VI. INDIVIDUALITY
§ 72. What is an individual? is a question which many readers will think it easy to answer. Yet it is a question that has led to much controversy among Zoologists and Botanists, and no quite satisfactory reply to it seems possible. As applied to a man, or to any one of the higher animals, which are all sharply-defined and independent, the word individual has a clear meaning: though even here, when we turn from average cases to exceptional cases--as a calf with two heads and two pairs of fore-limbs--we find ourselves in doubt whether to predicate one individuality or two. But when we extend our range of observation to the organic world at large, we find that difficulties allied to this exceptional one meets us everywhere under every variety of form.
Each uniaxial plant may perhaps fairly be regarded as a distinct individual; though there are botanists who do not make even this admission. What, however, are we to say of a multiaxial plant? It is, indeed, usual to speak of a tree with its many branches and shoots as singular; but strong reasons may be urged for considering it as plural. Every one of its axes has a more or less independent life, and when cut off and planted may grow into the likeness of its parent; or, by grafting and budding, parts of this tree may be developed upon another tree, and there manifest their specific peculiarities. Shall we regard all the growing axes thus resulting from slips and grafts and buds, as parts of one individual or as distinct individuals? If a strawberry-plant sends out runners carrying buds at their ends, which strike root and grow into independent plants that separate from the original one by decay of the runners, must we not say that they possess separate individualities; and yet if we do this, are we not at a loss to say when their separate individualities were established, unless we admit that each bud was from the beginning an individual? Commenting on such perplexities Schleiden says--"Much has been written and disputed concerning the conception of the individual, without, however, elucidating the subject, principally owing to the misconception that still exists as to the origin of the conception. Now the individual is no conception, but the mere subjective comprehension of an actual object, presented to us under some given specific conception, and on this latter it alone depends whether the object is or is not an individual. Under the specific conception of the solar system, ours is an individual: in relation to the specific conception of a planetary body, it is an aggregate of many individuals." ... "I think, however, that looking at the indubitable facts already mentioned, and the relations treated of in the course of these considerations, it will appear most advantageous and most useful, in a scientific point of view, to consider the vegetable cell as the general type of the plant (simple plant of the first order). Under this conception, Protococcus and other plants consisting of only one cell, and the spore and pollen-granule, will appear as individuals. Such individuals may, however, again, with a partial renunciation of their individual independence, combine under definite laws into definite forms (somewhat as the individual animals do in the globe of the Volvox globator[25]). These again appear empirically as individual beings, under a conception of a species (simple plants of the second order) derived from the form of the normal connexion of the elementary individuals. But we cannot stop here, since Nature herself combines these individuals, under a definite form, into larger associations, whence we draw the third conception of the plant, from a connexion, as it were, of the second power (compound plants--plants of the third order). The simple plant proceeding from the combination of the elementary individuals is then termed a bud (gemma), in the composition of plants of the third order."
