is tacitly assumed by both that, as in the hypothesis of Weismann so in the counter-hypothesis, a particular part of the germ-plasm gives origin to a particular part of the developed organism. But nothing of the kind is implied. The nature of the counter-hypothesis (at any rate as held by me) is entirely misapprehended. Anyone who turns back to the chapters in the first volume where the conception of physiological units (or constitutional units) was set forth, or who re-reads the foregoing appendix, will see that there is altogether excluded any idea of correlation between certain parts of the germ and certain parts of the resulting organism. The units are supposed to be all alike, and during the progressive embryological changes local groups of them are supposed to take on different forms and structures under the combined forces, general and local, brought to bear on them. This conception is necessitated by all the evidence. The fact disclosed by the experiments of Driesch, Wilson, and Chabry, that from fractions of an ovum structures may be obtained like that obtained from the whole ovum, only smaller, necessitates it. The fact that any sufficiently large fragment of a polyp or planarian, no matter from what part of the body taken, will develop into a complete polyp or planarian necessitates it. The fact that from an undifferentiated portion of a plant, even so small as a scale, a complete plant may arise necessitates it. And it is necessitated by the fact that among plants, roots are produced by imbedded shoots and shoots by roots, as well as by the fact that low animals, such as hydroids, if deprived of both head and root, will develop a head from the root part and a root from the head part, if their respective conditions are inverted. All this evidence shows conclusively that the component units of each species, whether existing in the germ or in the developed organism, are, when not yet differentiated by local conditions, all alike, and that the notion of special parts of the germ-plasm correlated with special parts of the resulting organism, is entirely alien to the hypothesis.

“But how do the units of a modified organ affect the units of the germ in such wise that these produce an inherited modification of the organ?” will be asked. This difficulty has been dealt with in §§ 97_d_, 97_e_, where the analogy between the social organism and the individual organism has been brought in aid: serving, if not to furnish a conception, yet to furnish an adumbration. Regarding citizens as the units of an unfolding society, say a colony, it was pointed out that the nature they inherit from a mother-society gives them a proclivity towards a society of like structure, the traits of which are progressively assumed as the colony grows sufficiently large to make them possible. At the same time it was pointed out that while the influence of the entire aggregate on the individuals is seen in this forming of them into a society of the inherited type, the influences of local circumstances, and of individuals on one another, in each group, make them differentiate into appropriate social structures, taking on fit occupations and industries: the implication being that in virtue of their inherited natures they all have partial capacities for the various activities they undertake; so that an immigrant clerk sets up a tavern, a compositor takes to carpentering, and a university man rides after cattle or is employed on a sheep farm. Evidence was given in that place, as in the above paragraph, that the constitutional units of an organism similarly have all of them potentialities for taking on this or that structure and mode of action which local conditions determine. It was further argued that as citizens are continually being remoulded by their society into congruity with it, and, if circumstances change them, tend to remould their society; so in the individual organism, there is this reciprocal action of the whole on the units and of the units on the whole. Hence it was inferred that the modified units in any modified part tend to diffuse modifications like their own through the units at large: being aided by the circulation of protoplasm, as suggested in §§ 54_d_ and 97_f_. And it was urged that, however inconceivably complex such a process may be, yet it seems not incredible when we recognise the probability that an organism is more or less permeable to undulations propagated by its molecules: Rontgen rays giving warrant. If such units throughout the tissues may take in and send out ethereal waves which bring it into rhythmical relations with others of its kind and tend to produce congruity, it becomes, if not conceivable still supposable, that throughout the circulating protoplasm there goes on a continual harmonization of its components--a moulding of each by all and of all by each. Should it be said that such a process is too marvellous to be reasonably assumed, the reply is that it is not more marvellous than heredity itself, which, were it not familiar to us, would be thought incredible.

* * * * *

But as I have said in the place referred to--“At last then we are obliged to admit that the actual organizing process transcends conception. It is not enough to say that we cannot know it; we must say that we cannot even conceive it:” can only conceive the possibility of a suggested interpretation.

Hence we have to rely upon evidences of other kinds. Among these, some which I think dispose absolutely of the fashionable hypothesis while they harmonize with the opposed hypothesis, have now to be named. That their implication should not have been generally recognized would have seemed to me incomprehensible were it not that I have myself only now observed this implication. The facts are these:--

“Verlot mentions a gardener who could distinguish 150 kinds of camellia, when not in flower; and it has been positively asserted that the famous old Dutch florist Voorhelm, who kept above 1,200 varieties of the hyacinth, was hardly ever deceived in knowing each variety by the bulb alone. Hence we must conclude that the bulbs of the hyacinth and the branches and leaves of the camellia, though appearing to an unpractised eye absolutely undistinguishable, yet really differ.” (Darwin, Variation of Animals and Plants, &c., vol. ii, p. 251.)

More recently testimony to like effect has been given by Dr. Maxwell Masters, and has already been quoted by me in a note to § 286 in illustration of another truth. He says concerning such variations:--

“To the untrained eye, the primordial differences noted are often very slight; even the botanist, unless his attention be specially directed to the matter, fails to see minute differences which are perceptible enough to the raiser or his workmen.... These apparently trifling morphological differences are often associated with physiological variations which render some varieties, say of wheat, much better enabled to resist mildew and disease generally than others. Some, again, prove to be better adapted for certain soils or for some climates than others; some are less liable to injury from predatory birds than others, and so on.”

In his Vegetable Teratology, p. 493, Dr. Masters names another fact having a like implication--the fact that among seedling stocks which have not yet flowered, those which will produce double flowers are distinguishable. He says:--

“This separation of the single from the double-flowered plants, M. Chatié tells us is not so difficult as might be supposed. The single stocks, he explains, have deep green leaves (glabrous in certain species), rounded at the top, the heart being in the form of a shuttlecock, and the plant stout and thick-set in its general aspect, while the plants yielding double flowers have very long leaves of a light green colour, hairy and curled at the edges, the heart consisting of whitish leaves, curved so that they enclose it completely.”

What is the general truth implied? Clearly that there exists no such thing as an independent local variation. Some marked change in the form or colour of a flower or a fruit draws attention; and, being a change which interests the florist or gardener, pecuniarily or otherwise, not only draws attention but usually monopolizes attention: the natural impression produced being that this variation stands there by itself--is without relation to variations elsewhere. But now it turns out that there are concomitant variations all over the plant. Even in underground bulbs certain appreciable differences go along with certain conspicuous differences in the flowers. And if along with a striking change in a flower which the florist contemplates, there go changes all over the plant not obvious to careless observers but visible to him, we must infer that there are everywhere minute differences which even the florist cannot perceive: the whole constitution of the plant has diverged in some measure from the constitutions of kindred plants. Every local variation implies a change pervading the entire organism, manifested in concomitant variations everywhere else.

If so, what becomes of the hypothesis of determinants--the hypothesis that there is a special element in the germ-plasm which results in a special local modification in the adult organism? That there are no facts supporting it has been all along manifest; but now it is manifest that the facts directly contradict it.

At the same time it may be remarked that while the facts are wholly incongruous with the hypothesis of determinants and its accompanying elaborate speculation, they are not incongruous with the alternative hypothesis. Impossible though it may be to imagine the natures of those ultimate units peculiar to each species, which have proclivities towards the particular form of organization characterizing it, yet that a change of structure arising in one part of the organism is accompanied by multitudinous changes of structure in other parts of the organism, is not only congruous with the belief that there exist such constitutional units, but yields it distinct support. For if, as above argued, a conspicuous local variation is not the result of any modification of units special to the locality, but is the result of a modification of the units at large, then it must happen that such modification must have its effects on all other parts of the organism; so that there cannot fail to result all those small concomitant variations above indicated.

May we not also say that it becomes less incomprehensible that structural changes caused by use and disuse are inherited? If, as we see, a local variation spontaneously arising is accompanied by multitudinous other local variations, implying a necessary correlation between each local variation and the general constitution of the organism; then it may be argued that if a marked change of function in an organ causes increase or decrease of it, this general correlation implies that there must be a reciprocal reaction between the part and the whole, tending to re-establish their congruity. The constitution at large will in so far be changed, and along with its change will go corresponding changes in the sperm-cells and germ-cells.

* * * * *

Finally let me add, not another argument, but another fact of observation, of the kind which opponents demand, but which, when they are from time to time furnished, are severally pooh-poohed as not enough. Each of them is spoken of as a solitary fact and slighted as inadequate; and when by and by another is named, this is treated in the same way; so that the facts which if brought together would be recognized as sufficient are never brought together. That to which I refer is set forth in a pamphlet by M. Leo Errera, Professor at the University of Brussels, entitled “Hérédite d’un Caractère acquis chez un Champignon pluricellulaire;” being an account of experiments of Dr. Hunger, at the Botanical Institute in Brussels. First enumerating various instances of adaptations to climate, as those of plants which, fitted to northern regions, preserve their constitutional rapidity of growth and seeding when brought south, and do this for several generations, he goes on to detail the culture-experiments of M. Hunger, and sums up the results of these in the following words:--

“On déduit de là que:

“1^o Les conidies d’Aspergillus niger sont adaptées à la concentration du milieu où a vécu l’individu qui les porte; cet effet est encore plus marqué après deux générations passées dans un milieu donné (Expér. I et II);

“2^o II s’agit d’une véritable adaptation et non pas simplement d’un accroissement de vigueur chez les conidies provenant des liquides concentrés, car ces mêmes conidies germent moins rapidement et donnent des plantes moins vigoureuses que les conidies normales lorsqu’on les sème de nouveau sur le milieu-type: en s’adaptant aux liquides concentrés, elles se sont désadaptées du liquide normal (Expér. III);

“3^o Une génération passée sur le liquide normal n’efface pas l’influence d’une ou de deux générations antérieures passées sur une liquide plus concentré (Expér. IV).

“Tous ces résultats concordent: ils montrent une légère, mais incontestable transmission héréditaire de l’adaptation au milieu.”

SUBJECT-INDEX.

(For this Index as it appeared in previous editions the Author is indebted to F. HOWARD COLLINS, Esq., of Edgbaston, Birmingham. It has now been adjusted to suit the present revised and enlarged edition.)

Acacia, foliar organs, =II=, 41, 264.

Acalephæ: environment, =I=, 105; water in, =I=, 173.

Acari: special creation and effects of, =I=, 428; direct transformations, =I=, 706; segmentation, =II=, 111.

Acorus calamus, agamic propagation, =I=, 642.

Acquired characters, inheritance of: functionally-produced modifications in plants and animals, =I=, 307–13, 318, 526, 541, 562, 692–5; =II=, 618–22; conceivability of, on the hypothesis of physiological units, =I=, 368–71, 695; =II=, 618–22; diminution of jaw, =I=, 541–2, 693; current views on, =I=, 559–60; cessation of selection, =I=, 560–3; Eimer’s theory of orthogenesis, =I=, 560; species differentiation, =I=, 573; location of mammalian testes, =I=, 573; tactual perceptiveness, =I=, 602–8, 633, 665, 666, 672–3, 692; blindness of cave-animals, =I=, 612–3, 647–9; co-adaptation of co-operative parts, =I=, 621, 663–5; transmission of disease, =I=, 622–3; hypothesis supported by telegony, =I=, 624–8, 644–6, 649–50; views of Darwin and neo-Darwinists, =I=, 630, 685, 690; why facts in support are meagre, =I=, 632; degradation of little toe, =I=, 652–3, 673; neuter forms of social insects, =I=, 658–9, 663–4, 670, 675; degenerated instinct in ants, =I=, 660–2; rudimentary limbs of whale, =I=, 669, 692; importance of question, =I=, 672, 690; monstrous development of honey-ants, =I=, 683–4; osteology of Punjabis, =I=, 689; summary of evidences in support, =I=, 692–5; genesis of vertebrate skull, =II=, 227; false joints, =II=, 371, 372; conceivability of rival hypotheses, =II=, 618–22; adaptation to environment in Aspergillus, =II=, 623.

Acrogens, the term, =II=, 55–6. (See Archegoniateæ.)

Actinophrys: a primary aggregate, =II=, 76; genesis, =II=, 452.

Actinozoa: multiaxial development, =I=, 166; waste and repair, =I=, 213, 219; differentiation, =I=, 391; parasitism, =I=, 397; integration, =II=, 92; symmetry, =II=, 189, 192; growth and genesis, =II=, 444.

Activity: the principle of, the essential element in Life, =I=, 113, 114, 122; not inherent in living matter, =I=, 120; nutrition and genesis, résumé, =II=, 497–9; and evolution, =II=, 501–4.

Adaptation: general truths, =I=, 227–33, 233–5; botanical, =I=, 227; physiological, =I=, 228–33; psychological, =I=, 229, 230–3; structural, functional, and interdependence, =I=, 235–9, 240–1, 318; social and organic stability, =I=, 240–2; résumé, =I=, 242–3; to varied media, =I=, 479–81, 489, 556; multiplication of effects, =I=, 512–3, 550; direct equilibration, =I=, 522–3; natural selection and equilibration, =I=, 530–5; non-adaptive specific characters, =I=, 565; time required for effecting, =I=, 565–6; an obstacle to re-adaptation, =II=, 11; of skin and skeleton, =II=, 215, 217; outer tissue, =II=, 312–4, 387; skin and mucous membrane differentiation, =II=, 321–2, 389; vascular system, =II=, 343–4; osseous, =II=, 352; muscular, =II=, 368–9, 391; persistence of force and physiological, =II=, 394; of reproductive activity to conditions, =II=, 411–6; vertebræ development, =II=, 563–6. (See also Co-adaptation.)

Africa, effect of climate on inhabitants, =I=, 30.

Agamogenesis: alternation with gamogenesis, =I=, 266–7, 272–3, 284–94, 336, 592; =II=, 415; parallelism in karyokinesis, =I=, 267–8; a process of disintegration, =I=, 276–7; conditions determining its continuance, =I=, 284–94, 295–7, 330; physiological units, =I=, 351; =II=, 613; spontaneous fission, =I=, 582, 584–7, 589–92, 595–6, 599; remarkable extent of, under favourable conditions, =I=, 591–2, 640–1; in Actinozoa, =II=, 92; in Hydrozoa, =II=, 102; in Annelida, =II=, 103; innutrition, =II=, 179–80.

Agaricinæ, =II=, 139, 257.

Agassiz, L. J. R., zoological classification, =I=, 380.

Aggregates, Animal and Plant (see Morphology).

Agility, a vital attribute, =I=, 578.

Agrimony, floral symmetry, =II=, 42, 167, 170.

Air, in vegetal tissues, =II=, 567–8, 583, 591, 593.

“Air plants,” =I=, 208.

Albumen: properties, =I=, 12; Lieberkühn’s formula, =I=, 13; diffusibility, =I=, 19; in organic tissues, =I=, 41.

Alcohols, properties, =I=, 10–12.

Algæ: reproduction, and the dynamic element in life, =I=, 118–9; multicentral development, =I=, 163, 164; axial development, =I=, 165; locomotive powers of minute forms, =I=, 196; uniform tissue and function, =I=, 200, 586; gamogenesis, =I=, 271, 279, 280, 283; =II=, 448, 449, 450; fertility, =I=, 582; =II=, 440, 441; fission, =I=, 584, 585; unicellular forms, =II=, 22; integration in Confervoideæ and Conjugateæ, =II=, 25; pseudo-foliar and axial development, =II=, 28–33, 57; foliar development, =II=, 76, 91; branch symmetry, =II=, 145; cell metamorphoses, =II=, 176; tissue differentiation, =II=, 244, 246, 251, 252, 256, 272, 385–6; adaptation of reproductive activity to conditions, =II=, 289; integration, =II=, 292; indefiniteness, =II=, 295; genesis and development, =II=, 463.

Alimentary canal: metabolic processes and agents, =I=, 68–9, 74; structural traits, =I=, 192; progressive development, =I=, 195; relation to environment, =I=, 196; function, =I=, 205; segmentation in annelids, =II=, 125; differentiation, =II=, 301, 302, 321–2, 323–5, 389; specializations in birds, =II=, 325; in ruminants, =II=, 327–9; differentiation of liver, =II=, 329–33; muscularity, =II=, 364.

Allotropism: of organic constituents, =I=, 4, 9; muscular action, =I=, 59.

Alloys, melting point of, =I=, 339.

Alternation of generations, misleading application of term, =II=, 84. (See Agamogenesis and Gamogenesis.)

Amitosis, occurrence of, in morbid tissues, =I=, 264.

Ammonia: properties, =I=, 7, 9; nerve stimulation, =I=, 55.

Amœba: central development, =I=, 163; a primary aggregate, =II=, 86; symmetry of encysted, =II=, 186; symbiosis, =II=, 400.

Amphibia: classification of, =I=, 392; embryonic respiratory system, =I=, 457; structure and media, =I=, 483; limb locomotion, =II=, 15; segmentation, =II=, 122, 225; outer tissues, =II=, 311; respiration, =II=, 334, 338; Owen on skeleton, =II=, 552, 557, 558.

Amphioxus: separation of segmentation spheres of egg, =I=, 691; embryogeny, =II=, 121; local segmentation, =II=, 125–7, 605; genesis of vertebrate axis, =II=, 213–6, 218, 222; development, =II=, 564.

Amphipnous cuchia, vascular air-sacs, =II=, 337.

Anabas scandens, the climbing fish, =I=, 480, 483.

Anacharis (see Eloidea).

Anæsthetics, diverse effects of, =I=, 55.

Angræcum, assimilative function of root, =II=, 255.

“Animal Spirits,” vitalism and, =I=, 115.

Animals: nutrition and molecular re-arrangement, =I=, 36–7; nitrogenous character, =I=, 39–41; sensible motion, =I=, 57; metabolism, =I=, 62–77; multiplication of energies, =I=, 75; contrasted traits of plants and, =I=, 196; what is an individual? =I=, 246–7; solar influence, =I=, 500, 556; geologic changes affecting, =I=, 501–4, 549, 550, 556; interdependence with plants, =I=, 504–6, 514; =II=, 398–401; complexity of influences affecting, =I=, 506; geographical isolation and origin of species, =I=, 568–9; vital attributes, =I=, 577–9; distribution and antiquity of plant and animal types, =II=, 297; mutual dependence of organisms at large, =II=, 397–408; hypothetical plant-animal type, =II=, 397; progressive increase of size, =II=, 401; laws of multiplication, =II=, 411–6; rhythm in numbers, =II=, 419; law of weights and dimensions, =II=, 434.

Animals, domesticated: variation, =I=, 324, 326, 560, 563, 693; interbreeding, =I=, 345–7, 354; =II=, 615; pure and mixed breeds, =I=, 354, 625.

Annelida: phosphorescence, =I=, 50; axial development, =I=, 165, 166; integration, =I=, 363; larval forms and phylogeny, =I=, 447; =II=, 115; segmental fission, =I=, 588–9; segmentation, =I=, 98–101, 103–4, 602–5; =II=, 107–9, 125–7; lateral gemmation, =II=, 105; embryogeny, =II=, 119; bilateral symmetry, =II=, 197–200; genesis, =II=, 444, 453.

Annulosa: regeneration, =I=, 361–2; distinctive traits, =I=, 392; origin of type, =II=, 98–110, 602–6; unit of composition, =II=, 105; application of term, =II=, 111; vertebrate symmetry compared, =II=, 203–6; segmental differentiation, =II=, 207–9; unintegrated function in Planaria, =II=, 373; development and genesis, =II=, 464; nutrition and genesis, =II=, 490. (See also Annelida and Arthropoda.)

Anthropomorphism, former prevalence of, =I=, 419.

Ants: utilization of aphids, =I=, 660–1; =II=, 403, 405; nest-mates, =II=, 405; castes in social species, =I=, 658–9, 670, 675; loss of self-feeding instinct in Amazons, =I=, 660–1, 663–4; monstrous development of Honey-ants, =I=, 683; bulk and fecundity, =II=, 492. (See also Termites.)

Aphis: individuality, =I=, 249, 250; =II=, 603; parthenogenesis, =I=, 274–5, 289; fertility, =I=, 582, 640–1; =II=, 476, 490; utilized by ants, =I=, 660–1; =II=, 403, 405; over-multiplication checked by lady-bird, =II=, 406.

Aquatic animals, large size attained by, =I=, 156.

Arachnida: avoidance of danger, =I=, 92; oviparous homogenesis, =I=, 271; segmentation, =I=, 469; =II=, 113, 114; integration and homology, =II=, 111, 121; bilateral symmetry, =II=, 198.

Arcella: symmetry, =II=, 186; outer tissue differentiation, =II=, 309.

Archegoniateæ: morphological composition, =II=, 32–5; growth and development, =II=, 50–6; tubular structure, =II=, 58, 62; alternating generation not distinctive, =II=, 84; asymmetry and environment, =II=, 140; integration, =II=, 293, 296; individuation and genesis, =II=, 441, 451, 463.

Archenteron: primitive externality, =II=, 301; formation of cœlom, =II=, 302.

Archiannelida: segmentation, =II=, 125.

Arenicola marina: polytrochal larvæ, =II=, 109.

Arm: embryogeny of human, =I=, 169; vicarious use of, =I=, 209.

Army, morphological analogy, =II=, 6.

Arteries (see Vascular System).

Arthropoda: uniaxial development, =I=, 165; protoplasmic continuity, =I=, 190, 629; excursiveness, =I=, 481; limb locomotion, =II=, 15; integration and homology, =II=, 111–4, 121; bilateral symmetry, =II=, 197–200; genesis, =II=, 445, 453.

Ascidians: multiaxial development, =I=, 165, 166; functional differentiation, =I=, 202; composite individuality of Doliolum, =I=, 247; self-fertilization, =I=, 342; integration, =II=, 94, 96, 97; symmetry, =II=, 194; origin of vertebrate type, =II=, 194, 598, 605.

Ascomycetes, reproduction, =II=, 450.

Assimilation: compared with reasoning, =I=, 81–7; a trait of vitality, =I=, 577.

Asteroidea, radial symmetry, =II=, 196.

Astronomy: growth of celestial bodies, =I=, 135; Schleiden on individuality, =I=, 245; evolution, =I=, 432, 435; classification of stars, =I=, 444; rhythm of, and organic change, =I=, 499–501, 557; law of equilibration, =I=, 519–20; co-operation of structure and function, =II=, 3.

Atavism: occurrence of, =I=, 305–6, 314; digital variation, =I=, 321–3.

Atoms: use of term, =I=, 6, 31; ethereal undulations and oscillations, =I=, 31–5.

Australia: settler’s usages, =I=, 364; ratio of jaw to skull in natives, =I=, 541.

Axillary buds, origin and development, =II=, 65–8.

Axis: “neutral” of mechanics, =II=, 210; genesis of vertebrate, =II=, 212–6, 224–7.

Bacteria: fission, =I=, 270; non-nucleated, =II=, 20; rate of increase, =II=, 443.

Baer, K. E. von: embryological formula, =I=, 171, 172, 451, 453, 461, 466; zoological classification, =I=, 383; on animal transitions, =I=, 480.

Balanophoræ, inner tissue, =II=, 274.

Bark: varied development, =II=, 247–9; physiological differentiation, =II=, 249–50, 258, 386.

Basidiomycetes, reproduction, =II=, 450.

Bat, infertility of, =II=, 473.

Bates, H. W., protective mimicry of butterflies, =I=, 398.

Batrachia (see Amphibia).

Bean, vascular system, =II=, 573, 591.

Beaver, tail and co-adapted structures, =I=, 616.

Bees (see Insects).

Begoniaceæ: multiplication =I=, 224, 317, 442; individuality, =I=, 251; development from scales, =I=, 282; symmetry, =II=, 159, 166; development, =II=, 271.

Berkeley, M. J., indefiniteness of mosses and ferns, =II=, 296.

Bile, arrest of excretion, =I=, 209.

Bilirubin and biliverdine, function of, =II=, 330, 333.

Biology: definition and divisions, =I=, 124–5; organic structural phenomena, =I=, 125–7; also functional, =I=, 127–9; actions and reactions of function and structure, =I=, 129–30; genesis, =I=, 130–1; limited knowledge of, =I=, 131; evolution, =I=, 432, 434; sociological analogies (see Sociology).

Biophors, Weismann’s germ-plasm units (see Weismann).

Birds: flesh-eating and grain-eating contrasted, =I=, 68; growth and expenditure of force, =I=, 142; size of egg and adult, =I=, 144; limitations on flight, =I=, 155; self-mobility, =I=, 175; temperature, =I=, 176; functional and structural differentiation, =I=, 201; food of starving pigeon, =I=, 215; viviparousness, =I=, 271; heredity and pigeon breeding, =I=, 305; atavism in pigeon, =I=, 314; osseous variation in pigeon, =I=, 321; classification, =I=, 392; migrations and change of habits, =I=, 399, 402, 500; distribution in time, =I=, 410; Darwin on petrels, =I=, 455; rudimentary teeth, =I=, 457; vertebræ, =I=, 471; =II=, 564; feather development, =I=, 473; habits of water ouzel, =I=, 485; egg shells and direct equilibration, =I=, 526; bones of waders and direct equilibration, =I=, 527; fertility and nervous development, =I=, 598; cellular continuity, =I=, 629; adaptation of structure to environment, =II=, 12; sexual selection, =II=, 269; wing spurs, =II=, 313; outer tissue differentiation, =II=, 314–5, 387; alimentary canal development, =II=, 325, 327; muscular colour and activity, =II=, 365–9; nutrition, =II=, 433; cost of genesis, =II=, 436; growth and genesis, =II=, 454, 458; heat expenditure and genesis, =II=, 468–9, 474; activity and genesis, =II=, 470–2, 474; contrasted mammalian fertility, =II=, 470; eggs of wild and tame, =II=, 478; fertility of blackbird and linnet compared, =II=, 503; Owen on skeleton of, =II=, 559, 560, 561.

Bischoff, embryogeny of human arm, =I=, 169.

Bison, modifications entailed by increased weight of head, =I=, 512.

Blackbird, contrasted with linnet in development, =II=, 503.

Blainville, de, definition of life, =I=, 79, 93.

Blastosphere, independence of cells in Echinoderm larvæ, =I=, 185.

Blastula, definition of life and formation of, =I=, 112.