The animal kingdom presents still greater difficulties. When, from sundry points on the body of a common polype, there bud out young polypes which, after acquiring mouths and tentacles and closing up the communications between their stomachs and the stomach of the parent, finally separate from the parent; we may with propriety regard them as distinct individuals. But when in the allied compound Hydrozoa, we find that these young polypes continue permanently connected with the parent; and when by this continuous budding-out there is presently produced a tree-like aggregation, having a common alimentary canal into which the digestive cavity of each polype opens; it is no longer so clear that these little sacs, furnished with mouths and tentacles, are severally to be regarded as distinct individuals. We cannot deny a certain individuality to the polypedom. And on discovering that some of the buds, instead of unfolding in the same manner as the rest, are transformed into capsules in which eggs are developed--on discovering that certain of the incipient polypes thus become wholly dependent on the aggregate for their nutrition, and discharge functions which have nothing to do with their own maintenance, we have still clearer proof that the individualities of the members are partially merged in the individuality of the group. Other organisms belonging to the same order, display still more decidedly this transition from simple individualities to a complex individuality. In the Diphyes there is a special modification of one or more members of the polypedom into a swimming apparatus which, by its rhythmical contractions, propels itself through the water, drawing the polypedom after it. And in the more differentiated Physalia various organs result from the metamorphosis of parts which are the homologues of individual polypes. In this last instance, the individuality of the aggregate is so predominant that the individualities of its members are practically lost. This combination of individualities in such way as to produce a composite individual, meets us in other forms among the ascidians. While in some of these, as in the Clavelina and in the Botryllidæ, the animals associated are but little subordinated to the community they form, in others they are so combined as to form a compound individual. The pelagic ascidian Doliolum is an example. "Here we find a large individual which swims by contractions of circular muscular bands, carries a train of smaller individuals attached to a long dorsal process of the test. These are arranged in three rows: those constituting the lateral row have wide mouths and no sexual organs or organs of locomotion--they subserve the nutrition of the colony, a truth which is illustrated by the fact that as soon as they are properly developed the large individual (the mother) loses her alimentary canal;" while from the median row are eventually derived the sexual zoids.
On the hypothesis of Evolution, perplexities of this nature are just such as we might anticipate. If Life in general commenced with minute and simple forms, like those out of which all organisms, however complex, now originate; and if the transitions from these primordial units to organisms made up of groups of such units, and to higher organisms made up of groups of such groups took place by degrees; it is clear that individualities of the first and simplest order would merge gradually in those of a larger and more complex order, and these again in others of an order having still greater bulk and organization. Hence it would be impossible to say where the lower individualities ceased and the higher individualities commenced.
§ 73. To meet these difficulties, it has been proposed that the whole product of a single fertilized germ shall be regarded as a single individual; whether such whole product be organized into one mass, or whether it be organized into many masses that are partially or completely separate. It is urged that whether the development of the fertilized germ be continuous or discontinuous (§ 50) is a matter of secondary importance; that the totality of living tissue to which the fertilized germ gives rise in any one case, is the equivalent of the totality to which it gives rise in any other case; and that we must recognize this equivalence, whether such totality of living tissue takes a concrete or a discrete arrangement. In pursuance of this view, a zoological individual is constituted either by any such single animal as a mammal or bird, which may properly claim the title of a zoon, or by any such group of animals as the numerous Medusæ that have been developed from the same egg, which are to be severally distinguished as zooids.
Admitting it to be very desirable that there should be words for expressing these relations and this equivalence, it may be objected that to apply the word individual to a number of separate living bodies, is inconvenient: conflicting so much, as it does, with the ordinary conception which this word suggests. It seems a questionable use of language to say that the countless masses of Anacharis Alsinastrum (now Eloidea canadensis) which, within these few years, have grown up in our rivers, canals, and ponds, are all parts of one individual: and yet as this plant does not seed in England, these countless masses, having arisen by discontinuous development, must be so regarded if we accept the above definition.
It may be contended, too, that while it does violence to our established way of thinking, this mode of interpreting the facts is not without its difficulties. Something seems to be gained by restricting the application of the title individual, to organisms which, being in all respects fully developed, possess the power of producing their kind after the ordinary sexual method, and denying this title to those incomplete organisms which have not this power. But the definition does not really establish this distinction for us. On the one hand, we have cases in which, as in the working bee, the whole of the germ-product is aggregated into a single organism; and yet, though an individual according to the definition, this organism has no power of reproducing its kind. On the other hand, we have cases like that of the perfect Aphis, where the organism is but an infinitesimal part of the germ product, and yet has that completeness required for sexual reproduction. Further, it might be urged with some show of reason, that if the conception of individuality involves the conception of completeness, then, an organism which possesses an independent power of reproducing itself, being more complete than an organism in which this power is dependent on the aid of another organism, is more individual.
§ 74. There is, indeed, as already implied, no definition of individuality that is unobjectionable. All we can do is to make the best practicable compromise.