Blood: similarity of iron peroxide, =I=, 17; metabolic processes, =I=, 69; segregation of abnormal constituents, =I=, 180; protozoon life of corpuscles, =I=, 186–7; morbid changes, =I=, 221, 701; assimilative power and organic repair, =I=, 221–2; respiratory tissue differentiation, =II=, 310–1; pressure in mammals, =II=, 340. (See also Vascular System.)

Blow-fly, Weismann on nutrition and genesis in, =I=, 678–9.

Boers, Cape, habits and fertility, =II=, 508.

Boismont, A. B. de, on human fertility, =II=, 511.

Bone: growth and function, =I=, 151; adaptability, =I=, 230; =II=, 217–8; function and weight, =I=, 308, 693; mammalian cervical vertebræ, =I=, 394; evolution and vertebral column, =I=, 470–1; partial development, =I=, 473; size of head as influencing, =I=, 512, 536–9; direct equilibration and strength, =I=, 527; natural selection and co-adaptations, =I=, 614–21, 674, 677; rudimentary limbs of whale, =I=, 668, 685, 692; inheritance of acquired modifications in Punjabis, =I=, 689; skull development, =II=, 222; theory of supernumerary, =II=, 223; Cope on origin of vertebrate osseous system, =II=, 225–7; differentiation, =II=, 344–56; false joints, =II=, 370–2; Owen’s theory of vertebrate skeleton, =II=, 548–66.

Book-worm, food of, =I=, 77.

Born, G., experiments on frog larvæ, =I=, 365.

Botany, biological classification, =I=, 124, 125. (See Plants.)

Bothriocephalus, development, =II=, 490.

Botryllidæ: development, =I=, 166; independence of components, =I=, 247; agamogenesis, =I=, 641.

Bower, Prof., on alternation of generations, =II=, 84.

Brachiopoda, rude vascular system, =II=, 340.

Bradbury, J. B., on vaso-dilators, =I=, 55.

Brain: natural selection and mental evolution, =I=, 553; analysis of substance, =I=, 596; weight in higher animals, =I=, 598–9; size in civilized and uncivilized, =II=, 530.

Branches (see Morphology).

Branchiæ (see Respiratory System).

Brass, effect of antimony on, =I=, 121.

Bread, diamagnetism, =I=, 370.

Breeding: heredity, =I=, 304–5; in-and-in, =I=, 344–7, 353; =II=, 615; pure and mixed, =I=, 354, 625.

Bricks, changed equilibrium shown by, =I=, 38, 42.

Brodie, T. G., cell chemistry, =I=, 260.

Brownell, Miss J. L., on birth-rate in United States, =II=, 520.

Brown-Séquard, on inherited epilepsy, =I=, 312, 624.

Bryophyllum, peculiar proliferation, =II=, 295.

Bryophyta, large size attained by some, =I=, 138.

Bryozoa, gemmation, =I=, 588.

Budding (see Gemmation).

Buds: development, =I=, 167–8; theories of heredity and cauline, =I=, 358–9, 360; axillary, =II=, 65–9; effects of nutrition, =II=, 73–4.

Butterfly: protective mimicry, =I=, 398; instance of tame, =I=, 684.

Cabbage, varieties of, =I=, 302.

Cactaceæ: foliar and axial development, =II=, 47–9; differentiation in, =II=, 258, 276, 282; vascular system, =II=, 282; dye permeability and circulation, =II=, 571, 572; wood formation, =II=, 575, 577, 578, 580.

“Callus,” budding from, =I=, 358, 359.

Camel, natural selection and hump of, =I=, 534.

Canadians, French, fertility of, =II=, 509.

Cancer, the definition of life, =I=, 111; œsophageal, =II=, 324; and vascular system, =II=, 343.

Caoutchouc, leaf-structure, =II=, 589.

Capillaries (see Vascular System).

Capillarity, and vegetal vascular system, =II=, 279–80, 286, 568, 570, 585, 587, 592–6.

Carbohydrates: instability, =I=, 10; the term “hydro-carbon,” ib.; molecular changes in, =I=, 42–3; organic transformation, =I=, 43, 48; metabolic processes, =I=, 63–77, 262–3; =II=, 362.

Carbon: properties, =I=, 3–5, 20; compounds, =I=, 6, 7, 9, 10–12, 13, 24–5.

Carbonic acid (carbon dioxide): properties, =I=, 6, 7, 9; in animal and plant functions, =I=, 62, 214; =II=, 398; diffusibility, =II=, 331.

Carbonic oxide, properties, =I=, 6.

Carnivores: nitrogenous food, =I=, 47, 68; katabolic process, =I=, 71; restricted environment, =I=, 396; their beneficial effects on animal life, =II=, 405–6.

Carpenter, W. B.: on functional specialization, =I=, 208; reproduction of seaweed, =I=, 582; vegetal cell multiplication, =I=, 585; structure and multiplication of compound organisms, =I=, 586–9; on fundamental traits of sex, =I=, 595; nutritive system of invertebrates, =I=, 595; Macrocystis, =II=, 450; nutrition and reproductive function, =II=, 460.

Cartilage (see Bone).

Castration, effect of, on growth, =II=, 459.

“Castration parasitaire,” Julin on, =II=, 493–6.

Catalysis, and vital metamorphosis, =I=, 39, 43.

Cattell, McKeen, on tactual perceptiveness, =I=, 666.

Caulerpa, simulation of higher plant-forms, =II=, 22.

Cave-animals, degeneration of eyes, =I=, 309, 612–3, 614, 647–9, 693.

Cell, the: incomprehensibility of forces at work in, =I=, 118; protoplasts and their traits, =I=, 181; the cell-theory, =I=, 184, 252; =II=, 17–21, 85; differentiation, =I=, 188–9, 194; the continuity of protoplasm, =I=, 190–2, 194, 628–30; =II=, 21; its structure, =I=, 253–5; function of centrosome, =I=, 254–5, 257; structure and function of nucleus, =I=, 255–6, 258–9; karyokinesis, =I=, 257–8; function of chromatin, =I=, 259–65; fertilization and function of polar bodies, =I=, 266–8; theories of heredity based on theory, =I=, 356; Weismann’s differentiation into reproductive and somatic, =I=, 622, 628–30, 633–44; nucleus absent or dispersed, =II=, 20, 85; morphological differentiation, =II=, 175–7; animal morphology, =II=, 228–30; morphological summary, =II=, 233; vegetal tissue differentiation, =II=, 249–50, 386; vascular development, =II=, 279–84, 389.

Centipede, bilateral symmetry, =II=, 198–200.

Cephalopoda: bilateral symmetry, =II=, 203; vascular system, =II=, 341.

Cercariæ (see Distoma).

Cereus, tissue differentiation, =II=, 276, 283.

Cesalpino, =I=, 377.

Cestoda (see Entozoa).

Chætopoda, segmentation, =II=, 98, 103, 605.

Chaja, wing spurs, =II=, 313.

Change, and definition of life, =I=, 81–90, 113.

Charles, R. H., on inheritance of acquired modifications in leg-bones of Punjabis, =I=, 689.

Chatié, on single and double stocks, =II=, 622.

Chemistry: properties of organic elements, =I=, 3–5, 20, 22; of diatomic compounds, =I=, 7–10; tri-atomic, =I=, 10–12; poly-atomic, =I=, 12–13, 25; traits of evolution, =I=, 23–4; ethereal undulations and atomic oscillation, =I=, 31–6; chemical affinity and organic change, =I=, 36–7, 38–43; oxidation and generation of heat, =I=, 46–9, 60; generation of nerve force, =I=, 52, 60; metabolism, =I=, 62–77; physiology and organic, =I=, 127; flesh constituents, =I=, 154; composition of organisms and environment, =I=, 173; organic development and differential assimilation, =I=, 179–80; chemical units, =I=, 225; =II=, 612; primitive ideas of elements, =I=, 417; evolution of organic compounds, =I=, 696–701, 703.

Chestnut, leaf symmetry, =II=, 149, 153.

Chiton: simulation of segmentation, =II=, 116, 118; symmetry, =II=, 202.

Chlorophyll: function, =I=, 65; =II=, 263; nutrition and absence of, =II=, 74; constitution, =II=, 262; symbiotic presence in animals, =II=, 400.

Chondracanthus gibbosus, enormous development of reproductive system, =II=, 487.

Chordata, affinities, =I=, 466.

Chromatin (see Cell).

Circle, the, and evolution hypothesis, =I=, 433.

Circulation (see Vascular System).

Cirrhipedia: Darwin on retrograde development, =I=, 458; remarkable transformation in Sacculina, =II=, 494–5.

Civilization, human evolution and genesis, =II=, 529–31.

Cladophora: integration, =II=, 25; axial development, =II=, 28.

Classification: subjective conception, =I=, 78; two purposes of, =I=, 374; a gradual process, =I=, 375; botanical, =I=, 377–80, 389–90; zoological, =I=, 380–9; incomplete equivalence of groups, =I=, 389, 445–6, 448, 555, 572; group attributes, =I=, 390–3; the truths interpreted, =I=, 393–4; ethnologic and linguistic evolution, =I=, 441–6; organic evolution, =I=, 443, 447, 555; differences in kind and degree, =I=, 444–6; antecedent structural similarity, =I=, 447, 448–9; Von Baer’s formula, =I=, 451–4, 555; organic, not uniserial, =II=, 115.

Classification of the Sciences, The, and evolution and dissolution, =II=, 5.

Claus, C, on segmentation in Annelids and Chætopods, =II=, 605.

Clover: flower and axial development, =II=, 45; symmetry, =II=, 152.

Co-adaptation of co-operative parts: principles underlying, =I=, 234–5, 511–3, 514–5; slow operation of the process, =I=, 236; sociological analogy, =I=, 237–40; reversion under original conditions, =I=, 240; the analogy continued, ib.; the case of bison’s head, =I=, 512; natural selection an inadequate explanation, =I=, 535, 614–21, 692; Romanes on “cessation of selection” as effecting, =I=, 560, 561–2; Weismann’s theories, =I=, 560–3, 663–5, 670, 674–5; natural selection and economy of growth, =I=, 562; physiological processes involved, =I=, 566–7; Wallace’s argument from artificial selection, =I=, 615; what are co-operative parts? =I=, 616–7; “intra-selection” examined, =I=, 676–8.

Coal, social effects of supply, =I=, 238–9, 241.

Cocoa-nut, growth and fertility, =II=, 457.

Coccospheres: vital problem presented by protective structures, =I=, 119; imbricated plates, =I=, 182.

Cockroach, ousting of European species, =I=, 399.

Cod: ova of, =II=, 435; growth and fertility, =II=, 454.

Codium: symmetry, =II=, 136; tissue differentiation, =II=, 246.

Cœlenterata: rudimentary contractile organs, =I=, 58; vital changes in polyp, =I=, 95; axial development, =I=, 165, 166; environment and structure, =I=, 173; self-mobility, =I=, 175; =II=, 14, 15; functional differentiation, =I=, 201, 391; inactivity and waste, =I=, 213; reparative power, =I=, 219, 224; individuality, =I=, 246, 247, 250; heterogenesis, =I=, 273, 277, 296; negative disintegration in Hydrozoa, =I=, 276, 587; reproductive tissue, =I=, 281; differentiation in Hydrozoa, =I=, 391; classificatory value, =I=, 446; regeneration of fragments, =II=, 90; integration, =II=, 90, 102, 105, 124; gemmation, =II=, 91; tertiary aggregation, =II=, 92, 95, 124; molluscan affinities, =II=, 115; radial symmetry, =II=, 188; symmetry of compound, =II=, 192–3; segmental differentiation, =II=, 207; physiological differentiation in Hydra and analogy, =II=, 300; ciliation of blastula, =II=, 301; tissue reduplication, =II=, 301–2, 389; outer tissue differentiation, =II=, 309; osmosis in Hydra, =II=, 339; vascular system in Hydra, =II=, 340, 376; functional co-ordination, =II=, 376; symbiosis, =II=, 400; asexual genesis, =II=, 443–4; growth and sexual genesis, =II=, 452; development and genesis, =II=, 462; nutrition and genesis, =II=, 476.

Cœlom, origin and function, =II=, 302–3.

Collins, F. Howard, jaws and teeth of savages and civilized, =I=, 541.

Colloids: T. Graham on, =I=, 15–8; diffusibility, =I=, 18–21; organic, =I=, 21, 25, 26; pliability and elasticity, =I=, 27; capillary affinity, =I=, 28; isomerism, =I=, 59; instability, =I=, 350; molecular mobility and diffusibility, =II=, 331; instability of, and nerve differentiation, =II=, 356–61; and muscular tissue, =II=, 361–4.

Colonies, autogenous development and parallel in heredity, =I=, 366–8; =II=, 620.

Colour: sensation of, =I=, 54; phœnogamic, =II=, 75, 265–6; light and vegetal, =II=, 261–2; floral fertilization, =II=, 267–9; sexual selection, =II=, 269; activity and muscular, =II=, 365–9; physiological units and mixture of, in offspring, =II=, 616, 617.

Commensalism, organic Integration as displayed in, =II=, 402–4.

Compositæ: floral symmetry, =II=, 173.

Condor, weight of, =I=, 155.

Confervoideæ, =I=, 279, 280; =II=, 25, 28, 449. (See Algæ.)

Conjugateæ, =II=, 449. (See Algæ.)

Conjugation, in Algæ, =I=, 279; in Protozoa, =I=, 280; =II=, 452; can fission persist without? =I=, 637; relation to growth, =II=, 449.

Connective tissue, Hertwig’s classification, =I=, 189.

Constitutional units, =I=, 369. (See Physiological Units.)

Consumption, hereditary transmission, =I=, 307.

Co-ordination of actions (see Life).

Cope, E. D., on origin of vertebrate structure, =II=, 225–7.

Cormophyta: slight internal differentiation, =II=, 273; vascular system, =II=, 280.

Corpuscula tactus, their function, =I=, 75.

Correspondence, use of word, =I=, 97. (See Life.)

Cousin-marriages, =I=, 345; =II=, 615.

Cow: what prompts her to mumble a bone? =I=, 120.

Cow-parsnip (see Heracleum).

Crab (see Crustacea).

Creation (see Special creation).

Crinoidea, symmetry, =II=, 195–6.

Crocodile, continuous growth, =I=, 154, 292.

Crookes, Sir W., hypothetical chemical unit “protyle,” =I=, 22, 23.

Cruciferæ, floral symmetry, =II=, 164, 171.

Crustacea: locomotion of lobster, =I=, 175; regeneration of limbs, =I=, 224, 360, 589; =II=, 76; homogenesis, =I=, 271; genesis and nutrition in Daphnidæ, =I=, 290–1; growth and genesis, =I=, 292; degeneration of eye in cave-inhabiting, =I=, 309, 614, 648; hermit-crab parasite, =I=, 397; changes of media, =I=, 401, 481–2; retrograde development in cirripedes, =I=, 458; segmentation, =I=, 468–9; =II=, 114; Darwin on jaws and legs, =I=, 471; survival of cirripedes, =I=, 517; integration and homology, =II=, 111–4, 121, 603; bilateral symmetry, =II=, 198–201; eyes, =II=, 318; dermal structure of hermit-crab, =II=, 322, 387; fertility, =II=, 453; nutrition and genesis in parasitic species, =II=, 487; “castration parasitaire,” =II=, 493–6.

Crystalloids: Prof. Graham on, =I=, 15–8; diffusibility, =I=, 18–21; organic, =I=, 21–2, 26.

Crystals: simulation of life in “storm glass,” =I=, 96; growth, =I=, 135–7, 577; segregation, =I=, 179, 221, 223; equilibration, =I=, 337; physiological units and polarity, =I=, 701–6; time and formation, =II=, 77.

Ctenodrilus, segmental individuality, =II=, 103, 603, 604.

Cube, bilateral symmetry, =II=, 132.

Cunningham, J. T., =I=, vi; =II=, vi; on non-adaptive specific characters, =I=, 565; food of blow-fly larvæ, =I=, 678; arthropod segmentation, =II=, 114; egg-production of Conger, =II=, 425.

Cuttle-fish, Individuality of Hectocotylus, =I=, 250.

Cuvier, zoological classification, =I=, 381.

Cyanogen, properties, =I=, 7, 9.

Cyclichthys, dermal structure, =II=, 306.

Dalyell, Sir J., regeneration in Dasychone, =I=, 361; propagation of Hydra, =II=, 476.

Daphnidæ, heterogenesis and nutrition, =I=, 290–1.

Darwin, C: Origin of Species, =I=, 129; =II=, 528; natural selection and function, =I=, 308–9, 693; atavism, =I=, 314; osseous variations in pigeons, =I=, 321; plant variation and domestication, =I=, 325; “spontaneous variation,” =I=, 328, 697; floral fertilization, =I=, 340; =II=, 168, 267, 407, 608; intercrossing and self-fertilization, =I=, 344, 345; intercrossing =I=, 347, 611, 669; his theory of pangenesis examined, =I=, 356–62, 370, 372; plant-fertilization and distribution, =I=, 397; habits of birds, =I=, 400; distribution and natural barriers, =I=, 402, 476; disappearance and non-reappearance of species, =I=, 406; distribution in time and space, =I=, 410; linguistic classification, =I=, 442; classification of organisms, =I=, 443; classification and descent, =I=, 448; on petrels, =I=, 455; suppression of organs, =I=, 457; development of Cirrhipedia, =I=, 438; jaws and legs of Crustacea, =I=, 471; aborted organs, =I=, 474, 563; relations of species in Galapagos archipelago, =I=, 478; opinions of E. Darwin and Lamarck, =I=, 491; the term “survival of the fittest,” =I=, 530; Indirect equilibration by natural selection, =I=, 530–5; inheritance of acquired characters, =I=, 535–42, 560, 630, 685, 690; Wallace on natural selection in man, =I=, 553; misleading connotations of term “natural selection,” =I=, 609, 695; caste gradations and jaws of driver ants, =I=, 658; attachment of climbing plants, =II=, 276–7; vegetal fructification, =II=, 294; earth-worm, =II=, 402; animal sterility and domestication, =II=, 480, 483; variation in hyacinth and camellia, =II=, 621.

Darwin, Dr. E., modifiability of organisms, =I=, 490, 492–7.

Death: an arrest of vital correspondence, =I=, 102; only limit to vegetal growth, =I=, 153; cessation of co-ordination of actions, =I=, 578, 579; Weismann’s hypothesis, =I=, 636–8; physiological integration, =II=, 374, 392; cause of natural, =II=, 413; relation to births, =II=, 417.

Definiteness: of vital change, =I=, 87–90, 106, 109; developmental, =I=, 178; functional, =I=, 212; segregation of evolution, =I=, 514–6.

Definition, difficulties of, =I=, 78; =II=, 17.

Degeneracy, morphological obscurations due to, =II=, 12, 13.

Dendrobium (see Orchids).

Desmidiaceæ: unicellular, =II=, 21; linear and central aggregation, =II=, 23; natural selection and symmetry, =II=, 134, 133; morphological differentiation, =II=, 177; tissue, =II=, 244; genesis, =II=, 440, 449.

Determinants, Weismann’s germ-plasm units (see Germ-plasm).

Development: an increase of structure, =I=, 162; =II=, 461; primarily central, =I=, 162, 166; uni- and multicentral, =I=, 163–4, 166–7; axial, =I=, 164, 167; uni- and multiaxial, =I=, 165–6; a change to coherent definite heterogeneity, =I=, 167–70, 179; Von Baer’s formula, =I=, 171–2; individual differentiation from environment, =I=, 172–8; cell-formation, =I=, 225; discontinuous, and agamogenesis, =I=, 275; Prof. Huxley’s classification, =I=, 276; sociological parallel to autogenous, =I=, 364–8; =II=, 620; retrograde, =I=, 457–8; inequalities among co-operative parts, =I=, 617; “heterochrony,” =I=, 655; continuous and discontinuous vegetal, =II=, 52; summary of physiological, =II=, 384–90; nutrition and genesis, résumé, =II=, 497–9; evolution, =II=, 501–5; commencement of genesis, =II=, 506; of vertebrate limbs, =II=, 553. (See also Multiplication.)

Development Hypothesis, The, =I=, 417.

Dialects (see Language).

Dialysis, and diffusibillty. =I=, 19, 20.

Diastase, decomposition of, =I=, 38, 40.

Diatomaceæ: tissue, =II=, 244; genesis, =II=, 440, 448.

Diatomic compounds (see Chemistry).

Dicotyledons: growth, =I=, 139, 143; =II=, 63–4, 69–72, 78, 82–3; uniaxial development, =I=, 165; stem and leaf functions, =II=, 257; mechanical stress and wood formation, =II=, 277; growth and genesis, =II=, 451.

Differentiation (see Morphology and Physiology).

Difflugia: primary aggregate, =II=, 86–7; symmetry, =II=, 186; outer tissue differentiation, =II=, 309.

Diffusion, of colloids and crystalloids, =I=, 18–20; =II=, 331.

Digestion: action of nitrogenous compounds, =I=, 69; obesity, =II=, 480–4; fertility, =II=, 514.

Dimorphism: floral, =I=, 534; sexual, in parasites, =I=, 315; social insects (see Insects).

Dinosaurs, size of, =I=, 139.

Diphyes: individuality, =I=, 246; symmetry, =II=, 192.

Disease: segregation of blood constituents, =I=, 179; changes in blood from, =I=, 221, 701; heredity, =I=, 306–7, 312–3, 622–3; belief in supernatural origin, =I=, 419; parasitism and special creation, =I=, 427; morbid products as specific characters, =I=, 567; telegony, =I=, 646; dermal structure, =II=, 306; intestinal muscular hypertrophy, =II=, 325; indigestion and alimentary canal development, =II=, 328; jaundice and bilirubin, =II=, 330; localization of excretion, =II=, 331; membranes in inflammatory, =II=, 343; osseous differentiation in rickets, =II=, 352; fatty degeneration, =II=, 482.

Disintegration, physiological (see Physiology).

Distoma: metagenesis, =I=, 273–4; disintegration of genesis, =I=, 276; cycle of generations, =II=, 489.

Distribution: physical limits, =I=, 396; organic environment, =I=, 396–8; parasitic conditions, =I=, 397–8; simultaneity of agencies affecting, =I=, 398; mutual encroachments of species, =I=, 398–401, 477, 489; facts disproving pre-adaptation to habitats, =I=, 401–3, 411–2; of animals and plants in time, =I=, 404–11, 412; ousting of native species in New Zealand, =I=, 477; local influences, =I=, 477–9, 489; through varied media, =I=, 479–85, 489, 556; past and present organic forms, =I=, 485–9, 556; complex organization and, =II=, 296–7.

Division of labour, physiological (see Labour).

Dog: contrasted lives of tortoise and, =I=, 103, 104; inherited habits, =I=, 309, 573; abnormal digits, =I=, 324; interbreeding of divergent varieties, =I=, 565; decrease of jaw, =I=, 615, 693; telegony, =I=, 645; conditions affecting fertility, =II=, 474, 479.

Dohrn, theory of vertebrate structure, =II=, 606.

Doliolum, combination of individualities, =I=, 247.

Domestication (see Animals).

Doubleday, E., on nutrition of genesis, =II=, 510–2.

Driesch, separation of segmentation spheres of Echinus ovum, =I=, 691; =II=, 618.

Dropsy (see Disease).

Drosera: individuality, =I=, 251; proliferous growth, =II=, 75.

Du Bois-Reymond, E. H., electricity from muscles and nerves, =I=, 50.

Dumas, antithesis of animals and plants, =I=, 62.

Dwarfs, Hindu family of, =I=, 316.

Ear, development of vertebrate, =II=, 318, 320.

Earth, climatic rhythm and organic change, =I=, 499–501, 557.

Earth-worm: bilateral symmetry, =II=, 199, 200; mould production, =II=, 402.

Echinococcus (see Entozoa).

Echinodermata: independence of blastosphere cells, =I=, 185; protoplasmic continuity in embryos, =I=, 190; separation of segmentation spheres of ovum, =I=, 691; =II=, 618; symmetry, =II=, 191, 195–6.

Economy: of growth in natural selection, =I=, 536, 562; a trait of organic evolution, =II=, 501, 504.

Ectoderm: functional differentiation, =I=, 202, 203; functional vicariousness, =I=, 209; reproductive function, =I=, 281.

Effects, Multiplication of: variation, =I=, 329–30, 333; organic evolution, =I=, 511–4, 515, 517, 549, 557; =II=, 405–6; morphological development, =II=, 7–9, 234; physiological differentiation, =II=, 390–1, 392.

Eggs (see Embryology).

Eimer, T., theory of orthogenesis, =I=, 563–4.

Elasmobranchii: protoplasmic continuity, =I=, 629; segmentation, =II=, 126.

Electricity: genesis in organic matter, =I=, 50–2, 60; muscular action, =I=, 59; incomprehensibility, =I=, 121.

Elephant: fertility, =I=, 583, 599; =II=, 459, 506; cerebro-spinal system, =I=, 598, 599.

Elk, Irish, horns and correlated parts, =I=, 537, 674.

Eloidea canadensis: individuality, =I=, 248; enormous agamic multiplication, =I=, 642.

Elongation, and locomotion in animals, =II=, 15.

Embryology: as aiding biology, =I=, 125–6; simulated growth, =I=, 136; initial and final organic bulks, =I=, 143, 158, 161; fœtal flesh constituents, =I=, 154; human arm development, =I=, 169; Von Baer’s formula, =I=, 170–2, 451–4, 466; embryonic heat, =I=, 177; spherical organic form, =I=, 177; unit-life in multicellular organisms, =I=, 185–6; functional differentiation, =I=, 203; individuality, =I=, 246–7; unspecialized reproductive tissue, =I=, 279–83, 317; changes following impregnation, =I=, 283–4; nutrition and vegetal growth, =I=, 285–8, 295–7; and animal growth, =I=, 289–94, 295–7; physiological units and heredity, =I=, 317–9; variation and parental functional condition, =I=, 324; uterine environment, =I=, 327–8; physiological units and variation, =I=, 330–4, 458; fertilized and unfertilized ova, =I=, 340–1; hermaphrodism, =I=, 341–2, 344; sociological parallel, =I=, 366–8; evolution hypothesis, =I=, 434, 436, 453, 454, 555; petrel development, =I=, 455; substitution and suppression of organs, =I=, 456–8, 466, 472–3; structural proclivities of physiological units, =I=, 458; abridgment of stages, =I=, 458–9, 464; disappearance of intermediate forms, =I=, 459–60, 463; pre-adaptation, =I=, 461–3; discrimination of species in early stages, =I=, 461; anomalous persistence of ancestral traits, =I=, 463–5; phylogeny, =I=, 486; egg-shell function, =I=, 527; genesis of grades in social insects, =I=, 654–6, 658–9, 679–80; determination of sex, =I=, 657; order of development qualified by needs, =I=, 679; osteology of Punjabis, =I=, 689; direct transformations and physiological units, =I=, 706; transformation of blastema, =II=, 20; arrest of growth and innutrition, =II=, 73; development of segmented animals, =II=, 100–2, 602; adaptive vertebrate segmentation, =II=, 118–23, 124, 223–4, 605–6; animal cell morphology, =II=, 228; primary differentiations of germinal layers, =II=, 300–2; lung development, =II=, 333–4; mammalian ova-maturation, =II=, 342–3; movements of ova, =II=, 356, 363; modifications in mole, =II=, 391; genesis and nutrition, =II=, 424, 425; fish ova, =II=, 435, 454; cost of genesis, =II=, 435–6; number of birds’ eggs, =II=, 454–6, 478; heat and genesis, =II=, 468, 474; activity and genesis in birds, =II=, 470–2, 474; vertebrate limb development, =II=, 553; ossification in vertebrates, =II=, 556; Owen’s vertebrate theory, =II=, 563; development of vertebræ, =II=, 564. (See also Multiplication.)

Embryology of conceptions, =I=, 451.

Emigrants, type of organization among, =I=, 364; =II=, 620.

Endoderm: functional differentiation, =I=, 202, 203; functional vicariousness, =I=, 209.

Endogen, application of term, =II=, 62, 78, 82. (See Monocotyledons.)

Energy: evolution of, in animals, =I=, 71–7; organic growth and expenditure, =I=, 141; functional transfer, =I=, 201–6; chromatin as the source of, in karyokinesis, =II=, 261–5. (See also Force.)

Entozoa: metagenesis, =I=, 273, 641; self-fertilization, =I=, 342; distribution, =I=, 398; and special creation, =I=, 428; fission in simple types, =I=, 584; nutrition and genesis, =I=, 641; =II=, 488; direct transformation, =I=, 706; integration, =II=, 102; segmentation, =II=, 107, 108; interdependence and organic integration, =II=, 404.

Environment: degree of life and complexity of, =I=, 104–7; relation to organic structure and function, =I=, 172–8; =II=, 12–5; adaptation to varied media an evidence of evolution, =I=, 479–81, 556; influence of solar system, =I=, 500, 556; inherited adaptation to, =II=, 623.

Eolis, branchiæ, =II=, 118.

Epidermis (see Skin).

Epilepsy: definition of life and movements in, =I=, 112; heredity, =I=, 312.

Epithelium: ciliated, =I=, 187; Hertwig’s classification, =I=, 189; reproductive function, =I=, 280; “pavement” and “cylinder,” =II=, 229.

Epizoa: distribution, =I=, 398; special creation and effects of, =I=, 428; interdependence and organic integration, =II=, 404; nutrition and genesis, =II=, 487.

Equilibration: variation and law of, =I=, 326, 334; molecular arrangement, =I=, 337–45; of organic change, =I=, 348, 347, 557; direct and indirect, =I=, 519–22, 573; adaptation by direct, =I=, 522–3, 551, 557; nutrition, defence, and fertilization of plants, =I=, 523–5; direct of animals, =I=, 525–8, 551, 557; natural selection and indirect, =I=, 530–4, 552, 557; of natural selection, =I=, 543–7, 552–3, 557; increasing importance of direct, =I=, 553; of forces acting on species, =I=, 571–2; =II=, 417–20; phenomena not accounted for by, =I=, 573; tissue differentiation, =II=, 245; genesis of nervous system, =II=, 307–8; functional, =II=, 391–4; laws of multiplication, =II=, 411–6; in human and social evolution, =II=, 537. (See also Acquired characters and Natural selection.)

Errera, L., on inherited adaptation to environment in Aspergillus, =II=, 623.

Ethnology: heredity, =I=, 303–4, 310; plasticity of mixed races, =I=, 354; primitive ideas, =I=, 417; evolution and classification, =I=, 441–3, 446; natural selection, =I=, 553.

Euphorbiaceæ: foliar and axial development, =II=, 47–8; physiological differentiation, =II=, 258; dye permeability and circulation, =II=, 571; wood formation, =II=, 575, 577, 578; foliar vascular system, =II=, 589–92, 596.

Evaporation: organic change, =I=, 28; vegetal circulation, =II=, 587.

Evolution: chemical elements and compounds, =I=, 22–4, 67; primordial form of living matter, =I=, 63–4, 181; =II=, 21–2; definitions of life, =I=, 107–10; growth the primary trait of, =I=, 135; comprehends growth and development, =I=, 162; illustrations in development, =I=, 167–70, 178–9; progressive structural differentiation, =I=, 181–4, 192–6, 211–2; life before organization, =I=, 210; heterogeneity of function, =I=, 211; stability of species, =I=, 242, 515, 518; individuality, =I=, 247; cell-organization, =I=, 262; genesis, heredity, and variation resulting from, =I=, 354–5; period required for organic, =I=, 407, 565–6; contrasted with special creation hypothesis, =I=, 415, 431–40; derivation of hypothesis, =I=, 431, 439, 554; increasing belief in, =I=, 431–3, 439; experiences supporting conceivability, =I=, 433–5, 439; direct evidence, =I=, 435–7, 439; malevolence not implied by, =I=, 437–9; evidence from classification, =I=, 443, 444, 449, 466, 555; embryology, =I=, 451–3, 466; substitution and suppression of organs, =I=, 456–8, 466, 472–3; insect segmentation, =I=, 468–9; vertebral column development, =I=, 470–2; rudimentary organs, =I=, 472–5; adaptation to varied media, =I=, 479–85, 556; growth of the theory of organic, =I=, 490–8; instability of the homogeneous, a cause, =I=, 509–11, 516, 550; multiplication of effects, =I=, 511–14, 517–8, 550; =II=, 405; segregation, and heterogeneity and definiteness of, =I=, 514–8, 550; natural selection and general doctrine of, =I=, 543–8, 557; factors tabulated, =I=, 551; inductive evidences summarized, =I=, 555–6; surviving disbelief in France, =I=, 559; current theories of organic, =I=, 559–74; Eimer’s theory of orthogenesis, =I=, 563–4; Gulick on monotypic and polytypic, =I=, 569; phenomena unexplained by theories, =I=, 573–4; inorganic and the System of Philosophy, =I=, 696; “spontaneous generation,” =I=, 696–701, 702; dissolution and problems of morphology, =II=, 4–6; morphology and formula, =II=, 7–9, 231–5; difficulties of definition, =II=, 17; cell-doctrine, =II=, 17–21, 85; unicellular origin of plants, =II=, 21–2; résumé of plant-morphology, =II=, 78–80; origin and differentiation of phænogamic type, =II=, 83; physiological problems, =II=, 239–43; tissue differentiation, =II=, 244–6, 385; integration of organic world, =II=, 396, 406; race and individual multiplication, =II=, 428–30; declining fertility and human, =II=, 431, 529–30; individuation, genesis, and, =II=, 501–5; human life, prospective, =II=, 522–5; forces influencing human, =II=, 525–8; future of population, =II=, 532–7; self-sufficingness of, =II=, 537; vertebral, =II=, 563–6.

Excretion: genesis of organs of, =II=, 303; localization of, =II=, 331–3.

Exogen, application of term, =II=, 82. (See Dicotyledons.)

Expenditure (see Multiplication).

Eye, the: molecular transformations in visual process, =I=, 75–6; progressive development, =I=, 195; =II=, 317–9; waste and repair, =I=, 218; transmitted defects, =I=, 306, 311, 694; degeneration in cave-animals, =I=, 309, 612–3, 614, 647–9, 693; late development in insects, =I=, 658; migration in flat fishes, =II=, 205.

Fabre, J. H., nutrition and sex in Osmia tricornis, =I=, 657.

False joints, =I=, 230; theories of heredity and, =I=, 362, 364; =II=, 371–2.

Fats, the: physical and chemical properties, =I=, 10–12; non-nitrogenous, =I=, 41; action of bile, =II=, 330.

Fatty degeneration, and failing vitality, =I=, 41.

Feathers, development, =I=, 474; =II=, 314–6.

Feet, heredity and size, =I=, 311.

Ferments, changes and nitrogenous character of, =I=, 38.

Ferns: foliar development and nutrition, =II=, 76; inner tissue differentiation, =II=, 273; indefiniteness, =II=, 296; genesis, =II=, 441, 463.

Fertility, the General Law of Animal, =I=, 577–601. (See Multiplication.)

Fertilization: unit-life of generative elements, =I=, 185–6; the function of chromatin, =I=, 260, 263–5; extrusion of polar bodies, =I=, 266–8; nature and functions of generative elements, =I=, 279–83, 317, 334, 342, 593–7; differentiation and variation effected by, =I=, 330–2; the essential object of, =I=, 340–1; =II=, 614–6; hermaphrodism and self-, =I=, 341–2; crossing and its effects, =I=, 343–7; isolation of species in respect of, =I=, 570; floral (see Flowers).

Ficus, foliar structure, =II=, 589, 596.

Fingers: embryogeny of human, =I=, 169; heredity and abnormal, =I=, 305, 314, 321–3; autogenous development of supernumerary, =I=, 363; rudimentary, =I=, 473.

Fishes: sizes of ova and adult, =I=, 143–4; growth of pike, =I=, 154, 292; size and environment, =I=, 156; temperature, =I=, 174; self-mobility, =I=, 175; continuity of blastomeres, =I=, 214; =II=, 327; genesis, =I=, 271; =II=, 435, 436; conditions affecting genesis, =I=, 292–3, 583, 598, 599; =II=, 454; classification, =I=, 392; change of media, =I=, 401, 480; distribution in time, =I=, 408–9; climbing species, =I=, 480, 482; migrations, =I=, 500; dermal structure, =I=, 526; =II=, 305–6, 315, 387; Cunningham on non-adaptive specific characters, =I=, 565; elongation and locomotion, =II=, 15; segmentation, =II=, 122, 225; bilateral symmetry, =II=, 203–5; eyes of Pleuronectidæ, =II=, 205; genesis of vertebrate axis, =II=, 212–6, 218–21, 225; ossification of paleozoic, =II=, 218; respiratory organs, =II=, 334–8; activity and muscular colour, =II=, 365–9; Owen on skeleton, =II=, 552, 557, 558–60, 562, 564.

Fission (see Agamogenesis).

Flint, Austin, on telegony, =I=, 644.

Flounder, symmetry and eyes, =II=, 205.

Flower, Sir W., on ferret, =II=, 480.

Flowers: pollen propulsion in orchids, =I=, 57; nature of reproductive elements, =I=, 283; insect fertilization, =I=, 340, 525; =II=, 168, 174, 267, 407; self- and mutual fertilization, =I=, 342–5, 570; Darwin on homologies, =I=, 472; direct equilibration and fertilization, =I=, 524–5; dimorphism, =I=, 534; foliar homology of petals, =II=, 43–6; symmetry, =II=, 132, 161, 162–4, 170, 174, 608; fertilization and symmetry, =II=, 164–70; clusters and components, =II=, 170–4; nutrition and inflorescence, =II=, 179–80, 541–2, 546–7; tissue differentiation, =II=, 265–9; separation of ancestral traits in hybrids, =II=, 616–7.

Fly, beneficial parasitism, =II=, 406.

Food (see Nutrition).

Food-cavity, genesis and development of, =I=, 188, 195.

Foraminifera: form, =I=, 173; primary aggregate, =II=, 87, 124; progressing integration, =II=, 89–90, 124.

Force: action on like and unlike units, =I=, 5; expenditure and organic growth, =I=, 149–54, 161; functional accumulation, transfer, and expenditure, =I=, 198–9, 201–3, 391; waste and expenditure, =I=, 214–5; distribution during strain, =II=, 209–12. (See also Energy, and Persistence of Force.)

Fossils (see Palæontology).

Foster, Sir M., on storage of glycogen, =I=, 70, 74; Increase of weight in hybernating dormouse, =I=, 214.

Fowls (see Gallinaceæ).

Foxglove: abnormal development, =I=, 287; =II=, 46; floral distribution, =II=, 141; nutrition and growth, =II=, 179.

France: surviving disbelief in organic evolution, =I=, 559; rate of multiplication, =II=, 509, 512.

Frankland, Sir E., on isomerism of protein, =I=, 700.

Fraser, Col. A. T., on family of Hindu dwarfs, =I=, 316.

Fries, E., multiplication of Reticularia, =I=, 582; =II=, 450.

Frog: vitality of detached heart, =I=, 111; of larval fragments, =I=, 365.

Fry, Sir E., on alternation of generations, =II=, 84.

Fuci: cell multiplication, =II=, 27; undifferentiated outer tissue, =II=, 256.

Function: as a basis of classification, =I=, 124–9, 129–31; simultaneous progress of structure and, =I=, 197, 211; divisions of, =I=, 198–200, 391; correlative complexity of structure, =I=, 200, 210–1; progressive differentiations, =I=, 201–4; concomitant integration, =I=, 205–8; specialization and vicariousness, =I=, 208–10; formula of evolution, =I=, 211; diminished ability and overwork, =I=, 215–6; growth and increased, =I=, 228–33, 234–5; interdependence of social and organic, =I=, 237–9, 240–2; structure and heredity, =I=, 306–13, 318–9 (see Acquired Characters); aids natural selection, =I=, 308; organic interdependence, =I=, 318–9; parental condition and variation, =I=, 324, 326; variation and altered, =I=, 325–6, 333–4; as causing variation, =I=, 334–5; effect on physiological units, =I=, 353–4; =II=, 620; zoological classification, =I=, 391–3; multiplication of effects, =I=, 512; law of equilibration, =I=, 519–22, 557; correlation of changes in, =I=, 529; structural effects of changing, =I=, 541–2; structural co-operation, =II=, 3, 217; vicarious vegetal, =II=, 270; vicariousness and specialization, =II=, 293; epidermic structure, =II=, 312–4, 387; structure and muscular, =II=, 369, 391; adaptive bone-structures, =II=, 370–1; equilibration and adaptation, =II=, 392; persistence of force and adaptation, =II=, 394. (See also Physiology.)

Fungi: nitrogenous character, =I=, 40; development, =I=, 163, 164, 165; conjugation, =I=, 279; =II=, 449; fission, =I=, 584, 585; integration, =II=, 24–5, 293; symmetry, =II=, 137–40, 146; puff-ball tissue, =II=, 246, 252, 386; tissue differentiation, =II=, 256; inner tissue, =II=, 279; indefiniteness, =II=, 295; growth and genesis, =II=, 459; nutrition and genesis, =II=, 487.

Gallinaceæ: conditions affecting fertility, =II=, 454–5, 469, 471; masculine traits of old hens, =II=, 495.

Galls: definition of life and, =I=, 111; Hertwig on, =I=, 690.

Galton, F., on variation outside the mean, =I=, 669.

Gamogenesis: homogenesis, =I=, 270, 271, 336; heterogenesis, =I=, 270, 336; independence of offspring, =I=, 278; reproductive tissue, =I=, 279–84; vegetal nutrition, =I=, 285–8, 293; =II=, 39; animal nutrition, =I=, 289–94, 297; when and why does it recur? =I=, 294–7, 336–40; effect on species, =I=, 347–9; leaf formation, =II=, 39; alternating generation in liverworts, =II=, 80–4; molluscan homogenesis, =II=, 116, 117–8; vertebrate, =II=, 118; growth, =II=, 266. (See also Fertilization, and Multiplication.)

Gasteropoda (see Mollusca).

Geddes and Thompson, on the determination of sex, =I=, 657.

Gelatine, nutritive value of, =I=, 77.

Gemmation: and genesis, =I=, 272–6; theories of heredity and, =I=, 361; annulose, =II=, 100–5, 106.

Generalization, impossibility of perfect, =I=, 450.

Generation, and genesis: the words, =I=, 269.

Genesis (see Multiplication).

Gentiana: floral arrangement, =II=, 608–11.

Genus: indefinite value, =I=, 389, 446; instability of homogeneous and heterogeneity of, =I=, 509–11, 515, 517–8, 550, 557.

Geology: growth displayed in, =I=, 135, 136; distribution in time, =I=, 404–11, 412; special creation, =I=, 419, 426; evolution, =I=, 432, 437; record congruous with evolution, =I=, 485–9, 556; organic influence of changes, =I=, 501–3, 549, 550, 557; climatic influence of changes, =I=, 503; time required for organic evolution, =I=, 565–6; rise of insect and plant relations, =II=, 407; human evolution and changes, =II=, 534.

Geometry, evolution illustrated by, =I=, 433–4.

Germ-cell: unspecialized nature, =I=, 279–83, 317; dissimilarity, =I=, 330, 332, 334, 342; equilibrium, =I=, 340. (See also Fertilization.)

Germ-plasm, Weismann’s theory of, =I=, 357–8; inconsistent with plant embryogeny, =I=, 359; regeneration of lost limbs, =I=, 362; variations in peacock’s tail feather, =I=, 372, 695; =II=, 618–9; alleged differentiation of reproductive and somatic cells, =I=, 622, 628–30, 633–44, 646; origin of variations in neuter insects, =I=, 659, 663–5, 671, 675; correlated variations in stag, =I=, 677; insuperable difficulties, =I=, 682; conceivability of hypothesis, =I=, 695; =II=, 619; correlated variations in cultivated plants, =II=, 621–2.

Ghost-theory, Vitalism and, =I=, 114.

Giraffe, co-adaptation of structures, =I=, 615.

Gizzard, development of birds, =II=, 320.

Glass, molecular re-arrangement, =I=, 337, 352, 704.

Glove, strain analogy, =II=, 575.

Glycogen, In animal metabolism, =I=, 70, 72.

Goethe, J. W. von: foliar homology, =II=, 43–4, 543, 544; archetypal hypothesis, =II=, 122; vegetal fructification and nutrition, =II=, 180; theory of supernumerary bones, =II=, 223; on the skull, =II=, 561.

Gold, effect of bismuth on, =I=, 121.

Gorilla, callosities, =II=, 312.

Gould, J., Birds of Australia, =II=, 469.

Gout (see Disease).

Grafting, Born’s experiments with frog larvæ, =I=, 365.

Graham, T., properties of water, =I=, 9; =II=, 359; colloids and crystalloids, =I=, 15–8; =II=, 356; their diffusibility, =I=, 18–20; sapid and insipid substances, =I=, 53.

Graminæ: foliar surfaces, =II=, 61, 263; floral symmetry, =II=, 165; physiological differentiation, =II=, 257.

Graminivores, food contrasted with that of carnivores, =I=, 68.

Grassi, on food habits of Termites, =I=, 686.

Gravity: its ultimate incomprehensibility, =I=, 121; vegetal circulation, =II=, 586. (See also Specific Gravity.)

Gregarina: central development, =I=, 163; primary aggregate, =II=, 87; symmetry, =II=, 186.

Grimaux, on artificial proteids, =I=, 39.

Growth: organic and inorganic, =I=, 135–7; simulation of, =I=, 136; limits to, =I=, 137, 155–7; structural complexity, =I=, 138–40, 145–7, 161; nutrition, =I=, 140, 147–9, 161; expenditure of energy, =I=, 141–3, 161; initial and final bulks, =I=, 143–4, 157–60, 161; final arrest of, =I=, 149–55, 639; where unceasing, =I=, 154; résumé with generalizations, =I=, 161; defined, =I=, 162; =II=, 461; increased function, =I=, 228–33, 234–5; functional interdependence, =I=, 235–9, 240; nutrition and vegetal, =I=, 293, 294–7, 336; =II=, 39; heterogenesis and animal nutrition, =I=, 289–93, 296, 336; homo- and heterogenesis and natural selection, =I=, 294–8; of acrogens, =II=, 56; cylindrical form of vegetal, =II=, 56–64; endogenous, =II=, 60–2, 78; exogenous, =II=, 63–4, 78; plant differentiation, =II=, 129–131; tissue differentiation, =II=, 370; formation of adaptive bone-structures, =II=, 370–2; progressive increase of size with evolution, =II=, 401–2; vegetal, and asexual genesis, =II=, 439–42; animal, and asexual genesis, =II=, 442–5; antagonistic to asexual genesis, =II=, 446; vegetal and sexual genesis, =II=, 448–51; animal and sexual genesis, =II=, 452–6, 495; antagonistic to sexual genesis, =II=, 457–8; nutrition and genesis, résumé, =II=, 497–9; evolution and, =II=, 501–5; commencement of genesis, =II=, 506; fertilization and restoration of growth-energy, =II=, 613.

Gulick, T.: on monotypic and polytypic evolution, =I=, 569; physiological selection, =I=, 569–71.

Gunpowder, nitrogenous instability, =I=, 8, 43.

Gymnotus, electricity of, =I=, 51.

Gyrodactylus elegans, rapid succession of generations, =I=, 641; =II=, 488.

Habit, change of, in plants, =I=, 308.

Hæmal, term applied to female element, =I=, 594–5.

Hairs: non-conductors of heat, =I=, 526; vegetal, and natural selection, =I=, 532; development, =II=, 314–6; tactual organs, =II=, 317.

Hand: embryogeny, =I=, 169; heredity and size of, =I=, 311; distribution of veins, =I=, 364.

Hardy, W. B., =I=, vii; =II=, vi.

Hare: activity and muscular colour, =II=, 365; expenditure and genesis, =II=, 472.

Hart, J. A., on “Parasol” ants, =I=, 687–8.

Havilland, G. D., collection of Termites, =I=, 687.

Haystack, chemical action in, =I=, 74.

Head, structural influence of size, =I=, 512, 537.

Hearing: the sense of, =I=, 54; multiplying agencies, =I=, 75.

Heart (see Vascular System).

Heat: action on di- and tri-atomic compounds, =I=, 7–8, 10–12, 23, 24; on colloids and crystalloids, =I=, 26; organic changes from evaporation, =I=, 29; chemical decomposition by, =I=, 33; organic oxidation, =I=, 46–9, 60; growth and organic, =I=, 152–3; animal, vegetal, and environment, =I=, 174–5, 177; alloy melting points, =I=, 339; organic effects of rhythm in terrestrial, =I=, 498, 557; effect on physiological units, =I=, 705; respiration in fishes, =II=, 337; animal preservation, =II=, 434; vertebrate expenditure and genesis, =II=, 468–9, 474; insect genesis, =II=, 476; seasonal variations and genesis, =II=, 484–5; in germination, =II=, 615.

Hebrew idea of creation, =I=, 421.

Hectocotylus, individuality, =I=, 250.

Hellin, D., on multiparity and twin-births, =II=, 457.

Hen, what prompts her to pick up egg-shell fragments? =I=, 120.

Henslow, Rev. G., inheritance of functionally-produced changes, =I=, 560.

Hepaticæ: Schleiden on, =II=, 51, 52; continuous and discontinuous development, =II=, 52; phyletic homologies, =II=, 80–4; meaning of so-called alternating generation, =II=, 84; vascular system, =II=, 280; genesis and development, =II=, 463.

Heredity: structural modification, =I=, 232; function of cell-nucleus in, =I=, 258–59; general truths, =I=, 301–4; transmission of congenital peculiarities, =I=, 304–7; structure and altered function, =I=, 307–13, 318–9 (see also Acquired Characters); atavism, or recurrence of ancestral traits, =I=, 314; sex limitation, =I=, 314–6; physiological units, résumé, =I=, 350–5; =II=, 612–6; Darwin’s and Weismann’s theories examined, =I=, 356 et seq., 559–61; =II=, 622; true theory must include plants, =I=, 358; inadequacy of theory of physiological units, =I=, 360–1; sociological parallel, =I=, 366–8; natural selection (q. v.), =I=, 545–7, 553, 557; ethnology and natural selection, =I=, 553; unsolved problems, =I=, 573–4; mutilations, =I=, 631; ultimate process incomprehensible, =I=, 695; cell-doctrine, =II=, 19; physiological development, =II=, 242; wood formation, =II=, 287; tissue differentiation, =II=, 304, 312–4; respiratory system, =II=, 311; osseous differentiation, =II=, 351; muscular adaptation, =II=, 367; persistence of force and physiological adaptation, =II=, 394; vegetal vascular system, =II=, 574, 582, 588, 596.

Hermaphrodism, =I=, 340–3.

Hertwig, O.: on Weismann’s germ-plasm theory, =I=, 690; cell characters, =I=, 691; meaning of fertilization, =II=, 613.

Hertwig, R., classification of tissues, =I=, 189.

Heterochrony of development, =I=, 655.

Heterogeneity: in chemical evolution, =I=, 23–4; of vital changes, =I=, 84–90; of development, =I=, 170, 178; functional, =I=, 204–8, 211–2; of organic matter, =I=, 350–5; organic and instability of homogeneous, =I=, 509–11, 517, 549, 557; segregation accompanying, =I=, 514–6, 517–8, 550.

Heterogenesis: occurrence, =I=, 270, 272–5, 336; animal nutrition, =I=, 289–91, 295–7; natural selection, =I=, 295–8; heredity, =I=, 301.

Hindus: food, =I=, 68; dwarf family, =I=, 316.

Histology (see Physiology).

Hofmeister, sporophytic generation of Archegoniates, =II=, 80.

Hollyhock, floral symmetry, =II=, 167, 169, 170.

Homogeneous, instability of the: variation, =I=, 330, 334, 342; evolution, =I=, 509–11, 517, 549, 557; morphological development, =II=, 7–9, 234; direction of vegetal growth, =II=, 181; radial symmetry, =II=, 190; physiological differentiation, =II=, 384, 392.

Homogenesis (see Gamogenesis).

Homology, simulation of, by analogy, =II=, 14.