As applied either to an animate or an inanimate object, the word individual ordinarily connotes union among the parts of the object and separateness from other objects. This fundamental element in the conception of individuality, we cannot with propriety ignore in the biological application of the word. That which we call an individual plant or animal must, therefore, be some concrete whole and not a discrete whole. If, however, we say that each concrete living whole is to be regarded as an individual, we are still met by the question--What constitutes a concrete living whole? A young organism arising by internal or external gemmation from a parent organism, passes gradually from a state in which it is an indistinguishable part of the parent organism to a state in which it is a separate organism of like structure with the parent. At what stage does it become an individual? And if its individuality be conceded only when it completely separates from the parent, must we deny individuality to all organisms thus produced which permanently retain their connexions with their parents? Or again, what must we say of the Hectocotylus, which is an arm of the Cuttle-fish that undergoes a special development and then, detaching itself, lives independently for a considerable period? And what must we say of the larval nemertine worm the pilidium of which with its nervous system is left to move about awhile after the developing worm has dropped out of it?
To answer such questions we must revert to the definition of life. The distinction between individual in its biological sense, and individual in its more general sense, must consist in the manifestation of Life, properly so called. Life we have seen to be, "the definite combination of heterogeneous change, both simultaneous and successive, in correspondence with external co-existences and sequences." Hence, a biological individual is any concrete whole having a structure which enables it, when placed in appropriate conditions, to continuously adjust its internal relations to external relations, so as to maintain the equilibrium of its functions. In pursuance of this conception, we must consider as individuals all those wholly or partially independent organized masses which arise by multicentral and multiaxial development that is either continuous or discontinuous (§ 50). We must accord the title to each separate aphis, each polype of a polypedom, each bud or shoot of a lowering plant, whether it detaches itself as a bulbil or remains attached as a branch.
By thus interpreting the facts we do not, indeed, avoid all anomalies. While, among flowering plants, the power of independent growth and development is usually possessed only by shoots or axes; yet, in some cases, as in that of the Begonia-leaf awhile since mentioned, the appendage of an axis, or even a small fragment of such appendage, is capable of initiating and carrying on the functions of life; and in other cases, as shown by M. Naudin in the Drosera intermedia, young plants are occasionally developed from the surfaces of leaves. Nor among forms like the compound Hydrozoa, does the definition enable us to decide where the line is to be drawn between the individuality of the group and the individualities of the members: merging into each other, as these do, in different degrees. But, as before said, such difficulties must necessarily present themselves if organic forms have arisen by insensible gradations. We must be content with a course which commits us to the smallest number of incongruities; and this course is, to consider as an individual any organized mass which is capable of independently carrying on that continuous adjustment of inner to outer relations which constitutes Life.
VI. ^A
CELL-LIFE AND CELL-MULTIPLICATION.
§ 74a. The progress of science is simultaneously towards simplification and towards complication. Analysis simplifies its conceptions by resolving phenomena into their factors, and by then showing how each simple mode of action may be traced under multitudinous forms; while, at the same time, synthesis shows how each factor, by cooperation with various other factors in countless modes and degrees, produces different results innumerable in their amounts and varieties. Of course this truth holds alike of processes and of products. Observation and the grouping into classes make it clear that through multitudinous things superficially unlike there run the same cardinal traits of structure; while, along with these major unities, examination discloses innumerable minor diversities.
A concomitant truth, or the same truth under another aspect, is that Nature everywhere presents us with complexities within complexities, which go on revealing themselves as we investigate smaller and smaller objects. In a preceding chapter (§§ 54a, 54b) it was pointed out that each primitive organism, in common with each of the units out of which the higher and larger organisms are built, was found a generation ago to consist of nucleus, protoplasm, and cell-wall. This general conception of a cell remained for a time the outcome of inquiry; but with the advance of microscopy it became manifest that within these minute structures processes and products of an astonishing nature are to be seen. These we have now to contemplate.