Hooker, Sir J. D., =I=, ix; European plants in New Zealand, =I=, 477; plant distribution, =I=, 479; adaptation of plants to varied media, =I=, 484; plant growth, =II=, 56; Balanophoræ and Raffiesiaceæ, =II=, 274; structural complexity, =II=, 295, 297; relative antiquity and distribution of plants and animals, =II=, 297; bean vascular system, =II=, 574.

Hooker, Sir W., on fructification in Jungermanniaceæ, =II=, 52.

Horns, natural selection and correlated variation, =I=, 537, 567, 674, 677.

Horse: ancestral types, =I=, 409; fertility, =I=, 598; weight of brain, =I=, 599; quagga markings, =I=, 624, 627.

Husbandry, co-ordination of actions in, =I=, 96, 579.

Hutchinson, Sir J., hereditary syphilis, =I=, 623.

Huxley, T. H., =I=, ix; “continuous” and “discontinuous” development, =I=, 164; classification of development, =I=, 276; hermaphrodism, =I=, 344; zoological classification, =I=, 383; on “Persistent Types,” =I=, 408–9; ancestral equine types, ib.; segmentation of articulates, =I=, 468–9; =II=, 113; agamic multiplication of Aphis and Entozoa, =I=, 640–1; =II=, 476; cell-doctrine, =II=, 21; vertebrate embryo, =II=, 119, 120; molluscan symmetry, =II=, 202; tegumentary organs, =II=, 314, 315; vertebrate sensory organs, =II=, 318, 319; Chondracanthus, =II=, 487; Owen’s vertebrate theory, =II=, 563.

Hyacinth: lateral spike, =II=, 42; symmetry, =II=, 141, 162.

Hybernation, waste and repair in, =I=, 214–5.

Hybrids, separation of ancestral traits in, =II=, 616–7.

Hydro-carbons: properties, =I=, 6–9; the term carbo-hydrates (q. v.), =I=, 10.

Hydrochloric acid, in gastric juice, =I=, 69.

Hydrogen: chemical and physical properties, =I=, 3–5; compounds, =I=, 6, 8, 9, 10–12, 12–13.

Hydrozoa (see Cœlenterata).

Hymenoptera (see Insects).

Hypertrophy (see Disease).

Hypospadias, telegonic transmission, =I=, 646.

Hypostasis of a relation, exemplified in explanations of fertilization, =II=, 613.

Ideas (see Psychology).

Individuality: the botanical, =I=, 244–6; the zoological, =I=, 246–7; the fertilized germ product, =I=, 248–9; definition of life, =I=, 250.

Individuation: and genesis, =I=, 583–4; =II=, 428–30, 499; total cost, =II=, 435–7; genesis and evolution, =II=, 501–5, 529, 530.

Infusoria: functional specialization, =I=, 391; primary aggregate, =II=, 87; asymmetry, =II=, 187, 188; differentiation, =II=, 299, 385; genesis, =II=, 442, 446, 452.

Injuries, repair of animal, =I=, 219, 222–4, 316; =II=, 102, 611.

Insanity, inherited, =I=, 314.

Insects: temperature, =I=, 47, 174; phosphorescence, =I=, 49; self-mobility, =I=, 175; parthenogenesis, =I=, 274–5, 277, 294, 592, 640; growth and reproduction, =I=, 292; species distribution determined by presence of, =I=, 396–7; eyes of cave-inhabiting, =I=, 309, 612–3, 614, 647–9, 693; persistent types, =I=, 408; retrograde development, =I=, 458; segmentation, =I=, 468–9; =II=, 114; aborted organs, =I=, 474; East Indian distribution, =I=, 478; floral fertilization, =I=, 525; =II=, 168–9, 406–7, 608; appliances for cleaning antennæ, =I=, 651; eyes, =I=, 658; =II=, 318; integration and homology, =II=, 111–3, 121; bilateral symmetry, =II=, 198; sexual selection, =II=, 269; eyes, =II=, 318; environment, =II=, 433; cost of genesis, =II=, 436, 437; development and genesis, =II=, 461; nutrition and genesis, =II=, 476, 490–2.

Insects, Social, origin of caste gradations in, =I=, 654–65, 670, 674, 675, 678–84, 686–8.

Instability of the homogeneous (see Homogeneous).

Instinct: organic evolution and co-ordination of, in mason-wasp, =I=, 574; a vital attribute, =I=, 578; loss of self-feeding, in Amazon ants, =I=, 660–1, 663–4.

Integration: in chemical evolution, =I=, 23; morphological composition, =II=, 4–6; arthropod, =II=, 111–4, 121; physiological, in plants, =II=, 292–5, 295–8, 390; of organic world, =II=, 396–408; genesis, =II=, 424, 426–9.

Intelligence, a vital attribute, =I=, 579.

Internodes: varied development, =II=, 45; nutrition and length, =II=, 178–9.

Intestine (see Alimentary Canal).

Intra-selection, Roux’s theory of, =I=, 562, 676–8.

Irish, nutrition and genesis, =II=, 510.

Iron: colloidal form of peroxide, =I=, 17, 20; molecular re-arrangement, =I=, 337, 704; vegetal absorption, =II=, 573.

Iron industry, interdependence of social function, =I=, 237–41.

Isolation, and species differentiation, =I=, 568–9.

Isomerism: of organic constituents, =I=, 4, 9, 25; tri- and poly-atomic compounds, =I=, 11, 13, 25; muscular action, =I=, 59; organic evolution, =I=, 700, 703; differentiation of nerve tissue, =II=, 356–60, 361; of muscular tissue, =II=, 361–4.

Jackson, J. Hughlings, on inheritance of nervous peculiarities, =I=, 313, 694.

Jaundice (see Disease).

Jaws, of uncivilized and civilized, =I=, 541–2, 612, 693.

Johnson, G. Lindsey, on inherited myopia, =I=, 694.

Jones, T. Rymer, on fission, =I=, 585, 590.

Julin, C., on “castration parasitaire” in Crustaceans, =II=, 493–6.

Jungermanniaceæ: morphology, =II=, 33–4; relations of high and low types, =II=, 35, 55; continuous and discontinuous development, =II=, 52–5, 92; tubular structure, =II=, 58, 62; proliferous growth, =II=, 67, 91; colour, =II=, 75, 265; symmetry, =II=, 140; fertility and growth, =II=, 441.

Jussieu, A. de, plant classification, =I=, 378.

Karyokinesis, =I=, 257, 259, 263–5.

Kerner, A., on cauline buds, =I=, 358; plant classification in Natural History of Plants, =I=, 378–9.

Kidd, Benj., his acceptance of Weismannism, =I=, 690.

Kitto, Dr., his visual memory and deafness, =I=, 230.

Klebs, on Hydrodictyon, =I=, 288; Vaucheria, =II=, 84.

Klein, E., multiplication of Bacteria, =II=, 443.

Korschelt, E., annulose segmentation, =II=, 103, 601–3, 605; Arenicola larvæ, =II=, 109.

Labour, physiological division of, =I=, 204, 207, 591; =II=, 373; its meaning and Weismann’s fallacious interpretation, =I=, 634–5.

Lacaze-Duthiers, on origin of annulose type, =II=, 110.

Lamarck: zoological classification, =I=, 382; opinions of E. Darwin and, =I=, 491, 493–7; neo-Darwinists and, =I=, 630–1.

Laminariaceæ: pseudo-foliar and axial development, =II=, 30; tissue, =II=, 247, 256, 272.

Language: and evolution, =I=, 442, 444, 446; perceptiveness of tongue-tip, =I=, 607.

Lankester, Sir E. Ray, absence of nucleus in Archerina, =I=, 183; diversity of Protozoa, ib.; zoological classification, =I=, 387; blindness of cave-animals, =I=, 647–8, 649.

Laugh, definition of life and, =I=, 112.

Laurel, leaves of, =II=, 149, 249.

Leaves: growth of shoot, =I=, 168; development and aggregation, =II=, 37–42, 76; stem-like stalks, =II=, 41; homologies, =II=, 42, 75–7, 83; nutrition and compound, =II=, 42; foliar and axial development, =II=, 46–50, 541–7; “adnate,” =II=, 58; proliferous growth, =II=, 67, 91; nutrition and development, =II=, 76–8; symmetry, and of branches, =II=, 148–50, 151; size and distribution of leaflets, =II=, 152–5; transition from compound to simple, =II=, 155–8; unsymmetrical form, =II=, 158–9; natural selection and distribution, =II=, 179; morphological summary, =II=, 234–5; tissue differentiation, =II=, 247; distribution, =II=, 249; outer tissues of stem and, =II=, 256–9, 270, 386; distribution of stomata, =II=, 260–1; wax deposit on, =II=, 260, 261; light and colour, =II=, 261–2; superficial differentiation, =II=, 263–5, 270, 387; abortive in parasitic plants, =II=, 274; submerged, in aquatic plants, =II=, 274–5; inner tissue differentiation, =II=, 278, 388; vascular tissue differentiation, =II=, 286, 288, 388; dye absorption and circulation, =II=, 570–4, 577; vascular system, =II=, 588–92, 596; arrangement, =II=, 608–11.

Lepidoptera (see Insects).

Lepidosiren: ossification, =II=, 218; respiration, =II=, 338; skeleton, =II=, 553, 555, 560.

Lepidosteus: armour, =I=, 526; air-bladder, =II=, 334.

Leroy-Beaulieu, Pierre, on Australian miners’ usages, =I=, 364.

Lessonia: Hooker on growth, =II=, 56; branch symmetry, =II=, 146.

Lewes, G. H., definition of life, =I=, 80.

Lichens: tissue, =I=, 586; cell multiplication, =II=, 27; Hooker on growth, =II=, 56; tubular structure, =II=, 57; integration, =II=, 293; dual nature, =II=, 399; reproduction, =II=, 450.

Liebig, Baron, nitrogenous food stuffs, =I=, 47–8.

Life: co-ordination of actions, =I=, 79, 89, 577–80; defined by Schelling, =I=, 78, 178; Richeraud, =I=, 79; De Blainville, =I=, 79, 93; Lewes, =I=, 80; definition yielded by contrasting most unlike kinds, =I=, 81–8; changes showing, =I=, 91; vital actions and environment, =I=, 92–3; resulting addition to conception, =I=, 93, 326; Comte’s definition, =I=, 93; correspondence of external and internal relations, =I=, 93–6, 100; =II=, 523; continuous adjustment of such relations, =I=, 99; completeness proportionate to correspondence, =I=, 101–4, 109, 349; length and complexity, =I=, 103; complexity of environment and degree of, =I=, 104–6; definitions of evolution and, =I=, 107–10; deficiencies of formula, =I=, 112–3; activity the essential element, =I=, 113; hypothesis of independent vital principle examined, =I=, 114–7; difficulties of physico-chemical theory, =I=, 117–20; ultimate incomprehensibility, =I=, 120–3, 373; validity of conclusions reached, =I=, 123; is organization produced by? =I=, 197; precedes organization, =I=, 210; definitions of individuality and, =I=, 250; effect of incident forces on, =I=, 348–9, 355; length in individuals and species, =I=, 422; equilibration of, =I=, 547, 557; final formulation of definition, =I=, 580; co-ordination of actions and sexual differentiation, =I=, 593; “absolute” commencement of, =I=, 699, 702; integration and augmentation, =II=, 426; prospective human, =II=, 522–5.

Light: influence on organisms, =I=, 30–6; =II=, 433; nitrogenous plants, =I=, 40; organic phosphorescence, =I=, 49; heliotropism, =I=, 92; =II=, 160; effects on organic matter, =I=, 149; plant adaptation, =I=, 227; rhythmical variation of, and organic life, =I=, 499, 557; vegetal influences, =II=, 130, 131, 147, 149, 158; influence on flowers, =II=, 167–8, 608–11; vegetal tissue differentiation, =II=, 253–5, 258, 259; action on leaves, =II=, 260–4; on plant vascular system, =II=, 288, 297, 586; development of sensory organs, =II=, 320.

Liliaceæ, floral symmetry, =II=, 170.

Lime, leaf forms, =II=, 158, 159.

Lindley, J., plant classification, =I=, 377.

Linnæus, C., classificatory system, =I=, 377, 380.

Linnet, contrasted with blackbird in development, =II=, 503.

Liver: metabolic processes, =I=, 69, 70; vitality of excised, =I=, 111; development, =II=, 329–33.

Liver-fluke (see Distoma).

Liverworts (see Hepaticæ).

Lizard, regeneration of lost tail, =I=, 360.

Locomotion (see Motion).

Logic, reasoning and definition of life, =I=, 81–6.

Logwood, vegetal staining, =II=, 569–74, 577–81, 584.

Longevity, and complexity of life, =I=, 102–3.

Lubbock, Sir J.: on growth and genesis in insects and crustaceans, =I=, 292; aquatic flies, =I=, 400.

Lungs (see Respiratory System).

Lymphatic system: amœboid cells, =I=, 187; structural traits, =I=, 192, 193.

MacBride, E. W., =I=, vi; =II=, vi; zoological phyla, =I=, 386–7; arthropod segmentation, =II=, 114; ctenidia of slug, =II=, 117; conjugation of Paramæcium, =II=, 452.

Macrocystis pyrifera, gigantic seaweed, =I=, 121.

Magenta, vegetal staining, =II=, 569–74, 577–81, 584.

Magnetism: muscular action, =I=, 59; incomprehensibility, =I=, 121.

Maillet, B. de, modifiability of organisms, =I=, 490, 496.

Mammalia: temperature and molecular change, =I=, 30; nutrition and growth, =I=, 141; expenditure of force, =I=, 142, 156; flesh constituents, =I=, 154; temperature, =I=, 174, 177; self-mobility, =I=, 175; functional and structural differentiation, =I=, 201; heart-function, =I=, 206; viviparous homogenesis, =I=, 271; variation and uterine environment, =I=, 327; classification, =I=, 392; cervical vertebræ, =I=, 394; =II=, 564; aquatic types, =I=, 400; fossil remains and rate of evolution, =I=, 407; ancient and modern forms contrasted, =I=, 408–10; embryonic respiratory system, =I=, 456; suppression of teeth, =I=, 457; arrested development, =I=, 473–4; simulated homologies, =I=, 485; natural selection and inactive parts, =I=, 534; re-development of rudimentary organs, =I=, 563; location of testes and current theories, =I=, 573; fertility and development, =I=, 583; =II=, 465; fertility and nervous development, =I=, 598–9; locomotion and elongated form, =II=, 15; symmetry, =II=, 204; tegumentary structure, =II=, 314; circulation, =II=, 340; vascularity and ova-maturation, =II=, 342–3; activity and muscular colour, =II=, 365–9; functional integration, =II=, 375; outer tissue differentiation, =II=, 387; growth and genesis, =II=, 456, 459; comparative fertility, =II=, 465, 470; heat expenditure and genesis, =II=, 467–9; activity and fertility, =II=, 472; nutrition and genesis, =II=, 479–80.

Man: effect of climate on vigour, =I=, 30; flesh and grain eaters compared, =I=, 68; longevity and life, =I=, 103; complex environment, =I=, 105; embryogeny of arm, =I=, 169; fertility and conditions affecting it, =I=, 300, 570, 583; =II=, 484, 506–21; inheritance of functionally-produced changes, =I=, 310–3, 541, 605, 608, 612, 652, 673, 689, 693–4; heredity and sex, =I=, 315–6; function of bilirubin, =I=, 330; cousin-marriages, =I=, 346; =II=, 615; primitive notions, =I=, 417–9; inutility of Appendix vermiformis, =I=, 474; diminution of jaw, =I=, 541, 612, 693; co-ordination of actions greatest in, =I=, 579; fundamental traits of sex, =I=, 594–7; obesity, =I=, 594; substance and weight of brain, =I=, 596, 599; distribution of tactual perceptiveness, =I=, 602–8, 665–6, 672–3, 692; telegony, =I=, 625, 644–5; degradation of little toe, =I=, 652, 673; transmitted osteological peculiarities of Punjabis, =I=, 689; traits of twin-bearing women, =II=, 457; comparative mammalian fertility, =II=, 465; future evolution, =II=, 522–37. (See also Language and Sociology.)

Manatee, nailless paddles, =I=, 473.

Manx cats, =I=, 303.

Marchantiaceæ: symmetry, =II=, 140; outer tissue differentiation, =II=, 252.

Marmot, hybernation and waste, =I=, 214–5.

Marriage (see Multiplication).

Marsh, O. C., on telegony, =I=, 644.

Masters, M. T., on foliar homology, =II=, 46–7; selection of inconspicuous variations in plants, =II=, 298, 621; separation of ancestral constitutions in plant hybrids, =II=, 616; single and double stocks, =II=, 622.

Matter, incomprehensibility of interactions, =I=, 121–2.

Mechanics: transverse strains, =II=, 209–12; genesis of vertebrate axis, =II=, 212–6, 216–8, 224, 225–7; osseous differentiation, =II=, 345–51; disintegrated motion, =II=, 375; analogy from locomotive, =II=, 517–9; future human evolution, =II=, 524; strain and vegetal structure, =II=, 574–88, 592–6.

Medusæ: contractile functions, =I=, 58; =II=, 374; individuality, =I=, 248; heterogenesis, =I=, 273; fertility, =I=, 582; strobilization, =I=, 592; symmetry, =II=, 188–91.

Mehnert, E., on feet of pentadactyle vertebrates, =I=, 461.

Mensel’s salt, temperature and isomerism, =I=, 77.

Metabolism: antithesis between plants and animals, =I=, 62–3; evolution hypothesis and primordial, =I=, 63–4; in plants, =I=, 64–7; animals, =I=, 67–77; nervo-muscular activities, =I=, 71–7; summary, =I=, 77; cell processes, =I=, 261.

Metals: remarkable interactions of some, =I=, 121; melting of alloys, =I=, 339; atomic re-arrangement, =I=, 352.

Metamerism (see Segmentation).

Metazoa: cellular structure, =I=, 184, 194; =II=, 21; subordination of units, =I=, 185–7; general characters of tissues, =I=, 188–9; protoplasmic continuity, =I=, 190–2, 194, 628; genesis of food-cavity and visual organ, =I=, 195; Weismann’s differentiation theory, =I=, 637–43.

Meteorology: non-vital changes shown in, =I=, 82, 84; crystallization of “storm glass,” =I=, 96; special creation, =I=, 419; rhythm in, and organic change, =I=, 499–501, 557; variations due to geologic change, =I=, 503.

Microstomida, segmental reproduction, =II=, 102.

Migration: of animal species, =I=, 396–401, 411; solar influences, =I=, 500; part played by, in organic evolution, =I=, 568; causes of, =II=, 533–4.

Milk, heat and supply of, =II=, 468.

Milne-Edwards, H., “physiological division of labour,” =I=, 204; Weismann’s erroneous application of it, =I=, 634; on ocular structure, =II=, 318.

Mind (see Psychology).

Mitosis (see Karyokinesis).

Mobility, molar and molecular, =I=, 14; environment and self-mobility, =I=, 177.

Mohl, on phænogamic growth, =II=, 82.

Mole, modifications due to habits, =II=, 391.

Molecules: mechanically considered, =I=, 14; stability, =I=, 337–40; nerve differentiation, =II=, 355–61, 379–82.

Mollusca: axial development, =I=, 165; genesis, =I=, 271; =II=, 444; hermaphrodism, =I=, 341; classificatory traits, =I=, 392; distribution in time, =I=, 405, 408, 410, 446–7; trochophore and its relationships, =I=, 447; =II=, 108, 109, 115; development, =I=, 460; amphibious and terrestrial, =I=, 481; indirect equilibration, =I=, 534; secondary aggregation, =II=, 115–7; symmetry, =II=, 201–3; outer tissue, =II=, 310, 387; alimentary system, =II=, 325; vascular system, =II=, 340–1.

Molluscoida, =II=, 598. (See Polyzoa and Tunicata.)

Monocotyledons: growth, =I=, 138, 139, 143; uniaxial development, =I=, 165; cotyledonous germination and endogenous growth, =II=, 59–62, 69–72, 82–3, 181–2; absence of helical phyllotaxy in Ravenala, =II=, 182; surface contrasts, =II=, 257; outer leaf tissue, =II=, 263; wood formation, =II=, 278; growth and genesis, =II=, 451.

Monstrosities, in plants, =II=, 78, 541, 546; vertebrate, =II=, 118.

Morgan, T. H., on regeneration of Planaria, =II=, 102, 611.

Morphology: facts comprised in, =I=, 125–6; morphological units, =I=, 190–2, 225; rudimentary organs, =I=, 472–5, 556; structural and functional co-operation, =II=, 3, 239; integration, =II=, 4–6, 181–96; change of shape, =II=, 6; formula of evolution, =II=, 7–9; as interpreted by phylogeny, =II=, 10–6; evolution and cell-doctrine, =II=, 17–21.

Morphology, Animal: evolution and segmentation of Articulata, =I=, 468–9; vertebral column development, =I=, 470; simulated homologies, =II=, 14–5; primary aggregates, =II=, 85–8, 123–4; secondary, =II=, 88–91, 124; tertiary, =II=, 91–3; integration and independence of individuality, =II=, 93–9, 124; annulose segmentation, =II=, 98–101, 106–10, 125–7, 602–7; progressive annulose integration, =II=, 100–5, 111–5, 121, 124, 223; unintegrated molluscan form, =II=, 115–7; vertebrate segmentation and integration, =II=, 117–23, 124–7, 223–4, 602, 606–7; motion and symmetry, =II=, 183–5; symmetry of primary and secondary aggregates, =II=, 186, 187–91; of compound Cœlenterata, =II=, 192–4; simulation of plant shapes, =II=, 192; symmetry of Polyzoa and Tunicata, =II=, 194; of Platyhelminthes and Echinoderms, =II=, 195–7; of Annulosa, =II=, 197–201; of molluscs, =II=, 201–3; of vertebrates, =II=, 203–6, 208; similarity of animal and plant, =II=, 206; cell-shapes, =II=, 228–30; evolution and generalizations summarized, =II=, 231–5. (See also Structure.)

Morphology, Vegetal: simulated homologies, =II=, 13–4; unicellular plants, =II=, 21; aggregation and integration, =II=, 22–6, 78–9; pseudo-foliar development, =II=, 26–8; pseudo-axial, =II=, 28–9; pseudo-foliar and axial, =II=, 30–2; composition of Archegoniates, =II=, 33–5; leaf development and aggregation, =II=, 37–42, 75–8; foliar homologies, =II=, 42–6, 75–8; foliar and axial development, =II=, 46–50, 541–7; growth and development of Archegoniates, =II=, 50–6; of Phænogams, =II=, 56–64, 78–80; axillary bud development, =II=, 65–9; phænogamic modes of growth, =II=, 69–72; homologies, =II=, 73–5, 80–4; development of foliar into axial organs, =II=, 75–8; résumé, =II=, 78–80; criticisms and replies, =II=, 80–4; can plant shapes be formulated? =II=, 128; growth and differentiation, =II=, 129–31; kinds of symmetry, =II=, 131–3; symmetry of primary aggregates, =II=, 134–7; of secondary, =II=, 137–40; tertiary, =II=, 140–3; symmetry and environing influences, =II=, 143–4; symmetry of branches, =II=, 145–8; leaf and branch symmetry, =II=, 148–50; phænogamic unit homology, =II=, 151; size and distribution of leaflets, =II=, 152–5; transition from compound to simple leaves, =II=, 155–8; unsymmetrical leaf development, =II=, 158–9; differentiation of homologous units, =II=, 159–60; floral symmetry, =II=, 161–74; cell-differentiation and metamorphosis, =II=, 175–7; nutrition and differentiation, =II=, 178; and inflorescence, =II=, 179; helical growth of phænogams, =II=, 180–1; summary of symmetry, =II=, 234; stress and structure, =II=, 275–9, 388. (See also Structure.)

Morton, Lord, quagga-marked foal, =I=, 624.

Moser, impressions produced by light on metals, =I=, 352.

Mosses: varied development, =II=, 501, 52; homologies, =II=, 80, 81; indefiniteness, =II=, 296; multiplication, =II=, 441.

Moth, clothes, food of larva, =I=, 77.

Motion: organic, and environment, =I=, 75–7, 175–8, 196; of animals and waste, =I=, 214, 220; simulation of locomotive structures, =II=, 15.

Motor organs, differentiation of, =I=, 262.

Mountains: climatic effects, =I=, 504; growth of trees on, =II=, 142.

Mouse: fertility of, =II=, 421, 473; tapeworm parasitism, =II=, 490; compared with rat, =II=, 503–4.

Mucor, =II=, 22, 123.

Mucous membrane, differentiation, =II=, 321–2, 389.

Multiplication: decline of fertility with evolution, =I=, 103; =II=, 431; vitalism, =I=, 116; phenomena classified, =I=, 130; the term “genesis,” =I=, 269; processes classified, =I=, 270–6, 336, 583; a process of disintegration, =I=, 276; reproductive tissue in gamogenesis, =I=, 278–84; nutrition and growth, =I=, 285–94, 295–7, 299; natural selection, =I=, 295–8; hermaphrodism, =I=, 340–4; in-and-in breeding, =I=, 344–7; physiological units, =I=, 350–5; law of race-maintenance, =I=, 581; =II=, 420–3, 430; effect of mental application, =I=, 597; =II=, 511–4, 516–9, 530; individuation antagonistic to, =I=, 598–600; =II=, 428–30, 435–7, 499, 501–5; checks put by carnivores on, =II=, 405; four factors in rate of, =II=, 416, 435; destructive and preservative forces, =II=, 417–20; rhythm of species, =II=, 419; nutrition and disintegration of, =II=, 424, 425, 430; integration and genesis, =II=, 426–8; influence of environment, =II=, 432–3; and variations of expenditure, =II=, 433–5; growth and asexual genesis, =II=, 439–46; asexual and sexual distinguished, =II=, 448; sexual genesis and growth, =II=, 448–58, 495; and development, =II=, 461–5; plant expenditure, =II=, 467; animal expenditure, =II=, 468–72; nutrition in plants, =II=, 475, 511; in animals, =II=, 476–84, 511; seasonal variations, =II=, 484–5; nutrition, résumé, =II=, 486, 497–9; nutrition and parasitic, =II=, 486–90; reversion to agamogenesis, =II=, 490–2; human fertility, =II=, 506–10; Doubleday on, =II=, 510–2; civilized and uncivilized, =II=, 514–6; human evolution and decline in, =II=, 529–31; the future of population, =II=, 532–7; equilibration and evolution, =II=, 537.

Muscle: electrical contrasts, =I=, 50; action of, =I=, 59; metabolism, =I=, 70, 71–4; definition of life and actions of, =I=, 112–3; growth and function, =I=, 151, 155; development, =I=, 170; Hertwig’s classification of tissues, =I=, 189; functional differentiation, =I=, 203–4; waste and repair, =I=, 215–7; modifiability and adaptability, =I=, 228–9, 230, 232; correlated variations, =I=, 536–9, 614–21, 676, 693; resistance to strains, =I=, 639; action on bones in Punjabis, =I=, 689; differentiation, =II=, 361–9; activity and colour, =II=, 365–9; integration, =II=, 376, 382; equilibration in action, =II=, 393; activity and fertility in birds, =II=, 470–2; future human evolution, =II=, 523; origin of vertebrate type, =II=, 598–600.

Music: limited adaptability of voice and ear, =I=, 231; inheritance of faculty, =I=, 311–2, 694.

Mutilations, the question of their inheritance, =I=, 631.

Mycetozoa, growth and reproduction, =I=, 298–9.

Myocommata (myotomes), and vertebrate skeleton, =II=, 216, 217–8, 222.

Myopia, inheritance of, =I=, 306, 694.

Myrianida fasciata, =I=, 361; =II=, 445.

Myriapoda: gemmation, =I=, 589; segmentation, =I=, 590; =II=, 113, 114, 601; degenerated eyes of cave-inhabiting, =I=, 649; integration and homology, =II=, 111–4; genesis, =II=, 445.

Myxothallophyta, =I=, 378.

Nails, mammalian, =I=, 473.

Nais: regeneration of detached parts, =I=, 219, 361.

Narcissus, separation of ancestral traits in hybrids, =II=, 617.

Natural selection: structural modification, =I=, 211; in cell processes, =I=, 263–4; multiplication, =I=, 295–8; aided by function, =I=, 308–10; special creation, =I=, 426–7; the term “survival of the fittest,” =I=, 530; indirect equilibration, =I=, 530–5, 552–3, 557, 571; changes unexplained by, =I=, 535–42, 571; =II=, 371; tendency to economy, =I=, 536, 562; decrease of jaw, =I=, 541, 693; general doctrine of evolution, =I=, 543–8, 557; unceasing operation, =I=, 552; human races, =I=, 553; current views, =I=, 559–60; panmixia and cessation of selection, =I=, 560–3; intra-selection, =I=, 562, 676–8; Eimer’s theory of orthogenesis, =I=, 564; Mr. Cunningham’s criticism, =I=, 565–6; location of mammalian testes, =I=, 573; co-ordinated instincts of mason-wasp, =I=, 574; tactual perceptiveness, =I=, 603–8, 633, 646, 665, 671, 672, 692; erroneously identified with artificial selection, =I=, 609, 695; reversed selection, =I=, 611; blindness of cave-animals, =I=, 613, 614, 647–8, 693; co-adaptation of co-operative parts, =I=, 614, 621, 663–5, 670, 674, 675, 689, 692; where operative, =I=, 632; Weismann on conceivability of process, =I=, 651; degeneration of little toe, =I=, 652–3, 673; genesis of caste gradations In social insects, =I=, 654–60, 663, 670, 675, 684; self-feeding instinct in ants, =I=, 660–2, 670; rudimentary organs, =I=, 667–9, 671, 692; horns of stag, =I=, 676–8, 692; musical faculty, =I=, 694; the neo-Darwinian position reviewed, =I=, 694–5; vegetal nutrition, =II=, 51–2; upright vegetal growth, =II=, 56–7; endogenous growth, =II=, 57–8; exogenous, =II=, 64; Navicula symmetry, =II=, 135; foliar, =II=, 158; foliar distribution, =II=, 167, 179; floral fertilization and symmetry, =II=, 168–70, 608–11; helical phænogamic growth, =II=, 181; Echinodermata and bilateral symmetry, =II=, 197; vertebrate structure, =II=, 214–20, 227; phænogamic tissue differentiation, =II=, 248; physiological differentiation, =II=, 252, 256; rootlets of ivy, =II=, 254; stomata and foliar surfaces, =II=, 261, 262; floral fertilization, =II=, 268–9; sexual selection, =II=, 269; vegetal tissue differentiation, =II=, 279; wood formation, =II=, 287–8, 290; animal tissue differentiation, =II=, 304–8; evolution of nervous system, =II=, 307–8; respiratory system, =II=, 311; dermal callosities, =II=, 312–4; sensory organ complexities, =II=, 321; skin and mucous membrane differentiation, =II=, 322; localization of excretion, =II=, 333; respiratory organs of fishes, =II=, 335–8; heart and vascular system, =II=, 341, 344; osseous differentiation, =II=, 355; also muscular, =II=, 363, 368–9; “false joints,” =II=, 371; insect nutrition and genesis, =II=, 499; economics of evolution, =II=, 501–5; author’s enunciation of survival of the fittest in 1852, =II=, 528–9; evils of interference with, in man, =II=, 532–3; vegetal tissue formation, =II=, 582, 594–6; origin of vertebrate type, =II=, 599.

Nature, more complex than supposed, =I=, 252, 450.

Navicula, symmetry, =II=, 134–5.

“Nebular Hypothesis,” =I=, 23.

Negation, inconceivability of, the ultimate test of truth, =I=, 675.

Negroes, telegony In United States, =I=, 644–5.

Nemertidæ: continuing vitality of pilidium, =I=, 250; bilateral symmetry, =II=, 195.

Neo-Darwinists, and Lamarck, =I=, 630; their position reviewed, =I=, 694–5.

Nerves: electrical phenomena, =I=, 51; generation of nerve force, =I=, 52–6, 60; corpuscula tactus, =I=, 75; Hertwig’s classification of tissues, =I=, 189; structural traits, =I=, 192, 193; environment and structure, =I=, 196; differentiation, =I=, 203; =II=, 355–61; vasomotor system, =I=, 206; vicarious function, =I=, 209; activity and waste, =I=, 216; adaptability, =I=, 229, 232, 236; parallelism in cell processes, =I=, 260–2; heredity, =I=, 313; effects of severance, =I=, 349; relative development in men and women, =I=, 594; analysis of brain substance, =I=, 596; individuation and development of, =I=, 598, 599, 600; distribution of tactual perceptiveness, =I=, 603–8, 633, 646, 665–6, 671, 672, 692; alleged costliness of tissue, =I=, 662; instinct degeneration in ants, ib.; “sensation areas,” =I=, 666; segmentation in Annelids, =II=, 125; ectodermal derivation, =II=, 303–4; co-operating factors in evolution of, =II=, 307–8; differentiation from muscle, =II=, 363. (See also Psychology.)

Nervousness, hereditary transmission, =I=, 307.

Neurine, =I=, 594, 597.

Neuter insects (see Insects).

New Zealand: invasion of alien species, =I=, 477; kinship of past and present forms, =I=, 489.

Nitrogen: properties, =I=, 3–5, 20, 24; compounds and their properties, =I=, 6, 8, 9, 12–14, 25–6, 39, 41, 42–3; =II=, 250; organic importance, =I=, 42–3; evolution of heat and oxidation, =I=, 47; violent organic effects of compounds, =I=, 54–5; function in metabolism, =I=, 63–4, 66, 68–76; presence in protoplasm, =I=, 66; action in digestion, =I=, 69; fat accumulation and fertility, =II=, 483.

Nitro-glycerine, violent effects of, =I=, 55, 122.

Notochord: segmentation, =II=, 125, 218–22; formation, =II=, 217–8, 600; in Permian vertebrates, =II=, 225.

Noumenon, life not manifested as, =I=, 580.

Nuclein, =II=, 21.

Nucleus: central development, =I=, 163; in simple organisms, =I=, 183; phenomena exhibited by, =I=, 255–8; current hypotheses of function, =I=, 258–9; properties and function of chromatin, =I=, 259–65; fusion in fertilization, =I=, 283–4; function in unicellular reproduction, =I=, 595–6; absence of, =II=, 20–1; diffused form, =II=, 85; macro- and micronucleus in conjugation, =II=, 452.

Nutrition: organic molecular re-arrangement, =I=, 36; nitrogenous and non-nitrogenous, =I=, 47–8, 68, 71–4, 77; =II=, 362; food assimilation and reasoning, =I=, 81; needful for vital change, =I=, 94; relation to growth, =I=, 140, 143, 144, 147–9, 150, 157, 161; expenditure of energy, =I=, 157, 391; fluid, =I=, 208; vegetal fructification, =I=, 267; =II=, 266; vegetal growth and genesis, =I=, 293, 294–7, 336; animal growth and genesis, =I=, 289–93, 295–7, 336; conditions qualifying antagonism of growth and genesis, =I=, 299; competition among parts of an organism for, =I=, 562, 566, 676; sex differentiation, =I=, 594–5; cell multiplication, =I=, 638; differentiation of neuter insects, =I=, 655–60, 670, 674, 686–8; monstrous ant forms, =I=, 683–4; leaf development, =II=, 39, 42, 73–8; vegetal development, =II=, 51–2, 178, 276; axillary buds, =II=, 65–9, 73–4; effect on animal aggregation, =II=, 93; internodes and inflorescence, =II=, 178–80; helical phænogamic growth, =II=, 181; action of bile, =II=, 330; osseous development, =II=, 349, 353; genesis, =II=, 419, 422, 427, 435–7, 452; parental loss in feeding young, =II=, 424, 429; diverse sources, =II=, 433; Carpenter on reproduction and, =II=, 460; animal development and genesis, =II=, 465; expenditure and genesis, =II=, 468; variations of genesis, =II=, 475–80, 511; obesity and genesis, =II=, 480–4, 511; general doctrine of genesis, =II=, 486; genesis and vegetal parasitism, =II=, 486; also animal, =II=, 487–90, 495; insect genesis, =II=, 490–2; genesis, résumé, =II=, 497–9; and evolution, =II=, 501–4; of blackbird and linnet, =II=, 503; genesis in human race, =II=, 508–10, 514–6; Doubleday on, =II=, 510–2; future human evolution, =II=, 526, 531; floral monstrosities, =II=, 542, 546, 547.

Obesity, nutrition and genesis, =II=, 480–4, 511.

Odoriferous glands, natural selection and, =I=, 534.

Odours: floral fertilization, =II=, 268–9; animal protection, =II=, 434.

Offspring: parental loss entailed by nurture, =II=, 424, 429; influence of age on, =II=, 507.

Oken, L., archetypal hypothesis, =II=, 122; theory of supernumerary bones, =II=, 223; on the skull, =II=, 561.

Oliver, F. W., classification of plants, =I=, 378–9.

Ophryotrocha puerilis, ciliation of segments, =II=, 109.

Orchids: pollen propulsion, =I=, 57; leaf formation in Dendrobium, =II=, 60–1; aërial roots and physiological differentiation, =II=, 255, 257; foliar surface, =II=, 264.

Organic matter: properties of elements, =I=, 3–5, 22; of compounds, =I=, 5–13, 25; molar and molecular mobility, =I=, 12–14; colloid and crystalloid form, =I=, 15–8, 25; their diffusibility, =I=, 18–21, 26; extreme complexity, =I=, 21; laws of evolution and genesis of, =I=, 22–4; modifiability, =I=, 27, 44; capillarity and osmosis, =I=, 28; effects of heat, =I=, 29; of light, =I=, 30–4; nitrogenous, =I=, 39–43; oxidation and evolution of heat, =I=, 46, 60; genesis of electricity, =I=, 50–2, 60; sensible motions in, =I=, 59; transformations and persistence of force, =I=, 61; metabolism, =I=, 62–77; artificial production of compounds, =I=, 64; contrasted with inorganic matter, =I=, 107–8; incomprehensibility of vital changes in, =I=, 122; instability, =I=, 149, 508; phosphorus in cell-organization, =I=, 260–1; heterogeneity, =I=, 350–5; “spontaneous generation” and evolution of, =I=, 696–701; cell-doctrine and evolution of, =II=, 17–21.

Organization (see Structure).

Ormerod, Dr., on sex and nutrition in wasps, =I=, 656.

Orthogenesis, Eimer’s theory of, =I=, 563–4.

Osmosis: organic effects, =I=, 28, 29; in animals, =I=, 58; in vascular system, =II=, 339; in vegetal tissue, =II=, 568, 575, 577, 585, 592–6.

Osteology (see Bone).

Ovum (see Egg and Fertilization).

Owen, Sir R.: metagenesis and parthenogenesis, =I=, 273–4; fossil mammals, =I=, 410; human parasites, =I=, 427; continuous operation of creative power, =I=, 492; fission in Infusoria, =I=, 584, 585, 595–6; parthenogenesis, =I=, 592; theory of vertebrate skeleton, =II=, 123, 548–66; theory of supernumerary bones, =II=, 223; Eschricht on Ascaris, =II=, 488.

Oxalis: radial symmetry, =II=, 152; foliar surface, =II=, 264.

Oxen: comparison with sheep, =I=, 158, 160; cerebro-spinal system, =I=, 508.

Oxidation (see Oxygen).

Oxygen: properties, =I=, 3–5, 20, 22; compounds, =I=, 6–7, 10–13, 22, 24–5; a crystalloid, =I=, 21; combining power and atomic weight, =I=, 33; organic change from, =I=, 37; heat generation, =I=, 46–9; phosphorescence, =I=, 49; nerve force dependent on, =I=, 53; animal metabolism, =I=, 72, 73; necessary to animal life, =I=, 94–5, 577; activity and amount inhaled, =I=, 214.

Packard, A. S., on eyes of cave-animals, =I=, 648–9, 693.

Paget, Sir J., blood changes in small-pox and scarlatina, =I=, 221, 701.

Palæontology: distribution in time, =I=, 404–11, 412; special creation, =I=, 425; congruity with evolution hypothesis, =I=, 485–9, 556; relations of present to extinct species, =II=, 10–11; scarcity of remains, =II=, 34–5; secondary thickening in plants, =II=, 56; Cope on osteology of Permian Vertebrates, =II=, 225–6.

Pangenesis, Darwin’s theory of, =I=, 356, 357, 359, 360, 362, 372.

Panmixia, Weismann’s hypothesis of: Its relation to Romanes’ “cessation of selection,” =I=, 560; alleged selective process denied, =I=, 561–3, 667, 685; distribution of tactual perceptiveness, =I=, 608; rudimentary eyes of cave fauna, =I=, 612–3, 647; Romanes on process, =I=, 649, 667; degeneration of self-feeding instinct In Amazon ants, =I=, 660–2, 670; rudimentary limbs of whale, =I=, 668–9, 685; a pure speculation, =I=, 671; markings on leg-bones of Punjabis, =I=, 689.

Paramœcium: parasite infesting, =I=, 427; reproduction, =II=, 443, 452.

Parasites: sexual dimorphism, =I=, 315; limits to distribution, =I=, 397; special creation and, =I=, 427–9, 438; retrograde development, =I=, 457; =II=, 12; aphis and ant, =I=, 660–1; =II=, 403, 405; as an integrating agency, =II=, 402–4; its comparative recency, =II=, 404; nutrition and genesis in vegetal, =II=, 486; in animal, =II=, 487–90, 493; “castration parasitaire” in crustaceans, =II=, 493–6.

Parasol Ants, origin of classes, =I=, 687–8.

Parthenogenesis: occurrence, =I=, 274–5; alternating with gamogenesis, =I=, 289–91; Owen on, =I=, 592; laws of multiplication, =II=, 415; in articulate animals, =II=, 445.

Pasteur, L., silkworm diseases, =I=, 622–3.

Peacock: theories of heredity and structure of tail feather, =I=, 372–3, 695; =II=, 618–9.

Pear, foreright shoots, =I=, 287.

Peloria: in gloxinia, =II=, 166; phænogams, =II=, 180.

Penguin, dermal structure, =II=, 314.

Pepsin, =I=, 69.

Pericyclic fibres of monocotyledons, =II=, 278.

Peripatus capensis, protoplasmic continuity, =I=, 629.

Peri-visceral sac, function and differentiation, =I=, 391.

Perkin, W. H., =I=, vi.

Persistence of force, corollaries from: properties of compounds, =I=, 3; organic transformation, =I=, 60; growth, =I=, 150; organic energy, =I=, 220; variation, =I=, 335; genesis, heredity, and variation, =I=, 354–5; morphological summary, =II=, 235; vegetal tissue differentiation, =II=, 245; physiological development, =II=, 394.

Petals: foliar homology, =II=, 43–6; “adnate,” =II=, 58.

Petrels, Darwin on, =I=, 455.

Phænogams: production of spermatozoids, =I=, 186; morphological composition, =II=, 37–79; leaf transitions, =II=, 37–42; foliar homologies, =II=, 42–9; origin of type, =II=, 49–84; vertical growth, =II=, 56–64; axillary buds, =II=, 66; cotyledonous germination and endogenous growth, =II=, 69–72; axial homologies, =II=, 73–5; irregular development, =II=, 75–8; degree of composition, =II=, 78; reproductive homology, =II=, 80–4; uni- and multiaxial symmetry, =II=, 141–3; unit of composition, =II=, 151; helical growth, =II=, 181; secondary thickening, =II=, 247; tissue and leaf differentiation, =II=, 247–9, 387; also bark and cambium, =II=, 249–50, 386; also outer tissue, =II=, 252, 256–9, 270, 386–7; wax deposit on leaves, =II=, 260–2; differentiation of inner tissues, =II=, 273–5, 388; vascular system development, =II=, 280–4, 388; integration, =II=, 293–5, 296, 390; insect fertilization, =II=, 407; multiplication, =II=, 441, 442; genesis and growth, =II=, 451, 457; and development, =II=, 464; and nutrition, =II=, 476, 477, 511; substitution of axial for foliar organs, =II=, 541–7.

Phenomenon, life manifested as, =I=, 580.

Philology (see Language).

Phoronis, individuality, =II=, 444.

Phosphorescence, organic, =I=, 49.

Phosphorus: allotropic, =I=, 4; in cell physiology, =I=, 259–62; cerebral activity, =I=, 596–7; organic evolution, =I=, 703.

Photogenes, visibility of, =I=, 218.

Phylogeny: as interpreting morphology, =II=, 10–12; difficulties of affiliation, =II=, 34–5. (See Embryology and Evolution.)

Physiological Units: definition, =I=, 226; genesis, =I=, 280–1, 316; heredity, =I=, 315–9; variation, =I=, 330, 331–2, 333; =II=, 619; stability, =I=, 340; =II=, 614; self-fertilization, =I=, 342–4, 353; interbreeding, =I=, 345, 353; =II=, 615; recapitulation of hypothesis, =I=, 350–5; =II=, 612–7; structural proclivities, =I=, 362, 364, 369–71; =II=, 613, 622; sociological analogy, =I=, 364; =II=, 620; complexity in organized types, =I=, 368–70; re-named “constitutional units,” =I=, 369; telegony, =I=, 650; “mechanical theory,” =I=, 701–6; morphological development, =II=, 7–9; cell-doctrine, =II=, 17–21; development, =II=, 76; “false joints,” =II=, 371–2; dissociation of ancestral traits in hybrids, =II=, 616–7; inheritance of acquired characters, =II=, 618–23.

Physiological division of labour (see Labour).

Physiological Selection, =I=, 569–71.

Physiology: and psychology, =I=, 127; subdivisions, =I=, 128; vicarious function, =I=, 208; primitive interpretations, =I=, 417; multiplication of effects exemplified, =I=, 512; =II=, 390; relations to morphology, =II=, 3, 239–41; evolutionary interpretation of phenomena, =II=, 241–5, 384–95; ultimate inconceivability of processes, =II=, 372; correlated integration and differentiation, =II=, 373.

Physiology, Animal: metabolism, =I=, 67–77; vertebrate internal symmetry, =II=, 108; tissue differentiation in Protozoa, =II=, 299, 385; primary tissue differentiation, =II=, 300–2, 382, 389; natural selection and tissue differentiation, =II=, 304–8; outer tissue in Cœlenterata, =II=, 309–10; respiratory organs, =II=, 310–1, 333–8; differentiation of animal epidermic tissue, =II=, 312–4, 387; development of tegumentary organs, =II=, 314–6; of sensory, =II=, 317–20; inner and outer tissue transition, =II=, 321–2, 389; alimentary canal differentiation, =II=, 323–5; gizzard development in birds, =II=, 325; alimentary canal of ruminants, =II=, 327–9; differentiation of liver, =II=, 329–33; of animal vascular system, =II=, 339–44; of osseous system, =II=, 344–55; of nerve tissue, =II=, 355–61; of muscle, =II=, 361–9; differentiation and integration, =II=, 373–6; in vascular system, =II=, 376–9, 383; in nerves, =II=, 379–82; origin of development, =II=, 384; differentiation and instability of homogeneous, =II=, 384–9, 392; summary of development, =II=, 384–94; multiplication of effects, =II=, 390–1, 392; equilibration, =II=, 391–4. (See also Function.)

Physiology, Plant: metabolism, =I=, 62–7; tissue differentiation in secondary aggregates, =II=, 246, 385; in phænogams, =II=, 247–9, 386; in bark and cambium, =II=, 249–50, 386; in free and fixed surfaces, =II=, 251–6, 270, 386; outer stem and leaf tissue, =II=, 256–9, 270, 386; superficial differentiation in leaves, =II=, 260–4, 270, 387; floral tissue differentiation, =II=, 265–9; outer tissue, résumé, =II=, 270; inner tissue differentiation, =II=, 273–5, 388; supporting tissue, =II=, 275–9, 285–8, 388; vascular system development, =II=, 273–5, 279–84, 285–8, 388; inner tissue, summary, =II=, 288–91, 388; integration, =II=, 292–8; differentiation and instability of homogeneous, =II=, 384–9, 392; multiplication of effects, =II=, 390–1, 392; equilibration, =II=, 391–4; circulation and wood formation, =II=, 564–97; dye permeability, =II=, 569–74, 577–81, 584, 586. (See also Function.)

Pickering, J. W., on artificial proteids, =I=, 39.

Pig: colour of muscles, =I=, 365–6; telegony, =I=, 627; fertility of domestic and wild sow, =II=, 479–80.

Pigeons: food of starving, =I=, 215; heredity and variation, =I=, 305, 321, 615; atavism, =I=, 314; fertility, =II=, 471–2, 478.

Pike, unceasing growth, =I=, 154, 292.

Pique-gouffe, commensal relations with buffalo, =II=, 403.

Plagiochila, evolution of stem, =II=, 62.

Planaria: integration, =II=, 101–2; Morgan on regeneration, =II=, 102, 611; segmentation, =II=, 107; symmetry, =II=, 195; unintegrated function, =II=, 373.

Plants: influence of heat, =I=, 29; effect of solar rays, =I=, 31–6, 500, 557; chemical composition, =I=, 40–1; heat generation, =I=, 47; phosphorescence, =I=, 49; electricity, =I=, 51; sensible motion, =I=, 56–7, 58; metabolism, =I=, 62–7, 70; vital changes, =I=, 86, 87, 91, 94; simulation by crystals, =I=, 96; vital adjustments, =I=, 102; length and complexity of life, =I=, 103–4; biological classification, =I=, 125; growth, =I=, 136, 138, 140, 143, 145–9, 153, 160–1; =II=, 401–2; development, =I=, 163–5, 167–70, 272; weight, temperature, and self-mobility, =I=, 174; function, =I=, 174–8; structure, =I=, 194–6; =II=, 21; animal structure contrasted, =I=, 196; function and structure, =I=, 200; vicarious function, =I=, 208–9; waste and repair, =I=, 213, 220; physiological units, =I=, 225–6, 317, 360; adaptation, =I=, 227; what is an individual? =I=, 244–6, 250–1; genesis, =I=, 270, 271, 272–3, 274, 276–8, 279–85; relation of nutrition to growth and genesis, =I=, 284–9, 294, 295–300, 642; =II=, 39; ovule homologues, =I=, 288; natural selection, =I=, 294–8, 532, 533; =II=, 51; heredity, =I=, 301–4, 308, 358–60; variation, =I=, 320, 323–4, 325–6; fertilization, =I=, 340–5; classification, =I=, 377–80, 389–90; distribution, =I=, 396–400, 401–3, 404–12, 478–9, 556; special creation and parasitism, =I=, 428; evolution hypothesis, =I=, 434, 443, 449–50; rudimentary organs, =I=, 474, 475, 556; varied media, =I=, 484; =II=, 32; alien and native species in New Zealand, =I=, 477; E. Darwin and Lamarck on evolution of, =I=, 490–8; geologic changes affecting, =I=, 501–3, 557; interdependence of animals and, =I=, 504–6, 514; =II=, 398; complexity of influences affecting, =I=, 506; direct equilibration, =I=, 523–5; indirect, =I=, 532, 533; seed distribution, =I=, 546; wood development, =II=, 285–7, 289, 567–97; interdependence, =II=, 402–3, 404; insect relations, =II=, 406–7; adaptation and multiplication, =II=, 411–6; rhythm in numbers, =II=, 419; growth and asexual genesis, =II=, 439–42; growth and sexual genesis, =II=, 448–51; expenditure, =II=, 467; horticulture, nutrition, and genesis, =II=, 477; tree development, =II=, 553; circulation and wood formation, =II=, 567–92; dye permeability and circulation, =II=, 569–74, 577–81, 584, 586; résumé on circulation and wood formation, =II=, 592–7. (See also Multiplication, Morphology, and Physiology.)

Plasmodium, dissolution of, =I=, 185.

Plato, ἰδέα of, =II=, 550.

Platyhelminthes: transverse fission, =II=, 101; segmented and non-segmented types, =II=, 102, 107; symmetry, =II=, 195, 197; multiplication and growth, =II=, 488–9.

Plethora, fertility and, =II=, 480–4, 511.

Pleurococcaceæ, unicellular form, =II=, 21, 134.

Pleuronectidæ: symmetry and location of eyes, =II=, 205; outer tissue, =II=, 387.

Plumatella: metagenesis, =I=, 277; symmetry, =II=, 195.

Podostemaceæ, undeveloped circulatory system, =II=, 274.

Polar bodies, hypothesis concerning extrusion of, =I=, 266–8.