In the passages just referred to it was said that the external layer or cell-wall is a non-essential, inanimate part produced by the animate contents. Itself a product of protoplasmic action, it takes no part in protoplasmic changes, and may therefore here be ignored.
§ 74b. One of the complexities within complexities was disclosed when it was found that the protoplasm itself has a complicated structure. Different observers have described it as constituted by a network or reticulum, a sponge-work, a foam-work. Of these the first may be rejected; since it implies a structure lying in one plane. If we accept the second we have to conceive the threads of protoplasm, corresponding to the fibres of the sponge, as leaving interstices filled either with liquid or solid. They cannot be filled with a continuous solid, since all motion of the protoplasm would be negatived; and that their content is not liquid seems shown by the fact that its parts move about under the form of granules or microsomes. But the conception of moving granules implies the conception of immersion in a liquid or semi-liquid substance in which they move--not a sponge-work of threads but a foam-work, consisting everywhere of septa interposed among the granules. This is the hypothesis which sundry microscopists espouse, and which seems mechanically the most feasible: the only one which consists with the "streaming" of protoplasm. Ordinarily the name protoplasm is applied to the aggregate mass--the semi-liquid, hyaline substance and the granules or microsomes it contains.
What these granules or microsomes are--whether, as some have contended, they are the essential living elements of the protoplasm, or whether, as is otherwise held, they are nutritive particles, is at present undecided. But the fact, alleged by sundry observers, that the microsomes often form rows, held together by intervening substance, seems to imply that these minute bodies are not inert. Leaving aside unsettled questions, however, one fact of significance is manifest--an immense multiplication of surfaces over which inter-action may take place. Anyone who drops into dilute sulphuric acid a small nail and then drops a pinch of iron filings, will be shown, by the rapid disappearance of the last and the long continuance of the first, how greatly the increasing of surfaces by multiplication of fragments facilitates change. The effect of subdivision in producing a large area in a small space, is shown in the lungs, where the air-cells on the sides of which the blood-vessels ramify, are less than 1/100th of an inch in diameter, while they number 700,000,000. In the composition of every tissue we see the same principle. The living part, or protoplasm, is divided into innumerable protoplasts, among which are distributed the materials and agencies producing changes. And now we find this principle carried still deeper in the structure of the protoplasm itself. Each microscopic portion of it is minutely divided in such ways that its threads or septa have multitudinous contacts with those included portions of matter which take part in its activities.
Concerning the protoplasm contained in each cell, named by some cytoplasm, it remains to say that it always includes a small body called the centrosome, which appears to have a directive function. Usually the centrosome lies outside the nucleus, but is alleged to be sometimes within it. During what is called the "resting stage," or what might more properly be called the growing stage (for clearly the occasional divisions imply that in the intervals between them there has been increase) the centrosome remains quiescent, save in the respect that it exercises some coercive influence on the protoplasm around. This results in the radially-arranged lines constituting an "aster." What is the nature of the coercion exercised by the centrosome--a body hardly distinguishable in size from the microsomes or granules of protoplasm around--is not known. It can scarcely be a repelling force; since, in a substance of liquid or semi-liquid kind, this could not produce approximately straight lines. That it is an attractive force seems more probable; and the nature of the attraction would be comprehensible did the centrosome augment in bulk with rapidity. For if integration were in progress, the drawing in of materials might well produce converging lines. But this seems scarcely a tenable interpretation; since, during the so-called "resting stage," this star-like structure exists--exists, that is, while no active growth of the centrosome is going on.
Respecting this small body we have further to note that, like the cell as a whole, it multiplies by fission, and that the bisection of it terminates the resting or growing stage and initiates those complicated processes by which two cells are produced out of one: the first step following the fission being the movement of the halves, with their respective completed asters, to the opposite sides of the nucleus.