Polarity, organic, of physiological units, =I=, 226, 315, 317, 332, 350–1, 701–6.

Polyatomic compounds (see Chemistry).

Polychætæ, anomalous development in Myrianida, =I=, 361.

Polycytharia, integration, =II=, 90, 124.

Polygastrica, aggregation, =I=, 586.

Polymerism: of compounds, =I=, 9, 11, 25; nerve tissue, =II=, 356.

Polypori, symmetry and environment, =II=, 139.

Polyps (see Cœlenterata).

Polyzoa: size, =I=, 140; multiaxial development, =I=, 165; structural indefiniteness, =I=, 173; functional differentiation, =I=, 202; trochophoral kinship, =I=, 447; integration, =II=, 93–4, 96, 124; symmetry, =II=, 194, 207; vascular system, =II=, 340; gemmation, =II=, 444.

Poor Laws, and natural selection, =II=, 532.

Population, A Theory of, =I=, 265, 577–601; =II=, 411.

Potato: simulated growth, =I=, 136; vicarious function of tuber, =I=, 209; =II=, 255; sub-species, =I=, 302; dye absorption, =II=, 279.

Preservation: fertility and self-, =I=, 581; =II=, 423, 430; nutrition, =II=, 493.

“Progress; its Law and Cause,” theory of species differentiation, =I=, 568.

Projectiles, factors in flight of, =I=, 450–1.

Proteids: metabolic function, =I=, 67, 68, 69, 72, 76; complexity of molecule, =I=, 122.

Protein: evolution, =I=, 23, 24; isomerism, =I=, 700, 703, 704.

Proteus, degeneration of eye, =I=, 613.

Protodrilus, intestine segmentation, =II=, 125.

Protophyta: internal movements, =I=, 56; limit of growth, =I=, 138; development, =I=, 164; structure, =I=, 173, 181–3; self-mobility, =I=, 175; individuality, =I=, 245; multiplication, =I=, 270, 276, 279, 581, 584–5; =II=, 439, 462; genesis and nutrition, =I=, 295; unicellular, =II=, 21; central aggregation, =II=, 24; symmetry, =II=, 134; tissues, =II=, 244, 249; primary differentiation, =II=, 385; primordial type, =II=, 398; symbiosis, =II=, 400.

Protoplasm: self-increasing function of primordial, =I=, 63–4; plant metabolism, =I=, 65–7; complexity, =I=, 122, 253–5; differentiation in simple organisms, =I=, 182–3; continuity and inter-circulation, =I=, 190–2, 371, 629; =II=, 21, 620; “streaming,” =I=, 253; structure, =I=, 253–5. (See also Cell.)

Protozoa: inorganic components, =I=, 17; locomotion, =I=, 58, 175; =II=, 14; vital changes shown by, =I=, 94; limitation of growth, =I=, 138; development, =I=, 164; structure, =I=, 173, 181–3; incipient differentiation, =I=, 198, 391; =II=, 299, 309; multiplication, =I=, 270, 276, 279, 280, 582, 584; =II=, 442, 451–2; genesis and nutrition, =I=, 295; distribution, =I=, 396; parasites infesting, =I=, 427; Weismann’s hypothesis of immortality, =I=, 637; “spontaneous generation,” =I=, 697–701; non-nucleated, =II=, 20; primary aggregate, =II=, 86–7, 124; progressing integration, =II=, 89–91, 124; symmetry, =II=, 186; primordial plant-animal type, =II=, 397–8; symbiosis, =II=, 400.

Protyle, hypothetical chemical unit, =I=, 22, 23.

Pseud-axial development, vegetal, =II=, 28–9, 30.

Pseudo-foliar development, vegetal, =II=, 26–8, 30.

Psychidæ: parthenogenesis, =I=, 275; sexual dimorphism, =I=, 683.

Psychology: reasoning and definition of life, =I=, 81–8; correspondence shown by recognition, =I=, 95; contrasted with physiology, =I=, 127; departments of, =I=, 127–8; vicarious function, =I=, 209; waste and repair in sensory organs, =I=, 217; sensory adaptability, =I=, 229, 231, 232; inheritance of sensory defects, =I=, 306; musical talent, =I=, 311–2; intellectual progress and special creation hypothesis, =I=, 417; special creation a pseud-idea, =I=, 420, 429, 433, 554; legitimacy of evolution hypothesis, =I=, 433–5, 439, 554; embryology of ideas, =I=, 450, 457; persistent formative power unrepresentable, =I=, 492; E. Darwin’s and Lamarck’s theory of desires, =I=, 494; natural selection and brain evolution, =I=, 553; genesis and cerebral activity, =I=, 594; =II=, 512–4, 516–9, 530; heredity and distribution of tactual perceptiveness, =I=, 602–8, 646, 665–6, 672, 692; inconceivability of the negation, =I=, 675; vitiation of evidence, =II=, 88; repetition and perception, =II=, 143; differentiation of sensory organs, =II=, 317–20; differentiation of nerve tissue, =II=, 355–61; functional integration, =II=, 376; also integration, =II=, 380–2; equilibration of nerve discharge, =II=, 393; human fertility and nerve development, =II=, 466, 532; future human evolution, =II=, 523–5, 527; human evolution and genesis, =II=, 529–31; future mental development, =II=, 535; origin of vertebrate type, =II=, 598–600.

Pteridophyta: size attained by, =I=, 138, 139; homologies, =II=, 80–1, 82; frond surface differentiation, =II=, 260.

Pteropoda: bilateral symmetry, =II=, 201; dermal respiration, =II=, 310.

Ptyaline, metabolic function, =I=, 69.

Punjabis, inheritance of acquired osteological peculiarities, =I=, 689.

Pyrosomidæ: phosphorescence, =I=, 47; integration, =I=, 588; =II=, 97.

Quagga, telegonic transmission of markings to offspring of mare, =I=, 624, 627, 646.

Quills, development, =II=, 314–6.

Rabbit: activity and muscle colour, =II=, 365; over-running checked by weasels, =II=, 405; expenditure and genesis, =II=, 472.

Radial, definition, =II=, 148.

Radiolaria: unicentral development, =I=, 163; secondary aggregation, =II=, 88; symmetry, =II=, 187.

Radula, development of roots from leaflets, =II=, 34.

Rafflesiaceæ: homogenesis, =I=, 272; tissue differentiation, =II=, 274; nutrition and genesis, =II=, 486.

Rat (see Rodentia).

Rathke, H., on vertebrate embryo, =II=, 119.

Ray, J., plant classification, =I=, 378.

Reasoning, compared with assimilation, =I=, 81–7.

Recapitulation, embryological, =I=, 453.

Regeneration (see Repair).

Rejuvenescence, and sexual fertilization, =I=, 637; =II=, 613.

Remak, R., vertebrate embryo, =II=, 120.

Repair: continuity of, =I=, 216–9; animal injuries, =I=, 219, 222–4; =II=, 102, 611; deductive interpretation, =I=, 221–2; theories of heredity and regenerative phenomena, =I=, 360–1.

Repetition of like parts, =II=, 126.

Reproduction (see Multiplication).

Reptilia: growth and expenditure of force, =I=, 142; sizes of ova and adult, =I=, 144; longevity of crocodile, =I=, 154; temperature, =I=, 174; waste, =I=, 214; distinctive characters, =I=, 392; distribution in time, =I=, 409, 412; vertebral segmentation, =I=, 470; rudimentary limbs of snakes, =I=, 473; fertility and development, =I=, 583, 598, 599; regeneration, =I=, 589; elongated form, =II=, 15; supernumerary vertebræ, =II=, 123, 564; bilateral symmetry, =II=, 203, 204; Cope on segmentation in extinct, =II=, 225, 226; activity and muscular colour, =II=, 365; functional integration, =II=, 375; outer tissue differentiation, =II=, 387; Owen on skeleton, =II=, 560.

Resistance of media to locomotion, =II=, 15.

Respiratory System: effect of light, =I=, 31; organic re-arrangement, =I=, 37; cutaneous, =I=, 209; air-cells of lungs, =I=, 254; embryonic branchiæ of salamander, =I=, 457; differentiation, =II=, 310–1, 333–8; physiological integration, =II=, 374–5, 382; vascular differentiation and integration, =II=, 377.

Retrograde metamorphoses, in animals, =II=, 12.

Retzius, G., superficial nerve-endings, =I=, 666.

Reversed Selection, =I=, 611, 612.

Rhabdospheres, calcareous armour and dynamic element in life, =I=, 119.

Rhizoids, foliar expansions, =II=, 50.

Rhizopoda: structure, =I=, 173; undifferentiated function, =I=, 200; a primary aggregate, =II=, 86; symmetry, =II=, 186; tissue differentiated, =II=, 299, 385; motion of sarcode, =II=, 356; symbiosis, =II=, 400.

Rhythm: astronomic and organic, =I=, 499, 557; law of equilibration, =I=, 520–1; in multiplication, =II=, 419.

Richeraud, Baron A., definition of life, =I=, 79.

Riley, C. V., on telegony, =I=, 645; Termites, =I=, 680, 681; pouch of Honey-ants, =I=, 684.

Rodentia: incursions, =I=, 399; American types, =I=, 403; fertility and development, =I=, 583, 599.

Rivinus, plant classification, =I=, 377.

Rokitansky, on false joints, =I=, 230.

Romanes, G. J.: on “cessation of selection,” =I=, 560–2; isolation and species differentiation, =I=, 569; “physiological selection,” =I=, 569–71; panmixia, =I=, 649, 667; influence of a previous sire on progeny, =I=, 649.

Röntgen rays, =I=, 121; =II=, 621.

Roots: developed from leaflets, =II=, 34; physiological differentiation, =II=, 253–5, 270; nutrition from leaves, =II=, 274; size and function, =II=, 276.

Rotiferæ: latent vitality of desiccated, =I=, 117; trochopore, =II=, 108, 109; molluscan relationship, =II=, 115; fertility and size, =II=, 453, 459.

Roux, W.: “intra-selection,” =I=, 676; functional adaptation, =II=, 354.

Rudimentary organs: the definition of life and, =I=, 112; natural selection and eyes of cave fauna, =I=, 309, 612–4, 647–9, 693; evolution hypothesis, =I=, 472–5, 556; limbs of whale, =I=, 668–9, 685, 693.

Ruminants, alimentary canal development, =II=, 327–9.

Salamander, embryonic branchiæ, =I=, 457.

Salmonidæ, reproduction and growth, =I=, 291–3; =II=, 454.

Salpidæ: heterogenesis, =I=, 272, 277; integration, =I=, 588; =II=, 97.

Sap (see Vascular system).

Sarcina: central aggregation, =II=, 24; fertility, =II=, 440.

Savage, Dr., on “Heredity and Neurosis,” =I=, 313.

Scenedesmus, individuation, =II=, 24.

Scent: natural selection and keenness of, =I=, 610; floral fertilization, =II=, 268–9; animal protection, =II=, 434.

Schelling, E. W. J. von, definition of life, =I=, 78, 178.

Schleiden, J. M., on individuality, =I=, 245; on liverworts, =II=, 50, 52; algal indefiniteness, =II=, 296.

Science, complex revelations of, =I=, 252, 369, 450.

Scyphomedusæ, strobilization, =II=, 108.

Sea: changes and movements in, =I=, 83; life in, lower than terrestrial, =I=, 104; distribution, =I=, 396, 517; change of media caused by, =I=, 481; geologic influence, =I=, 502.

Seals: nail-bearing toes, =I=, 473; vibrissæ, =II=, 317.

Seasons: reproductive periodicity, =I=, 299; variations of genesis with, =II=, 484–5.

Sedgwick, Adam: on continuity of protoplasm in animals, =I=, 190, 629; =II=, 21; zoological classification, =I=, 387; discrimination of species in embryonic stages, =I=, 461; persistence of ancestral traits, =I=, 463–4; Archiannelidan segmentation, =II=, 109.

Sedgwick, Wm.: heredity and sex, =I=, 305, 314; telegonic transmission of hypospadias, =I=, 646.

Seeds: nitrogenous, =I=, 40; temperature of germinating, =I=, 47; =II=, 615; vitalism and latent vitality of, =I=, 116–7; variation in environment, =I=, 327; natural selection among, =I=, 532.

Segmentation (metameric): special creation hypothesis, =I=, 468–9; Huxley on number of somites in higher articulates, ib.; in annulose animals, =II=, 98–110, 111–5, 601–5; simulated molluscan, =II=, 116; in vertebrates, =II=, 125–7, 225–7, 606–7; in elasmobranchs, =II=, 126.

Segregation: of growth, =I=, 136; of like units, =I=, 179; organic repair, =I=, 221; variation, =I=, 331, 334; heterogeneity, and definiteness of evolution, =I=, 514–6, 517–8; morphological development, =II=, 7–9; physiological units, =II=, 616.

Self-fertilization, animal and vegetal, =I=, 341–4, 353.

Senses, the (see Psychology).

Sex: in Ascidian colonies, =I=, 247; limitation of heredity by, =I=, 314–6; correlated traits, =I=, 371–2, 513; nutrition and determination of, in social insects, =I=, 655–60, 678–84, 686–9; neural and hæmal traits, =I=, 683; differentiation of organs, =II=, 303; castration and growth, =II=, 459; Julin on “castration parasitaire” in crustaceans, =II=, 493–6; the object of fertilization, =II=, 613. (See also Fertilization.)

Sexual Selection (see Natural Selection).

Sharp, D.: on insect somites, =I=, 469; food habits of Termites, =I=, 686–7.

Sheep: contrasted with oxen, =I=, 158, 160; crossing of English and French breeds, =I=, 625; nutrition and genesis, =II=, 480.

Sherrington, Prof., on effects of nerve severance, =I=, 349.

Ship-building, interdependence of social functions, =I=, 237–9, 241.

Shipley, A. E.: segmentation of Microstomida, =II=, 102; Protodrilus, =II=, 125.

Silica, colloid and crystalloid, =I=, 16.

Silicic acid: properties, =I=, 16; isomerism, =I=, 59.

Silicon, allotropic, =I=, 4.

Silkworm disease, =I=, 622–3.

Simulation: of homology by analogy, =II=, 14, 485; of segmented structure by molluscs, =II=, 116.

Siphonophora, specialization of component polyps, =II=, 95.

Sirenia, simulated fish form, =I=, 485.

Size (see Growth).

Skeleton, vertebrate (see Vertebrata).

Skin: respiratory function, =I=, 209; adaptability, =I=, 228; =II=, 312–4, 387; transmitted peculiarities, =I=, 306; Wallace on distribution of sensitiveness, =I=, 646–7; differentiation, =II=, 215, 217, 304–7; tegumentary development, =II=, 314–6, 387; differentiation of sensory organs, =II=, 317–20; and mucous membrane, =II=, 303–4, 321–2, 389.

“Skin friction,” and locomotion of aquatic animals, =I=, 156.

Skull (see Vertebrata).

Sleep, repair favoured by, =I=, 216.

Small-pox, blood changes from, =I=, 221.

Smith, Prof. W., on fertility of diatomaceæ, =II=, 440.

Smith, W. P., on telegony in calves and foals, =I=, 645.

Smith, W. W., on habits of Tetramorium, =I=, 660.

Snakes (see Reptilia).

“Social organism,” author’s essay on, =I=, 363, 676.

Sociology: environment and degree of life, =I=, 105–6; functional differentiation, =I=, 204; division of labour, =I=, 207, 363–4, 367; functional interdependence, =I=, 237–9, 240–2; autogenous development of units in colonies, =I=, 364, 367–8; =II=, 620; belief in social evolution, =I=, 432; natural selection, =I=, 553; =II=, 532; integration and differentiation, =II=, 378–9; effects of population, =II=, 535–6; equilibration, =II=, 537.

Soil, dependence of plant evolution on, =II=, 402.

Solanum jasminoides, organs of attachment, =II=, 276.

Solar system, autogenous development illustrated by distribution of forces in, =I=, 366.

Sole, symmetry and location of eyes, =II=, 205.

Soma-plasm, Weismann’s theory of differentiation from germ-plasm, =I=, 357, 622, 628–30, 633–44.

Somites (see Segmentation).

Special creation: and evolution, =I=, 412, 415, 431; improbabilities, =I=, 418–9, 430, 439, 554; inconceivability, =I=, 420, 429, 431, 554; of individuals and species, =I=, 421–4; the implication of beneficence, =I=, 425–9; summary, =I=, 429, 554; Von Baer’s formula, =I=, 451–6; vertebrate skeleton, =II=, 551, 556, 565.

Species: adaptation and stability, =I=, 242; hereditary transmission, =I=, 301–4; variation in wild and cultivated, =I=, 323–5, 326, 693; gamogenesis and life of, =I=, 347–9; physiological units, =I=, 362, 364, 369–71, 458; =II=, 613; indefiniteness, =I=, 389, 445, 572; special creation, =I=, 422–4; instability of homogeneous, and differentiation of, =I=, 509–11, 515, 517–8, 550, 557; persistence of, =I=, 516, 518; =II=, 10–11; natural selection and equilibration, =I=, 543–8, 553, 557; non-adaptive characters, =I=, 565; morbid products as marks of, =I=, 567; migration and isolation as causes of differentiation, =I=, 568–9; increasing multiformity of aggregate, =II=, 396.

Specific gravity, of organisms and environment, =I=, 174, 177.

Spermatozoa} Sperm-cell } (see Fertilization).

Sphere: tendency of units to form, =I=, 15; the embryonic form, =I=, 177; symmetry, =II=, 131.

Spheroid, symmetry, =II=, 132.

Spiders (see Arachnida).

Spine (see Vertebrata).

Sponge: structure and dynamic element in life, =I=, 119; multicentral development, =I=, 164; units and aggregate, =I=, 185; reproductive tissue, =I=, 283; integration, =I=, 586; =II=, 90, 383; physiological differentiation, =II=, 300, 386; development and genesis, =II=, 463; analogy from, =II=, 576.

Spontaneous generation: and heterogenesis, =I=, 270; and evolution, =I=, 696–701, 703.

Stag, horns and correlated structures, =I=, 567, 670, 676–7, 692.

Stamens, and foliar homology, =II=, 44.

Starches: properties, =I=, 11; transformations, =I=, 66, 68, 69, 70; =II=, 593.

Star-fishes (see Asteroidea).

Statoblasts, of Plumatella, =I=, 277.

Steenstrup, on “Alternate Generation,” =I=, 592.

Sterility (see Multiplication).

Stickleback: ova, =II=, 454; bothriocephalus in, =II=, 490.

Stomach (see Alimentary canal).

Stomata, distribution, =II=, 260–1.

Straight line, and evolution hypothesis, =I=, 433.

Strain: compression and tension of, =I=, 151; =II=, 209–12; relation to mass, =I=, 155–7; vegetal structure, =II=, 574–88, 592–6; origin of vertebrate type, =II=, 600.

Strawberry: multiaxial development, =I=, 166; multiplication, =II=, 441.

Strength, a vital attribute, =I=, 578.

Structure: appliances for generating motion, =I=, 75–7; biological classification, =I=, 125–7, 129; size and organic, =I=, 137; growth and complexity, =I=, 138, 145, 161; relation to environment, =I=, 172–8, 195–6; of unicellular organisms, =I=, 181–3; multicellular, =I=, 183–96; Hertwig’s classification of tissues, =I=, 189; continuity of units, =I=, 190–2; systems of organs, =I=, 192; division into universal and particular, =I=, 193–4; general truths, =I=, 194–5; plant and animal, contrasted, =I=, 195–6; precedence of function or, =I=, 197, 211; correlative complexity of function and, =I=, 200, 211; progressive concomitant differentiation, =I=, 201–4; physiological units, =I=, 225–6, 362, 364, 369–71; =II=, 613; social and organic interdependence, =I=, 235–42; varied by function, =I=, 334, 535; =II=, 217 (see Acquired Characters); zoological classification, =I=, 390–2; equilibration, =I=, 521, 557; progress of, and genesis, =I=, 590–1; =II=, 462; co-operation with function, =II=, 3; evolution and increased, =II=, 4; retrograde metamorphosis, =II=, 12; simulated homologies, =II=, 13–14; earliest organic forms, =II=, 19; cylindrical vegetal, =II=, 57–62; permanence and complexity, =II=, 295, 296; function and epidermic, =II=, 312–4, 387; and muscular, =II=, 369, 391; adaptation and equilibration, =II=, 392; persistence of force and physiological adaptation, =II=, 394; evolution, =II=, 501–4. (See also Morphology.)

Struggle, for nutriment among components of an organism, =I=, 562, 676; for existence (see Natural Selection).

Struthers, Sir J.: on heredity, =I=, 305, 314; digital variation, =I=, 321; rudimentary limbs of whale, =I=, 668.

Strychnine, effects of, =I=, 54, 55.

Sturgeon, size of ova and adult, =I=, 144.

Sugars: properties, =I=, 10–11; transformations, =I=, 38, 40, 66, 69, 70; =II=, 593.

Suicide, hereditary tendency to, =I=, 307.

Sulphur: allotropic, =I=, 4, 59; organic evolution, =I=, 703.

Sun (see Light).

Survival of the Fittest, the expression, =I=, 530, 610. (See Natural Selection.)

Swan, vertebræ of neck, =II=, 123.

Swiftness, a vital attribute, =I=, 578.

Syllis ramosa, lateral branching, =I=, 166, 361; =II=, 105, 108.

Symbiosis, =II=, 399, 400.

Symmetry (see Morphology).

Syphilis, hereditary transmission, =I=, 623.

Tactual, Perceptiveness, heredity and the distribution of, =I=, 602–8, 633, 665, 666, 672, 692.

Tænia (see Entozoa).

Tansley, A. G., =I=, vi; =II=, vi; adaptation of reproductive activity to conditions in Algæ, =I=, 288–9; shapes of Caulerpa, =II=, 22; stem-thickening in extinct Thallophytes, =II=, 56; natural selection and leaf-distribution, =II=, 179.

Tape-worm (see Entozoa).

Taste, dependent on chemical action, =I=, 54.

Teeth: hereditary transmission, =I=, 306; suppression of mammalian, =I=, 457; of uncivilized and civilized, =I=, 541, 693.

Tegumentary organs, origin of, =I=, 314–6.

Telegony, or the Influence of a previous sire on offspring, =I=, 624–7, 644–6, 649–50.

Temperature (see Heat).

Tension (see Strain).

Termites: fertility, =I=, 583; =II=, 493; late development of sexual organs, =I=, 680; nutrition and differentiation of forms, =I=, 681.

Tetramorium, utilization of aphides by, =I=, 660–1.

Thallophyta: size, =I=, 138, 139; low co-ordination of parts, =I=, 164; pseudo-foliar, =II=, 28; “transition place,” =II=, 30; simulation of higher types, =II=, 32; secondary thickening in extinct species, =II=, 56; sexual and asexual genesis, =II=, 84. (See also Algæ.)

Tickling, physiology of, =I=, 76.

Tide (see Sea).

Time, as a factor in growth, =II=, 77.

Tissue, Hertwig’s classification, =I=, 189. (See Physiology.)

Tongue, perceptiveness of tip, =I=, 606–8, 665, 672–3.

Tortoise: contrasted life of dog and, =I=, 103–4; natural selection and carapace, =I=, 534.

“Transcendental Physiology,” =I=, 176.

Tree, as symbolizing phylogeny, =I=, 428, 452–3. (See Plants.)

Trematoda: agamogenesis, =I=, 277; parasitism, =I=, 428; alternate generation, =I=, 592.

Trembley, A., on the polyp, =I=, 223.

Trichinosis, in Germany, =I=, 428.

Trochophore, phyletic relationships shown by, =I=, 447; =II=, 108–9.

Tubicolæ: development, =II=, 100; bilateral symmetry, =II=, 197.

Tunicata: gemmation, =I=, 588; =II=, 445; alternate generation, =I=, 592; integration, =II=, 93–4; tertiary aggregation, =II=, 124; symmetry, =II=, 194–5.

Tunny, size of ova and adult, =I=, 144.

Turbellaria: segmentation, =II=, 102; symbiosis, =II=, 400.

Turnip: chlorophyll in roots, =I=, 209; =II=, 254; vascular system, =II=, 281, 284, 578, 591, 596.

Twins: similarity of, =I=, 324; traits of women bearing, =II=, 457.

“Types, persistent,” Huxley on, =I=, 408.

Ulcer, dermal structure, =II=, 306.

Ultimate Reality, incomprehensibility of, =I=, 120.

Ulva: cell multiplication, =II=, 26; outer tissue, =II=, 256.

Umbelliferæ: floral symmetry, =II=, 171; axial and foliar organs, =II=, 541–6.

United States: cases of telegony, =I=, 644–5; birth-rate, =II=, 520.

Units: differentiation and dissimilarity, =I=, 20; “protyle,” =I=, 22–3; shapes in higher types, =I=, 164; differential assimilation, =I=, 180; primordial organic, =I=, 181; morphological composition, =I=, 184–7, 194, 252; =II=, 5, 7–9, 21, 79, 85–6; segregation and organic repair, =I=, 221–2, 222–6; chemical, morphological, and physiological, =I=, 225–6; =II=, 612; stability, =I=, 339; instability and heterogeneity of organic, =I=, 350; Darwin’s gemmules, =I=, 356–60, 362, 372; Weissmann’s germ-plasm (q. v.) ib.; sociological comparison, =I=, 363–8; specific proclivities in embryogeny, =I=, 458; phænogamic, =II=, 73, 151; annulose, =II=, 105; incident force and homologous, =II=, 159; morphological summary, =II=, 233. (See also Physiological Units.)

“Universal Postulate,” =I=, 675.

Unsymmetrical, definition, =II=, 131.

Urea, muscular energy and excretion, =I=, 72.

Van Beneden, P. J., on Tænia, =II=, 103.

Variation: digital, =I=, 331; effects of parental conditions, =I=, 324; of altered function, =I=, 325, 334, 693; dissimilarity of initial conditions, =I=, 327–32, 333; “spontaneous,” =I=, 328, 513, 697; =II=, 529; persistence of force, =I=, 335; physiological units, =I=, 348–54, 360, 369, 371–3; =II=, 614–7, 622–3; Weismann’s germ-plasm theory, =I=, 357–8, 372–3, 671, 677; =II=, 622; equilibration and vegetal, =I=, 523–5; Weismann’s panmixia theory, =I=, 561–3, 649, 667–9, 671, 685; reproductive organs, =I=, 570; natural selection and concomitant, =I=, 614–21, 653, 664, 674, 692; and disused organs, =I=, 648, 668; plus and minus. =I=, 667, 685; Masters on correlated, in plants, =II=, 298, 621–2; equilibration of favourable, =II=, 394.

Vascular System: effects of vegeto-alkalies, =I=, 55; nutrition, =I=, 146, 148; embryonic development, =I=, 169; structural traits, =I=, 192, 193; function, =I=, 199; of Ascidians. =I=, 202; functional differentiation and integration, =I=, 205–6; organic repair, =I=, 217, 221–2; effect of function, =I=, 229, 234–5, 236; equilibration, =I=, 535; community in compound organisms, =I=, 588; development of vegetal, =II=, 273–5, 279–84, 285–8, 388; differentiation of, summary, =II=, 288–90, 388; differentiation of animal, =II=, 339–44; osseous development, =II=, 347–51; muscularity, =II=, 364; muscular colour, =II=, 365–9; heart-motor apparatus, =II=, 374; differentiation and integration in animal, =II=, 376–9, 383; wood formation, =II=, 567–92; résumé of wood formation, =II=, 592–7.

Vaucheria, reproduction, =I=, 279, 289.

Vegetative System, co-ordination of actions in, =I=, 578.

Vegeto-alkalies, physiological effects of, =I=, 54–5.

Velocity, of moving bodies, =II=, 219–20.

Vertebrata: size, =I=, 139; size at birth and maturity, =I=, 144; axial structure, =I=, 165; embryonic development and self-mobility, =I=, 175; functional differentiation, =I=, 206, 591; reparative power, =I=, 219, 223, 589; homogenesis universal, =I=, 271; distinctive traits, =I=, 392; =II=, 35; distribution in time, =I=, 408; classificatory value, =I=, 446; embryonic mammalian respiratory system, =I=, 456; embryological pre-adaptation, =I=, 461; evolution and vertebral column, =I=, 470; rudimentary organs, =I=, 473; evolution and varied media, =I=, 479–85; size of head and vertebræ, =I=, 512, 537; segregation and evolution of vertebræ, =I=, 515; fertility and development, =I=, 583, 598–9; Weismann on reproductive cells, =I=, 635; limb locomotion, =II=, 15; adaptive segmentation, =II=, 117–23, 125–7, 223, 602, 605–7; supernumerary vertebræ, =II=, 123; bilateral symmetry, =II=, 203–6; internal organic symmetry, =II=, 208; genesis of rudimentary axis, =II=, 212–6; natural selection and genesis of structure, =II=, 216, 227; origin of notochord, =II=, 216–8; spinal segmentation, =II=, 218–22, 224; skull development, =II=, 222, 227; résumé of axis development, =II=, 224; Cope on author’s theory, =II=, 225–7; nerve differentiation, =II=, 304; sensory organs, =II=, 318; air-chambers, =II=, 334; osseous differentiation, =II=, 344–55; activity and muscular colour, =II=, 365–9; heart-motor apparatus, =II=, 374; cost of genesis, =II=, 436; agamogenesis unknown, =II=, 445; growth and genesis, =II=, 454; heat expenditure and genesis, =II=, 468–9, 474; Owen, theory of skeleton, =II=, 548–66; evolution of vertebræ, =II=, 563–6; origin of type, =II=, 598–600.

Vestiges of Creation, =I=, 491.

Vibrissæ, function of, =I=, 75.

Vitalism, hypothesis examined, =I=, 114–7.

Vittadini, C., on silkworm disease, =I=, 622–3.

Viviparons genesis, =I=, 271, 274–5, 278.

Voice, correlated sexual traits, =I=, 371–2.

Volcano, definition of life and, =I=, 85, 89.

Volvocineæ: unicentral development, =I=, 163; individuality, =I=, 245; disintegration of genesis, =I=, 276, 587; spherical aggregation, =II=, 24; symmetry, =II=, 137, 187; fertility, =II=, 441.

Vomiting, alimentary canal development, =II=, 328.

Vorticella: secondary aggregate, =II=, 90; symmetry, =II=, 188.

Wallace, A. R.: “The Origin of the Human Races,” =I=, 553; the expression “Survival of the Fittest,” =I=, 530; his association of natural with artificial selection, =I=, 609; co-adaptation in giraffe, =I=, 615; skin sensitiveness, =I=, 646.

Wasp: co-ordination of instincts in Mason-, =I=, 574, 679–80; genesis of worker, =I=, 654–7.

Waste, animal, =I=, 69, 213–5, 228; relation to activity, =I=, 196, 220–1; in plants, =I=, 213, 220.

Water: properties, =I=, 7, 9; colloidal affinity for, =I=, 28; organic change from, =I=, 29; organic need for, =I=, 147; proportion in mammalian adult and fœtus, =I=, 154; motion through, =I=, 156; organic development and environment, =I=, 173, 177, 479; terrestrial organisms inhabiting, =I=, 400; adaptation of organisms to change of media, =I=, 479–85; vegetal tissue differentiation, =II=, 253; molecular re-arrangement, =II=, 359; colloidal contraction, =II=, 361–2.

Water-weed, American, invasion of, =I=, 399.

Watts, Dr., on The Principles of Biology, =I=, ix.

Wax, foliar deposit, =II=, 260–1.

Weber, on tactual discriminativeness, =I=, 602.

Weight: relation to environment of organic, =I=, 174, 177; varying as cube of dimensions, =I=, 151; =II=, 434, 470.

Weismann, Aug.: reproductive tissue in Medusæ, =I=, 281; in Daphnidæ, =I=, 290; his theory of the differentiated germ-plasm and its fundamental units, =I=, 357, 622–3, 628–30, 633–44, 646; =II=, 618–9, 622; the alleged differentiation and plant-phenomena, =I=, 359–60; and regenerative processes, =I=, 360; false joints, =I=, 362; implied complexity of determinants, =I=, 370; theory inadequate to explain correlation of sexual traits, =I=, 372; and variations in peacock’s tail feather, =I=, 372–3, 695; =II=, 618; his view of natural selection as sole factor in organic evolution, =I=, 559; the doctrine of panmixia, =I=, 561–3, 612, 632, 649, 667–9, 671, 685, 689; arguments against inheritance of acquired characters, =I=, 612–3, 651–65, 669–71; blindness of cave-animals, =I=, 613; current acceptance of his views, =I=, 631, 690; cannot explain the process of natural selection, =I=, 651; the degradation of the little toe in man, =I=, 652, 669, 673; caste gradations of social insects, =I=, 654, 658–65, 670, 675, 678–84, 685; food-seeking instinct in Amazon ants, =I=, 660, 670; the co-adaptation of co-operative parts, =I=, 663–4, 670, 674, 675, 676; tactual discriminativeness, =I=, 665, 672; intra-selection, =I=, 676–8; effect of nutrition on fertility of blow-fly, =I=, 678–9.

Whale: weight of brain, =I=, 599; rudimentary limbs, =I=, 668–9, 685, 693.

Wheat, adaptive variations, =II=, 298.

Whistling, definition of life and, =I=, 112.

White-Cooper, Mr., on inheritance of abnormal vision, =I=, 306.

Willow, nutrition and growth, =I=, 294.

Wilson, E. B.: composition of chromatin, =I=, 260; separation of segmentation spheres of Amphioxus ovum, =I=, 691.

Wind: and vegetal bilateral symmetry, =II=, 142; and inner vegetal tissue differentiation, =II=, 275–9, 285, 288, 388; and proliferation of Bryophyllum, =II=, 295; and vegetal sap movement, =II=, 583, 584, 587; résumé, 592–6.

Wolff, C.: vegetal fructification and nutrition, =I=, 283; =II=, 179–80; vegetal vascular system, =II=, 283.

Women (see Man).

Wood (see Plants).

Yeast: fermentation, =I=, 38; fertility, =I=, 581; =II=, 440; linear aggregation, =I=, 587; =II=, 23.

Zebra marks in horses, =I=, 314.

Zoology, classification, =I=, 124–5, 380–9.

Zoophytes, structural indefiniteness, =I=, 173.

Zoospores, unit-life of, =I=, 185.

Zygote, of conjugating Algæ, =I=, 283.

THE END.

FOOTNOTES:

[1] It seems needful here to say, that allusion is made in this paragraph to a proposition respecting the ultimate natures of Evolution and Dissolution, which is contained in an essay on The Classification of the Sciences, published in March, 1864. When the opportunity comes, I hope to make the definition there arrived at, the basis of a re-organization of the second part of First Principles: giving to that work a higher development, and a greater cohesion, than it at present possesses. [The intention here indicated was duly carried out in 1867.]

[2] Let me here refer those who are interested in this question, to Prof. Huxley’s criticism on the cell-doctrine, published in the Medico-Chirurgical Review in 1853.

A critic who thinks the above statements are “rather misleading” admits that the lowest types of organisms yield them support, saying that “there are certainly masses of protoplasm containing many nuclei, but no trace of cellular structure, in both animals and plants. Such non-cellular masses may exist during development and later become separated up into cells, but there are certain low organisms in which such masses exist in the adult state. They are called by some botanists non-cellular, by others multi-nucleate cells. Clearly the difference lies in the criteria of a cell. There are also some Protozoa, and the Bacteria, in which no nucleus has certainly been demonstrated. But it is usual to consider the bodies of such organisms as cells nevertheless, and it is supposed that such cells represent a stage of development in which the nucleus has not yet been evolved, though the chemical substance ‘nuclein’ has been formed in some of them.”

Perhaps it will be most correct to say that, excluding the minute, non-nucleated organisms, all the higher organisms--Metazoa and Metaphyta--are composed throughout of cells, or of tissues originally cellular, or of materials which have in the course of development been derived from cells. It must, however, be borne in mind that, according to sundry leading biologists, cells in the strict sense are not the immediate products either of the primitive fissions or of subsequent fissions; but that the multiplying so-called cells are nucleated masses of protoplasm which remain connected by strands of protoplasm, and which acquire limiting membranes by a secondary process. So that, in the view of Mr. Adam Sedgwick and others, the substance of an organism is in fact a continuous mass of vacuolated protoplasm.

[3] In further illustration, Mr. Tansley names the fact that in the genus Caulerpa we have extremely complicated forms often of considerable size produced in the same way. The various species simulate very perfectly the members of different groups among the higher plants, such as Horse-tails, Mosses, Cactuses, Conifers and the like.

[4] It may be objected that in Cladophora the separate compartments of the thallus severally contain many nuclei, making it doubtful whether they descend from uni-nucleate cells. If, however, they do not they simply illustrate another form of integration.

[5] The great mass of early ancestral types--plant and animal--consisting of soft tissues, have left no remains whatever, and we have no reason to suppose that those which left remains fell within the direct ancestral lines of any existing forms. Contrariwise, we have reason to suppose that they fell within lines of evolution out of which the lines ending in existing forms diverged. We must therefore infer that the difficulties of affiliation which arise if we contemplate divergent types now existing, would not arise if we had before us all the early intermediate types. The Mammalia differ in sundry respects from all other kinds of Vertebrata--Fishes, Reptiles, Birds; and if the absence of hair, mammæ, and two occipital condyles, in these other vertebrates were taken to imply a fundamental distinction, it might, in the absence of any known fossil links, be inferred that the Mammalia belonged to a separate phylum. But these differences are not held to negative the assumed relationship. Similarly among plants. We must not reject an hypothesis respecting a certain supposed type, because the existing types it must have been akin to present traits which it could not have had. We are justified in assuming, within limits, a hypothetical type, unlike existing types in traits of some importance. Hence results the answer to a criticism passed on the above argument, that it implies relations between the undeveloped and developed forms of the Jungermanniaceæ such as the facts do not show us. This objection is met on remembering that the types in which the supposed transition took place disappeared myriads of years ago.

[6] There is much force in the criticism passed on the above paragraph, and by implication on some preceding paragraphs, that though in plants which tend to produce compound leaves the production is largely dependent on the supply of nutriment, yet the unqualified statement of this relation as a general one, is negatived by the existence of plants which bear only simple leaves, however much high nutrition causes growth. But mostly valid though this objection is, it is probably not universally valid. I am led to say this by what occasionally occurs in flowers. The flowering stem of the Hyacinth is single; but I have seen a cultivated Hyacinth in which one of the flowers had developed into a lateral spike. Still more striking evidence was once supplied to me by Agrimony. All samples of this plant previously seen had single flowering spikes, but some years ago I met with one, extremely luxuriant, in which some flowers of the primitive spike were replaced by lateral spikes; and I am not sure that some of these, again, did not bear lateral spikes. Now if in plants which, in probably millions of cases, have their flowering stems single, excessive nutrition changes certain of their flowers into new spikes, it is a reasonable supposition that in like manner plants which are thought invariably to bear only single leaves, will, under kindred conditions, bear compound leaves.

[7] See British and Foreign Medico-Chirurgical Review for January, 1862.

[8] Schleiden, who chooses to regard as an axis that which Mr. Berkeley, with more obvious truth, calls a mid-rib, says:--“The flat stem of the Liverworts presents many varieties, consisting frequently of one simple layer of thin-walled cells, or it exhibits in its axis the elements of the ordinary stem.” This passage exemplifies the wholly gratuitous hypotheses which men will sometimes espouse, to escape hypotheses they dislike. Schleiden, with the positiveness characteristic of him, asserts the primordial distinction between axial organs and foliar organs. In the higher Archegoniates he sees an undeniable stem. In the lower Archegoniates, clearly allied to them by their fructification, there is no structure having the remotest resemblance to a stem. But to save his hypothesis, Schleiden calls that “a flat stem,” which is obviously a structure in which stem and leaf are not differentiated. He is the more to be blamed for this unphilosophical assumption, since he is merciless in his strictures on the unphilosophical assumptions of other botanists.

[9] To this interpretation it is objected that “the more-developed Jungermanniaceæ” do not appear to have arisen from the lower forms of Jungermanniaceæ--that is to say, from such lower forms as are now existing. It may, however, be contended that this fact does not exclude the interpretation given; since the higher forms may well have been evolved, not from any of the lower forms we now know, but from lower forms which have become extinct. This, indeed, is the implication of the evolutionary process as pointed out in the note to Chap. I. If then we assume some early type of intermediate structure, the explanation may not improbably hold.

[10] I am indebted to Dr. Hooker for pointing out further facts supporting this view. In his Flora Antarctica, he describes the genus Lessonia (see Fig. 37), and especially L. ovata, as having a mode of growth simulating that of the dicotyledonous trees, not only in general form but in internal structure. The tall vertical stem thickens as it grows, by the periodical addition of layers to its periphery. That even Thallophytes should thus, under certain conditions, present a transversely-increasing axis, shows that there is nothing absolutely characteristic of Phanerogams in their habit of stem-thickening. Mr. Tansley gives me further verification by the statement that “it is also now certain that members of the Equisetineæ and Lycopodineæ, as well as some Ferns which flourished in Carboniferous times, had secondary thickening in their stems quite comparable to that of modern Dicotyledonous trees.”

[11] See note at the end of the chapter.

[12] Since this paragraph was put in type [this refers to the first edition], I have observed that in some varieties of Cineraria, as probably in other plants, a single individual furnishes all these forms of leaves--all gradations between unstipulated leaves on long petioles, and leaves that embrace the axis. It may be added that the distribution of these various forms is quite in harmony with the rationale above given.

[13] Since these figures were put on the block, it has occurred to me that the relations would be still clearer, were the primary frond represented as not taking part in these processes of modification, which have been described as giving rise to the erect form; as, indeed, the rooting of its under surface will prevent it from doing in any considerable degree. In such case, each of the Figs. 111 to 117, should have a horizontal rooted frond at its base, homologous with the pro-embryo among Acrogens. This primary frond would then more manifestly stand in the same relation to the rest, as the cotyledon does to the plumule--both by position, and as a supplier of nutriment. Fig. 117_a_, which I am enabled to add, shows that this would complete the interpretation. Of the dicotyledonous series, it is needful to add no further explanation than that the difference in habit of growth, will permit the second frond to root itself as well as the first; and so to become an additional source of nutriment, similarly circumstanced to the first and equal with it.

[14] How the element of time modifies the result, is shown by the familiar fact that crystals rapidly formed are small, and become relatively large when left to form more slowly. If the quantity of molecules contained in a solution is relatively great, so that the mutual polarities of the molecules crowded together in every place throughout the solution are intense, there arises a crystalline aggregation around local axes; whereas, in proportion as the local action of molecules on one another is rendered less intense by their wider dispersion, they become relatively more subordinate to the forces exerted on them by the larger aggregates of molecules that are at greater distances, and thus are left to arrange themselves round fewer axes into larger crystals.

[15] It is objected that these transformations should be much commoner than they are, were they caused solely by the variations of nutrition described. The reply is that they are comparatively rare in uncultivated plants, where such variations are not frequent. The occurrence of them is chiefly among cultivated plants which, being artificially manured, are specially liable to immense accessions of nutriment, caused now by sudden supplies of fertilizing matters, and now by sudden arrival of the roots at such matters already deposited in the soil. It is to these great changes of nutrition, especially apt to take place in gardens, that these monstrosities are ascribed; and it seems to me that they are as frequent as may be expected.

[16] Since this paragraph was published in 1865, much has been learned concerning cell-structure, as is shown in Chapter VI^A of Part I. While some assert that there exist portions of living protoplasm without nuclei, others assert that a nucleus is in every case present, and that where it does not exist in a definite aggregated form it exists in a dispersed form. As remarked in the chapter named, “the evidence is somewhat strained to justify this dogma.” Words are taken in their non-natural senses, if one which connotes an individualized body is applied to the widely-diffused components of such a body; and this perverting of proper meanings leads to obscuration of what may perhaps be an essential truth. As argued in the chapter named (§§ 74_e_, 74_f_), nuclear matter is, as shown by its chemical character, an extremely unstable substance, the molecular changes of which, perpetually going on, initiate shocks, producing changes all around. In the earlier stages of cell-evolution this unstable substance is dispersed throughout the cytoplasm; whereas in the more advanced stages it is gathered together in one mass. If so, instead of saying there is a dispersed nucleus we should say there are the materials of a nucleus not yet integrated.

[17] This statement seems at variance with the figure; but the figure is very inaccurate. Its inaccuracy curiously illustrates the vitiation of evidence. When I saw the drawing on the block, I pointed out to the draughtsman, that he had made the surrounding curves much more obviously related to the contained bodies, than they were in the original (in Dr. Carpenter’s Foraminifera); and having looked on while he in great measure remedied this defect, thought no further care was needed. Now, however, on seeing the figure in the printer’s proof, I find that the engraver, swayed by the same supposition as the draughtsman that such a relation was meant to be shown, has made his lines represent it still more decidedly than those of the draughtsman before they were corrected. Thus, vague linear representations, like vague verbal ones, are apt to grow more definite when repeated. Hypothesis warps perceptions as it warps thoughts.

[18] Though the subdivision into chambers of the shell does not correspond to the subdivision into cell-units it may still be held that since in the solitary types the subdivision of the nucleus is followed by formation of new individuals which separate, and since in the compound types the subdivision of the nucleus is followed by growth and formation of new chambers, the compound type must be regarded as an aggregate of the second order.

[19] A critic says the question is “what are the forces internal or external which produce union or separation.” A proximate reply is--degree of nutrition. As in a plant new individuals or rudiments of them are cast off where nutrition is failing, so in a compound animal. The connecting part dwindles if it ceases to carry nutriment.

[20] It has been pointed out that I have here understated the evidence of physiological integration. An instance of it among Hydrozoa is shown in Fig. 151, but by a strange oversight I have forgotten to name the various cases furnished by the Siphonophora in which the individual polypes of a compound aggregate are greatly specialized in adaptation to different functions.

[21] Recently Mr. T. H. Morgan has made elaborate experiments which show that Planaria Maculata may be cut into many pieces from various parts and of various shapes--even a slice out of the side--and each, if not too small, will produce a perfect animal.

[22] Since this was written in 1865 there has come to light evidence more completely to the point than any at that time known. In the subdivision of Platyhelminthes known as Turbellaria, there are some, the Microstomida which, by a process of segmentation form “chains of 4, then 8, then 16, and sometimes even 32 individuals.” “Each forms a mouth [lateral] and for some time the chain persists, but the individuals ultimately become sexually matured and then separate.” (Shipley, Zoology of the Invertebrata, p. 92.) Here it should be remarked that the lateral mouths enable the members of a string to feed separately, and that nutrition not being interfered with they doubtless gain some advantage by temporary maintenance of their union--probably in creeping.

[23] I find that the reasons for regarding the segment of a Tænia as answering to an individual of the second order of aggregation, are much stronger than I supposed when writing the above. Van Beneden says:--“Le Proglottis (segment) ayant acquis tout son développement, se détache ordinairement de la colonie et continue encore à croître dans l’intestin du même animal; il change même souvent de forme et semble doué d’une nouvelle vie; ses angles s’effacent, tout le corps s’arrondit, et il nage comme une Planaire au milieu des muscosités intestinales.”

[24] Though this was doubtful in 1865 it is no longer doubtful. In an individual Ctenodrilus monostylus, which multiplies by dividing and subdividing itself, “parts arise which are destitute of both head and anus and at times consist of only a single segment.” In another species, C. pardalis, there is separation into many segments; and each segment before separating forms a budding zone out of which other segments are afterwards produced, completing the animal (Korschelt and Heider, Embryology, i, 301–2).

[25] In place of those originally here instanced about which there are disputes, I may give an undoubted one described by McIntosh, the Syllis ramosa, a species of chætopod living in hexactinellid sponges from the Arafura Sea, which branches laterally repeatedly so as to extend in all directions through the canals of the sponge. In most cases the buds terminate in oval segments with two long cirri each. But male and female buds were found, provided each with a head, and containing ovaries and testes. Sometimes these sexual buds had become separate from the branched stock.

[26] The name Annulosa, once used to embrace the Annelida and Arthropoda, has of late ceased to be used. It seems to me better than Appendiculata, both as being more obviously descriptive and as being more exclusive.

[27] The fusion of the segments forming the Arthropod head and the extreme changes, or perhaps in some cases disappearances, of their appendages, put great difficulties in the way of identification; so that there are differences of opinion respecting the number of included segments. Prof. MacBride writes:--“It is highly probable that a primary head (præoral lobe or præstomium) has been derived from annelid ancestors, but the secondary fusion of body-segments with this head, in other words the formation of a secondary head, has gone on independently in the different classes of the phylum Arthropoda, viz., Arachnida, Crustacea, and Tracheata (including Insects and Myriapods). Judged by the number of appendages (which gives an inferior limit) the head of a malacostracous Crustacean consists of præstomium and 8 segments; the head of an insect of præstomium and 4 segments; the head of a Myriapod of præstomium and 3 segments; and the head of an Arachnid of præstomium and 3 segments.” Again, the comment of Mr. J. T. Cunningham is:--“According to Claus and most modern authorities there are only 5 segments in the head of an Arthropod, the eyes not counting as appendages; and further it should be noted that the second pair of antennæ are wanting in Insects.”

Of course difference of opinion respecting the number of somites in the head involves difference of opinion respecting the number constituting the entire body, which, in the higher Arthropods, is said by some to be 19 and by others 20. But those who thus differ in detail, agree in regarding all the segments of head and body as homologous, and this is the essential point with which we are here concerned.

[28] Prof. MacBride corrects this statement by saying that “The ctenidia or gills (which in Mollusca generally are represented only by a single pair) are here represented by a large number of pairs; they do not, however, correspond in either number or position to the shell plates.” It may, I think, be contended that if these had any morphological significance, they would not differ in arrangement from the shell plates, and would not be limited to this special type of Mollusc.

[29] Though it is alleged that at a later stage the posterior part of the skull is formed by fusion of divisions which are assumed to represent vertebræ, yet it is admitted that the anterior part of the skull never shows any signs of such division. Moreover in both parts the bones show no trace of primitive segmentation.

[30] See note at the end of the chapter.

[31] A qualifying fact should be named. When the production of vertebral segments has become constitutionally established, so that there is an innate tendency to form them, there arises a liability to form supernumerary ones; and this, from time to time recurring, may lengthen the series, as in the body of a snake or the neck of a swan. This qualification, however, affects equally the hypothesis of an ideal type and the hypothesis of mechanical genesis.

[32] Here and throughout, the word radial is applied equally to the spiral and the whorled structures. These, as being alike on all sides, are similarly distinguished from arrangements that are alike on two sides only.

[33] It should be added that this change of distribution is not due to change in the relative positions of the insertions of the leaves but to their twistings.

[34] We may note that some of these leaves, as those of the Lime, furnish indications of the ratio which exists between the effects of individual circumstances and those of typical tendencies. On the one hand, the leaves borne by these drooping branches of the Lime are with hardly an exception unsymmetrical more or less decidedly, even in positions where the causes of unsymmetry are not in action: a fact showing us the repetition of the type irrespective of the conditions. On the other hand, the degree of deviation from symmetry is extremely variable, even on the same shoot: a fact proving that the circumstances of the individual leaf are influential in modifying its form. But the most striking evidence of this direct modification is afforded by the suckers of the Lime. Growing, as these do, in approximately upright attitudes, the leaves they bear do not stand to one another in the way above described, and the causes of unsymmetry are not in action; and here, though there is a general leaning to the unsymmetrical form, a large proportion of the leaves become quite symmetrical.

[35] It was by an observation on the forms of leaves, that I was first led to the views set forth in the preceding and succeeding chapters on the morphological differentiation of plants and animals. In the year 1851, during a country ramble in which the structures of plants had been a topic of conversation with a friend--Mr. G. H. Lewes--I happened to pick up the leaf of a buttercup, and, drawing it by its foot-stalk through my fingers so as to thrust together its deeply-cleft divisions, observed that its palmate and almost radial form was changed into a bilateral one; and that were the divisions to grow together in this new position, an ordinary bilateral leaf would result. Joining this observation with the familiar fact that leaves, in common with the larger members of plants, habitually turn themselves to the light, it occurred to me that a natural change in the circumstances of the leaf might readily cause such a modification of form as that which I had produced artificially. If, as they often do with plants, soil and climate were greatly to change the habit of the buttercup, making it branched and shrub-like; and if these palmate leaves were thus much overshadowed by one another; would not the inner segments of the leaves grow towards the periphery of the plant where the light was greatest, and so change the palmate form into a more decidedly bilateral form? Immediately I began to look round for evidence of the relation between the forms of leaves and the general characters of the plants they belong to; and soon found some signs of connexion. Certain anomalies, or seeming anomalies, however, prevented me from then pursuing the inquiry much further. But consideration cleared up these difficulties; and the idea afterwards widened into the general doctrine here elaborated. Occupation with other things prevented me from giving expression to this general doctrine until Jan. 1859; when I published an outline of it in the Medico-Chirugical Review.