§ 74c. With the hypothesis, now general, that the nucleus or kernel of a cell is its essential part, there has not unnaturally grown up the dogma that it is always present; but there is reason to think that the evidence is somewhat strained to justify this dogma.
In the first place, beyond the cases in which the nucleus, though ordinarily invisible, is said to have been rendered visible by a re-agent, there are cases, as in the already-named Archerina, where no re-agent makes one visible. In the second place, there is the admitted fact that some nuclei are diffused; as in Trachelocerca and some other Infusoria. In them the numerous scattered granules are supposed to constitute a nucleus: an interpretation obviously biassed by the desire to save the generalization. In the third place, the nucleus is frequently multiple in cells of low types; as in some families of Algæ and predominantly among Fungi. Once more, the so-called nucleus is occasionally a branching structure scarcely to be called a "kernel."
The facts as thus grouped suggest that the nucleus has arisen in conformity with the law of evolution--that the primitive protoplast, though not homogeneous in the full sense, was homogeneous in the sense of being a uniformly granular protoplasm; and that the protoplasts with diffused nuclei, together with those which are multi-nucleate, and those which have nuclei of a branching form, represent stages in that process by which the relatively homogeneous protoplast passed into the relatively heterogeneous one now almost universal.
Concerning the structure and composition of the developed nucleus, the primary fact to be named is that, like the surrounding granular cytoplasm, it is formed of two distinct elements. It has a groundwork or matrix not differing much from that of the cytoplasm, and at some periods continuous with it; and immersed in this it has a special matter named chromatin, distinguished from its matrix by becoming dyed more or less deeply when exposed to fit re-agents. During the "resting stage," or period of growth and activity which comes between periods of division, the chromatin is dispersed throughout the ground-substance, either in discrete portions or in such way as to form an irregular network or sponge-work, various in appearance. When the time for fission is approaching this dispersed chromatin begins to gather itself together: reaching its eventual concentration through several stages. By its concentration are produced the chromosomes, constant in number in each species of plant or animal. It is alleged that the substance of the chromosomes is not continuous, but consists of separate elements or granules, which have been named chromomeres; and it is also alleged that, whether in the dispersed or integrated form, each chromosome retains its individuality--that the chromomeres composing it, now spreading out into a network and now uniting into a worm-like body, form a group which never loses its identity. Be this as it may, however, the essential fact is that during the growth-period the chromatin substance is widely distributed, and concentration of it is one of the chief steps towards a division of the nucleus and presently of the cell.
During this process of mitosis or karyokinesis, the dispersed chromatin having passed through the coil-stage, reaches presently the star-stage, in which the chromosomes are arranged symmetrically about the equatorial plane of the nucleus. Meanwhile in each of them there has been a preparation for splitting longitudinally in such way that the halves when separated contain (or are assumed to contain) equal numbers of the granules or chromomeres, which some think are the ultimate morphological units of the chromosomes. A simultaneous change has occurred: there has been in course of formation a structure known as the amphiaster. The two centrosomes which, as before said, place themselves on opposite sides of the nucleus, become the terminal poles of a spindle-shaped arrangement of fibres, arising mainly from the groundwork of the nucleus, now continuous with the groundwork of the cytoplasm. A conception of this structure may be formed by supposing that the radiating fibres of the respective asters, meeting one another and uniting in the intermediate space, thereafter exercise a tractive force; since it is clear that, while the central fibres of the bundle will form straight lines, the outer ones, pulling against one another not in straight lines, will form curved lines, becoming more pronounced in their curvatures as the distance from the axis increases. That a tractive force is at work seems inferable from the results. For the separated halves of the split chromosomes, which now form clusters on the two sides of the equatorial plane, gradually part company, and are apparently drawn as clusters towards the opposing centrosomes. As this change progresses the original nucleus loses its individuality. The new chromosomes, halves of the previous chromosomes, concentrate to found two new nuclei; and, by something like a reversal of the stages above described, the chromatin becomes dispersed throughout the substance of each new nucleus. While this is going on the cell itself, undergoing constriction round its equator, divides into two.