[36] It is objected to the above interpretation that “many flowers of sizes intermediate between the Hollyhock and the Agrimony are radially symmetrical and yet grow sideways. I may mention various Liliaceæ, e.g. Chlorophytum, Eucomis, Muscari, Anthericum. Sagittaria, also, has many of its flowers in this position. Further, if the higher insects alight on flowers in a definite way, as they do, the parts of the flower must bear different relations to the visiting insect, however large, so that flowers unvisited ought all to be zygomorphic.” My reply is that in the sense which here concerns us, the different petals of the Hollyhock-flower do not bear different relations to the visiting insect; since, practically, the upper and lateral petals bear no physical relations at all: in so far as the visiting bee is concerned they are non-existent. The argument implies that change in the form of a flower from the radial to the bilateral is likely to take place only when the contact-relations of the petals to the visiting insect, are such as to make some forms facilitate its action more than others; and the large petals of the Hollyhock cannot facilitate its action at all. In respect of the Liliaceæ instanced, it is needful to inquire whether the structures are such that this alleged cause of bilateral symmetry can come into play.

[37] I had intended here to insert a figure exhibiting these differences; but as the Cow-parsnip does not flower till July, and as I can find no drawing of the umbel which adequately represents its details, I am obliged to take another instance.

[38] It has been pointed out to me that “the extreme development of the corolla so often found in the outer flowers or on the outer side of the outer flowers in closely-packed inflorescences, associated as it often is with disappearance of stamens or carpels or both, is usually put down to specialization of these outer flowers for attractive purposes. Since the whole inflorescence is increased in conspicuousness by such a modification, it is supposed that natural selection favoured those plants which sacrificed a portion of their seed-bearing capacity for the supposed greater advantage of securing more insect visits.” But granting this interpretation, it may still be held that increase of attractiveness due to increase of area must be achieved by florets at the periphery, and that their ability to achieve it depends on their having an outer, unoccupied, space which the inner florets have not; so that, though in a more indirect way, their different development is determined by different exposure to conditions.

[39] One of my critics writes:--“This chapter might of course be enormously extended, not only as in the preceding ones by citation of quite similar cases, but by the introduction of fresh groups of cases.”

[40] Natural selection may have operated in establishing a constitutional tendency to other sudden abridgments. Mr. Tansley alleges that this is a part-cause of the varying distribution of leaves. He says:--“I have myself made some observations on the length of internodes in the Beech, and am satisfied that it follows quite other laws, connected with the suitable disposition of the leaves on the branch. Although I have not had the opportunity of following up this line of work so as in any way to generalize the results, I suspect that ‘indirect equilibration’ is a widespread cause of such variation.”

[41] It is but just to the memory of Wolff, here to point out that he was immensely in advance of Goethe in his rationale of these metamorphoses. Whatever greater elaboration Goethe gave to the theory considered as an induction, seems to me more than counter-balanced by the irrationality of his deductive interpretation; which unites mediæval physiology with Platonic philosophy. A dominant idea with him is that leaves exist for the purpose of carrying off crude juices--that “as long as there are crude juices to be carried off, the plant must be provided with organs competent to effect the task”; that while “the less pure fluids are got rid of, purer ones are introduced” and that “if nourishment is withheld, that operation of nature (flowering) is facilitated and hastened; the organs of the nodes (leaves) become more refined in texture, the action of the purified juices becomes stronger, and the transformation of parts having now become possible, takes place without delay.” This being the proximate explanation, the ultimate explanation is, that Nature wishes to form flowers--that when a plant flowers it “attains the end prescribed to it by nature”; and that so “Nature at length attains her object.” Instead of vitiating his induction by a teleology that is as unwarranted in its assigned object as in its assigned means, Wolff ascribes the phenomena to a cause which, whether sufficient or not, is strictly scientific in its character. Variation of nutrition is unquestionably a “true cause” of variation in plant-structure. We have here no imaginary action of a fictitious agency; but an ascertained action of a known agency.

[42] The Natural History Review for July, 1865, contained an article on the doctrine of morphological composition set forth in the foregoing Chaps. I. to III. In this article, which unites exposition and criticism in a way that is unhappily not common with reviewers, it is suggested that the spiral structure may be caused by natural selection. When this article appeared, the foregoing five pages were standing over in type, as surplus from No. 14, issued in June, 1865.

[43] A verifying comment on this paragraph runs as follows:--“In the Hypotricha Infusoria, which creep over solid surfaces, there is a differentiation between ventral and dorsal surface and an approach to bilateral symmetry. The ventral surface is provided with movable cilia, the dorsal with immobile setæ.”

[44] Criticisms on the above passage have shown the need for naming sundry complications. These complications chiefly, if not wholly, arise from changes in modes of life--changes from the locomotive to the stationary, and from the stationary to the locomotive. Referring to my statement that (ignoring the spherical) the radial type is the lowest and must be taken as antecedent to the bilateral type, it is alleged that all existing “radial animals above Protozoa are probably derived from free-swimming, bilaterally-symmetrical animals.” If this is intended to include the planulæ of the hydroid polyps, then it seems rather a straining of the evidence. These locomotive embryos, described as severally having the structure of a gastrula with a closed mouth, can be said to show bilateralness only because the first two tentacles make their appearance on opposite sides of the mouth--a bilateralness which lasts only till two other tentacles make their appearance in a plane at right angles, so giving the radial structure. I think the criticism applies only to cases furnished by Echinoderms. The larvæ of these creatures have bilaterally-symmetrical structures, which they retain as long as they swim about and which such of them as fix themselves lose by becoming similarly related to conditions all round: the radial structure being retained by those types which, becoming subsequently detached, move about miscellaneously. But, as happens in some of the Sea-urchins and still more among the Holothurians, the structure is again made bilaterally-symmetrical by a locomotive life pursued with one end foremost. Should it be contended that the conditions and the forms are reciprocally influential--that either may initiate the other, it still remains unquestionable that ordinarily the conditions are the antecedents, as is so abundantly shown by plants.

[45] Should it be proved that the Ascidian is a degraded vertebrate, then the argument will be strengthened; since loss of bilateral symmetry has gone along with change to asymmetrical conditions.

[46] A critical comment made on this sentence runs as follows:--“The aërial roots of most epiphytic orchids contain chlorophyll in their cortex throughout their length, but the cortex being covered by a ‘velamen’ of air-containing cells which break up and reflect incident light, the green colour is not visible through this opaque coat. When moistened the cells of the velamen take up water and the green colour immediately shows through. Such roots do not however possess stomata. The roots of certain species of Angræcum, however, contain the whole of the assimilating tissue of the plant.”

[47] The current doctrine that chlorophyll is the special substance concerned in vegetal assimilation, either as an agent or as an incidental product, must be taken with considerable qualification. Besides the fact that among the Algæ there are many red and brown kinds which thrive; and besides the fact that among the lower Archegoniates there are species which are purple or chocolate-coloured; there is the fact that Phænogams are not all green. We have the Copper-Beech, we have the black-purple Coleus Verschaffeltii, and we have the red variety of Cabbage, which seems to flourish as well as the other varieties. Chlorophyll, then, must be regarded simply as the most general of the colouring matters found in those parts of plants in which assimilation is being effected by the agency of light. Though it is always present along with the red and brown pigments, yet there is much evidence to show that these are the actual assimilative pigments.

[48] This seems as fit a place as any for noting the fact, that the greater part of what we call beauty in the organic world, is in some way dependent on the sexual relation. It is not only so with the colours and odours of flowers. It is so, too, with the brilliant plumage of birds; and it is probable that the colours of the more conspicuous insects are in part similarly determined. The remarkable circumstance is, that these characteristics, which have originated by furthering the production of the best offspring, while they are naturally those which render the organisms possessing them attractive to one another, directly or indirectly, should also be those which are so generally attractive to us--those without which the fields and woods would lose half their charm. It is interesting, too, to observe how the conception of human beauty is in a considerable degree thus originated. And the trite observation that the element of beauty which grows out of the sexual relation is so predominant in æsthetic products--in music, in the drama, in fiction, in poetry--gains a new meaning when we see how deep down in organic nature this connexion extends.

[49] Students of vegetal physiology, familiar with the controversies respecting sundry points dealt with in this chapter, will probably be surprised to find taken for granted in it, propositions which they have habitually regarded as open to doubt. Hence it seems needful to say that the conclusions here set forth, have resulted from investigations undertaken for the purpose of forming opinions on several unsettled questions which I had to treat, but which I could find in books no adequate data for treating. The details of these investigations, and the entire argument of which this chapter is partly an abstract, will be found in Appendix C.

[50] To this implied inference it is objected that “excess of nutritive material does not necessarily lead to correspondingly increased growth.” My reply is that a concomitant factor is activity of the tissue, and that in its absence growth is not to be expected.

[51] In recent years (since 1890) Prof. Wilhelm Roux, in essays on functional adaptation, has set forth some views akin to the foregoing in respect to the general belief they imply, though differing in respect of the physiological processes he indicates. The following relevant passage has been translated for me from an article of his in the Real-Encyclopädie der gesammten Heilkunde:--“A more complete theory of functional adaptation by the author is founded on the assumption that the ‘functional’ stimulus, or ‘the act of exercising the function’ (in muscles and glands), and especially, in the case of bones, the concussion and tension caused by stress and strain, exert a ‘trophic’ stimulus on the cells, in consequence of which, and along with an increased absorption of nutriment, they grow and eventually increase (or the osteoblasts at the point of greater stimulus form more bone); while, conversely, with continued inactivity, by absence of these stimuli the nourishment of the cell declines so that the waste is insufficiently replaced (or otherwise that the bone-substance gradually loses its power of resistance to the osteoblasts formed as a result of inactivity”).

[52] An outline of the doctrine set forth in the following chapters, was originally published in the Westminster Review for April, 1852, under the title--A Theory of Population deduced from the General Law of Animal Fertility; and was shortly afterwards republished with a prefatory note stating that it must be accepted as a sketch which I hoped at some future time to elaborate. In now revising and completing it, I have omitted a non-essential part of the argument, while I have expanded the remainder by adding to the number of facts put in evidence, by meeting objections which want of space before obliged me to pass over, and by drawing various secondary conclusions. The original paper, with omissions, will be found in Appendix A to Volume I of this work.

[53] I was here thinking only of the cases which are general among insects, but it seems that vertebrate animals, too, furnish cases. Mr. Cunningham writes:--“There is a curious instance of this in the Conger: the female grows to 6 or 7 feet long and a weight of 60 lbs. and upwards and then ceases to feed for 6 months while the eggs develop, and when the eggs are shed dies.”

[54] I say “normal” for the purpose of excluding not only morbid growths but excess of fat.

[55] To meet a possible criticism it should be remarked that this calculation assumes that the power of asexual reproduction is not exhausted by the end of the month. It has been found that “the successive fissions of Paramœcium cannot continue indefinitely. After some hundreds of generations the products of fission are small, have no mouth, and die unless before this they have been allowed to conjugate with individuals of another brood.” It may, however, be fairly taken for granted that “some hundreds of generations” would take longer than a month.

[56] Even this number is far exceeded. Dr. Edward Klein, in a lecture he gave at the Royal Institution on June 2, 1898, asserted that 246 bacteria in a cubic centimetre of nutritive liquid would multiply to 20,000,000 in the course of twenty-four hours: a rate which, at the end of the third day, would give, as the offspring of one individual, 537,367,797,000,000.

[57] It has since been shown that in Myrianida fasciata as many as 29 attached groups exist. See Cambridge Natural History, Vol. II, Worms, Rotifers and Polyzoa, p. 280.

[58] To this passage Prof. MacBride appends the remark:--“This is quite proven now, and the statement as it stands is quite correct; but far better and more minutely worked out cases are to be found amongst the Infusoria. In Paramœcium for example, there are normally present a large macronucleus and a small micronucleus lying alongside of it. When two individuals adhere preparatory to conjugation, the macronucleus breaks up into fragments which are absorbed: the micronucleus--which has some time previously divided into two--begins to break up further and eventually forms eight bodies; all of these except one disappear; this last piece then divides into two; of these two one represents a male genital cell, for it passes over into the body of the other Paramœcium and fuses with one of the two corresponding nuclei there; thus each of the two individuals which adhere fertilizes the other. The two individuals then separate and the nucleus (result of fusion of male and female nuclei) in each divides into four. Of these, two move to one end of the animal and two to the other. The animal then divides into two transversely--each of the products thus having two nuclei which form the micro-and macronucleus of it. Thus it appears that the function of sexual union is simply to give increased vigour to all the vital processes including fission. Since as mentioned above (p. 443) if it is prevented, the products of fission are eventually unable to feed themselves.”

[59] A passage translated for me from the German may be here given in verification. Dr. Dionys Hellin in an essay on the origin of Multiparity and Twin-births, refers to the thesis above set forth, and says that “the fact that it is generally women of small growth who bear twins is in complete agreement with it.” He adds that “Puech is right in his opinion that twin pregnancies are a direct result of relatively large ovaries (i.e., in comparison with the whole body). He has observed that for the same size of body the ovarium of a pluriparous animal is always of greater volume than that of a uniparous animal ... a sow has ovaries as large as a cow’s; but while the latter bears only one calf [at a time], the sow brings forth 6–15 at each litter. Even in animals of the same species but belonging to different races these relations may be verified,” e.g., Barbary sheep and ordinary sheep.

[60] When, after having held for some years the general doctrine elaborated in these chapters, I agreed, early in 1852, to prepare an outline of it for the Westminster Review, I consulted, among other works, the just-issued third edition of Dr. Carpenter’s Principles of Physiology, General and Comparative--seeking in it for facts illustrating the different degrees of fertility of different organisms, I met with a passage, quoted above in § 339, which seemed tacitly to assert that individual aggrandizement is at variance with the propagation of the race; but nowhere found a distinct enunciation of this truth. I did not then read the Chapter entitled “General View of the Functions,” which held out no promise of such evidence as I was looking for. But on since referring to this chapter, I discovered in it the definite statement that--“there is a certain degree of antagonism between the Nutritive and Reproductive functions, the one being executed at the expense of the other. The reproductive apparatus derives the materials of its operations through the nutritive system, and is entirely dependent upon it for the continuance of its function. If, therefore, it be in a state of excessive activity, it will necessarily draw off from the individual fabric some portion of the aliment destined for its maintenance. It may be universally observed that, when the nutritive functions are particularly active in supporting the individual, the reproductive system is in a corresponding degree undeveloped,--and vice versâ.” P. 592.

[61] The climate, the locality, and the kind of food, are of course all factors; and hence, probably, the differences between the statements of different authorities concerning these several cases. Prof. MacBride writes:--

“According to Flower (Mammals, Living and Extinct) the Ferret is a domesticated variety of the common polecat, which has 3 to 8 young. Darwin (Animals and Plants) says that the wild sow often breeds twice a year and produces a litter of 4 to 8, and sometimes even 12. The domestic sow breeds twice and would breed oftener if permitted, and if any good at all produces 8 in litter.”

[62] It is worth while inquiring whether unfitness of the food given to them, is not the chief cause of that sterility which, as Mr. Darwin says, “is the great bar to the domestication of animals.” He remarks that “when animals and plants are removed from their natural conditions, they are extremely liable to have their reproductive systems seriously affected.” Possibly the relative or absolute arrest of genesis, is less due to a direct effect on the reproductive system, than to a changed nutrition of which the reproductive system most clearly shows the results. The matters required for forming an embryo are in a greater proportion nitrogenous than are the matters required for maintaining an adult. Hence, an animal forced to live on insufficiently-nitrogenized food, may have its surplus for reproduction cut off, but still have a sufficiency to keep its own tissues in repair, and appear to be in good health--meanwhile increasing in bulk from excess of the non-nitrogenous matters it eats.

[63] Huxley, Anatomy of Invertebrated Animals, p. 274.

[64] Shipley, Zoology of Invertebrata, p. 112.

[65] I am told that “Wagner, who described the larva, found that it bored into the bark of trees. It attacks also the wheat plant, and is a most destructive parasite.” Apparently this statement is at variance with the foregoing inference. It is clear, however, that since these heaps of nitrogenous refuse in which it has been found are artificial and recent, they cannot be its natural habitats; and it seems not improbable that these larvæ, suddenly supplied with a more nutritive food in unlimited amount, may have as a consequence acquired this habit of agamogenetic multiplication which did not characterize the species under its natural conditions and relatively low nutrition.

[66] This is exactly the reverse of Mr. Doubleday’s doctrine; which is that throughout both the animal and vegetable kingdoms, “over-feeding checks increase; whilst, on the other hand, a limited or deficient nutriment stimulates and adds to it.” Or, as he elsewhere says--“Be the range of the natural power to increase in any species what it may, the plethoric state invariably checks it, and the deplethoric state invariably develops it; and this happens in the exact ratio of the intensity and completeness of each state, until each state be carried so far as to bring about the actual death of the animal or plant itself.”

I have space here only to indicate the misinterpretations on which Mr. Doubleday has based his argument.

In the first place, he has confounded normal plethora with what I have, in § 355, distinguished as abnormal plethora. The cases of infertility accompanying fatness, which he cites in proof that over-feeding checks increase, are not cases of high nutrition properly so-called; but cases of such defective absorption or assimilation as constitutes low nutrition. In Chap. IX, abundant proof was given that a truly plethoric state is an unusually fertile state. It may be added that much of the evidence by which Mr. Doubleday seeks to show that among men, highly-fed classes are infertile classes, may be out-balanced by counter-evidence. Many years ago Mr. G. H. Lewes pointed this out: extracting from a book on the peerage, the names of 16 peers who had, at that time, 186 children; giving an average of 11·6 in a family.

Mr. Doubleday insists much on the support given to his theory by the barrenness of very luxuriant plants, and the fruitfulness produced in plants by depletion. Had he been aware that the change from barrenness to fruitfulness in plants, is a change from agamogenesis to gamogenesis--had it been as well known at the time when he wrote as it is now, that a tree which goes on putting out sexless shoots, is thus producing new individuals; and that when it begins to bear fruit, it simply begins to produce new individuals after another manner--he would have perceived that facts of this class do not tell in his favour.

In the law which Mr. Doubleday alleges, he sees a guarantee for the maintenance of species. He argues that the plethoric state of the individuals constituting any race of organisms, presupposes conditions so favourable to life that the race can be in no danger; and that rapidity of multiplication becomes needless. Conversely, he argues that a deplethoric state implies unfavourable conditions--implies, consequently, unusual mortality; that is--implies a necessity for increased fertility to prevent the race from dying out. It may be readily shown, however, that such an arrangement would be the reverse of self-adjusting. Suppose a species, too numerous for its food, to be in the resulting deplethoric state. It will, according to Mr. Doubleday, become unusually fertile; and the next generation will be more numerous rather than less numerous. For, by the hypothesis, the unusual fertility due to the deplethoric state, is the cause of undue increase of population. But if the next generation is more numerous while the supply of food has not increased in proportion, then this next generation will be in a still more deplethoric state, and will be still more fertile. Thus there will go on an ever-increasing rate of multiplication, and an ever-decreasing share of food, for each person, until the species disappears. Suppose, on the other hand, the members of a species to be in an unusually plethoric state. Their rate of multiplication, ordinarily sufficient to maintain their numbers, will become insufficient to maintain their numbers. In the next generation, therefore, there will be fewer to eat the already abundant food, which becoming relatively still more abundant, will render the fewer members of the species still more plethoric, and still less fertile, than their parents. And the actions and reactions continuing, the species will presently die out from absolute barrenness.

[67] A good deal of this chapter retains its original form; and the above paragraph is reprinted verbatim from the Westminster Review for April, 1852, in which the views developed in the foregoing hundred pages were first sketched out. This paragraph shows how near one may be to a great generalization without seeing it. Though the struggle for life is the alleged motive force; though the process of natural selection is recognized; and though to it is ascribed a share in the evolution of a higher type; yet the conception is not that which Mr. Darwin has worked out with such wonderful skill and knowledge. In the first place, natural selection is here described only as furthering direct adaptation--only as aiding progress by the preservation of individuals in whom functionally-produced modifications have gone on most favourably. In the second place, there is no trace of the idea that natural selection may by co-operation with the cause assigned, or with other causes, produce divergences of structure; and of course, in the absence of this idea, there is no implication that natural selection has anything to do with the origin of species. And in the third place, the all-important factor of variation--“spontaneous,” or incidental as we may otherwise call it--is wholly ignored. Though use and disuse are, I think, much more potent causes of organic modification than Mr. Darwin supposes--though, while pursuing the inquiry in detail, I have been led to believe that direct equilibration has played a more active part even than I had myself at one time thought; yet I hold Mr. Darwin to have shown beyond question, that a great part of the facts--perhaps the greater part--are explicable only as resulting from the survival of individuals which have deviated in some indirectly-caused way from the ancestral type. Thus, the above paragraph contains merely a passing recognition of the selective process; and indicates no suspicion of the enormous range of its effects, or of the conditions under which a large part of its effects are produced.

[68] For the information of those who may wish to examine metamorphoses of these kinds, I may here state that I have found nearly all the examples described, in the neighbourhood of the sea--the last-named, on the shore of Locheil, near Fort William. Whether it is that I have sought more diligently for cases when in such localities, or whether it is that the sea-air favours that excessive nutrition whence these transformations result, I am unable to say.

[69] These two dyes have affinities for different components of the tissues, and may be advantageously used in different cases. Magenta is rapidly taken up by woody matter and other secondary deposits; while logwood colours the cell-membranes, and takes but reluctantly to the substances seized by magenta. By trying both of them on the same structure, we may guard ourselves against any error arising from selective combination.

[70] Those who repeat these experiments must be prepared for great irregularities in the rates of absorption. Succulent structures in general absorb much more slowly than others, and sometimes will scarcely take up the dye at all. The differences between different structures, and the same structure at different times, probably depend on the degrees in which the tissues are charged with liquid and the rates at which they are losing it by evaporation.

[71] It may be added here that, on considering the mechanical actions that must go on, we are enabled in some measure to understand both how such inosculating channels are initiated, and how the structures of their component cells are explicable. What must happen to one of these elongated prosenchyma-cells if, in the course of its development, it is subject to intermittent compressions? Its squeezed-out liquid while partially escaping laterally, will more largely escape upwards and downwards; and while repeated lateral escape will tend to form lateral channels communicating with laterally-adjacent cells, repeated longitudinal escape will tend to form channels communicating with longitudinally-adjacent cells--so producing continuous though irregular longitudinal canals. Meanwhile each cell into and out of which the nutritive liquid is from time to time squeezed through small openings in its walls, cannot thicken internally in an even manner: deposition will be interfered with by the passage of the currents through the pores. The rush to or from each pore will tend to maintain a funnel-shaped depression in the deposit around; and the opening from cell to cell will so acquire just that shape which the microscope shows up--two hollow cones with their apices meeting at the point where the cell-membranes are in contact. Moreover, as confirming this interpretation, it may be remarked that we are thus supplied with a reason for the differences of shape between these passages from one pitted cell to another, and the analogous passages that exist between cells otherwise formed and otherwise conditioned. In the cells of the medulla, and others which are but little exposed to compression, the passages are severally formed more like a tube with two trumpet-mouths, one in each cell. This is just the form which might be expected where the nutritive fluid passes from cell to cell in moderate currents, and not by the violent rushes caused by intermittent pressures. Of course it is not meant that in each individual cell these structures are determined by these mechanical actions. The facts clearly negative any such conclusion, showing us, as they in many cases do, that these structures are assumed in advance of these mechanical actions. The implication is, that such mechanical actions initiated modifications that have, with the aid of natural selection, been accumulated from generation to generation; until, in conformity with ordinary embryological laws, the cells of the parts exposed to such actions assume these special structures irrespective of the actions--the actions, however, still serving to aid and complete the assumption of the inherited type.

[72] Some exceptions to this occur in plants that have retrograded in the character of their tissues towards the simpler vegetal types. Certain very succulent leaves, such as those of Sempervivum, in which the cellular tissue is immensely developed in comparison with the vascular tissue, seem to have resumed to a considerable extent what we must regard as the primitive form of vegetal circulation--simple absorption from cell to cell. These, when they have lost much of their water, will take up the dye to some distance through their general substance, or rather through its interstices, even neglecting the vessels. At other times, in the same leaves, the vessels will become charged while comparatively little absorption takes place through the cellular tissue. Even in these exceptional cases, however, the movement through cellular tissue is nothing like as fast as the movement through vessels.

[73] It seems probable, however, that osmotic distention is here, especially, the more important of the two factors. The rising of the sap in spring may indirectly result, like the sprouting of the seed, from the transformation of starch into sugar. During germination, this change of an oxy-hydro-carbon from an insoluble into a soluble form, leads to rapid endosmose; consequently to great distention of the seed; and therefore to a force which thrusts the contained liquids into the plumule and radicle, and gives them power to displace the soil in their way: it sets up an active internal movement when neither evaporation nor the change which light produces can be operative. And similarly, if, in the spring, the starch stored-up in the roots of a tree passes into the form of sugar, the unusual osmotic absorption that arises will cause an unusual distention--a distention which, being resisted by the tough bark of the roots and stem, will result in a powerful upward thrust of the contained liquid.

Transcriber’s Note:

1. Obvious printers’, spelling and punctuation errors have been silently corrected.

2. Where appropriate, original spelling has been retained.

3. Both hyphenated and non-hyphenated versions of the same words have been retained where deemed appropriate.

4. Superscripts are represented using the caret character, e.g. D^r.

5. Italics are shown as xxx, bold print is shown as =xxx=.

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