Many parts of this complex process are still imperfectly understood, and various opinions concerning them are current. But the essential facts are that this peculiar substance, the chromatin, at other times existing dispersed, is, when division is approaching, gathered together and dealt with in such manner as apparently to insure equal quantities being bequeathed by the mother-cell to the two daughter-cells.
§ 74d. What is the physiological interpretation of these structures and changes? What function does the nucleus discharge; and, more especially, what is the function discharged by the chromatin? There have been to these questions sundry speculative answers.
The theory espoused by some, that the nucleus is the regulative organ of the cell, is met by difficulties. One of them is that, as pointed out in the chapter on "Structure," the nucleus, though morphologically central, is not central geometrically considered; and that its position, often near to some parts of the periphery and remote from others, almost of itself negatives the conclusion that its function is directive in the ordinary sense of the word. It could not well control the cytoplasm in the same ways in all directions and at different distances. A further difficulty is that the cytoplasm when deprived of its nucleus can perform for some time various of its actions, though it eventually dies without reproducing itself.
For the hypothesis that the nucleus is a vehicle for transmitting hereditary characters, the evidence seems strong. When it was shown that the head of a spermatozoon is simply a detached nucleus, and that its fusion with the nucleus of an ovum is the essential process initiating the development of a new organism, the legitimate inference appeared to be that these two nuclei convey respectively the paternal and maternal traits which are mingled in the offspring. And when there came to be discerned the karyokinesis by which the chromatin is, during cell-fission, exactly halved between the nuclei of the daughter-cells, the conclusion was drawn that the chromatin is more especially the agent of inheritance. But though, taken by themselves, the phenomena of fertilization seem to warrant this inference, the inference does not seem congruous with the phenomena of ordinary cell-multiplication--phenomena which have nothing to do with fertilization and the transmission of hereditary characters. No explanation is yielded of the fact that ordinary cell-multiplication exhibits an elaborate process for exact halving of the chromatin. Why should this substance be so carefully portioned out among the cells of tissues which are not even remotely concerned with propagation of the species? If it be said that the end achieved is the conveyance of paternal and maternal qualities in equal degrees to every tissue; then the reply is that they do not seem to be conveyed in equal degrees. In the offspring there is not a uniform diffusion of the two sets of traits throughout all parts, but an irregular mixture of traits of the one with traits of the other.
In presence of these two suggested hypotheses and these respective difficulties, may we not suspect that the action of the chromatin is one which in a way fulfils both functions? Let us consider what action may do this.
§ 74e. The chemical composition of chromatin is highly complex, and its complexity, apart from other traits, implies relative instability. This is further implied by the special natures of its components. Various analyses have shown that it consists of an organic acid (which has been called nucleic acid) rich in phosphorus, combined with an albuminous substance: probably a combination of various proteids. And the evidence, as summarised by Wilson, seems to show that where the proportion of phosphorized acid is high the activity of the substance is great, as in the heads of spermatozoa; while, conversely, where the quantity of phosphorus is relatively small, the substance approximates in character to the cytoplasm. Now (like sulphur, present in the albuminoid base), phosphorus is an element which, besides having several allotropic forms, has a great affinity for oxygen; and an organic compound into which it enters, beyond the instability otherwise caused, has a special instability caused by its presence. The tendency to undergo change will therefore be great when the proportion of the phosphorized component is great. Hence the statement that "the chemical differences between chromatin and cytoplasm, striking and constant as they are, are differences of degree only;" and the conclusion that the activity of the chromatin is specially associated with the phosphorus.[26]
What, now, are the implications? Molecular agitation results from decomposition of each phosphorized molecule: shocks are continually propagated around. From the chromatin, units of which are thus ever falling into stabler states, there are ever being diffused waves of molecular motion, setting up molecular changes in the cytoplasm. The chromatin stands towards the other contents of the cell in the same relation that a nerve-element stands to any element of an organism which it excites: an interpretation congruous with the fact that the chromatin is as near to as, and indeed nearer than, a nerve-ending to any minute structure stimulated by it.
Several confirmatory facts may be named. During the intervals between cell-fissions, when growth and the usual cell-activities are being carried on, the chromatin is dispersed throughout the nucleus into an irregular network: thus greatly increasing the surface of contact between its substance and the substances in which it is imbedded. As has been remarked, this wide distribution furthers metabolism--a metabolism which in this case has, as we infer, the function of generating, not special matters but special motions. Moreover, just as the wave of disturbance a nerve carries produces an effect which is determined, not by anything which is peculiar in itself, but by the peculiar nature of the organ to which it is carried--muscular, glandular or other; so here, the waves diffused from the chromatin do not determine the kinds of changes in the cytoplasm, but simply excite it: its particular activities, whether of movement, absorption, or structural excretion, being determined by its constitution. And then, further, we observe a parallelism between the metabolic changes in the two cases; for, on the one hand, "diminished staining capacity of the chromatin [implying a decreased amount of phosphorus, which gives the staining capacity] occurs during a period of intense constructive activity in the cytoplasm;" and, on the other hand, in high organisms having nervous systems, the intensity of nervous action is measured by the excretion of phosphates--by the using up of the phosphorus contained in nerve-cells.
For thus interpreting the respective functions of chromatin and cytoplasm, yet a further reason may be given. One of the earliest general steps in the evolution of the Metazoa, is the differentiation of parts which act from parts which make them act. The Hydrozoa show us this. In the hydroid stage there are no specialized contractile organs: these are but incipient: individual ectoderm cells have muscular processes. Nor is there any "special aggregation of nerve-cells." If any stimulating units exist they are scattered. But in the Medusa-stage nerve-matter is collected into a ring round the edge of the umbrella. That is to say, in the undeveloped form such motor action as occurs is not effected by a specialized part which excites another part; but in the developed form a differentiation of the two has taken place. All higher types exhibit this differentiation. Be it muscle or gland or other operating organ, the cause of its activity lies not in itself but in a nervous agent, local or central, with which it is connected. Hence, then, there is congruity between the above interpretation and certain general truths displayed by animal organization at large. We may infer that in a way parallel to that just indicated, cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes: leaving behind the substance excited, now distinguished as cytoplasm.
§ 74f. Some further general aspects of the phenomena appear to be in harmony with this interpretation. Let us glance at them.
In Chapters III and IIIA of the First Part, reasons were given for concluding that in the animal organism nitrogenous substances play the part of decomposing agents to the carbo-hydrates--that the molecular disturbance set up by the collapse of a proteid molecule destroys the equilibrium of sundry adjacent carbo-hydrate molecules, and causes that evolution of energy which accompanies their fall into molecules of simpler compounds. Here, if the foregoing argument is valid, we may conclude that this highly complex phosphorized compound which chromatin contains, plays the same part to the adjacent nitrogenous compounds as these play to the carbo-hydrates. If so, we see arising a stage earlier that "general physiological method" illustrated in § 23f. It was there pointed out that in animal organisms the various structures are so arranged that evolution of a small amount of energy in one, sets up evolution of a larger amount of energy in another; and often this multiplied energy undergoes a second multiplication of like kind. If this view is tenable, we may now suspect that this method displayed in the structures of the Metazoa was initiated in the structures of the Protozoa, and consequently characterizes those homologues of them which compose the Metazoa.
When contemplated from the suggested point of view, karyokinesis appears to be not wholly incomprehensible. For if the chromatin yields the energy which initiates changes throughout the rest of the cell, we may see why there eventually arises a process for exact halving of the chromatin in a mother-cell between two daughter-cells. To make clear the reason, let us suppose the portioning out of the chromatin leaves one of the two with a sensibly smaller amount than the other. What must result? Its source of activity being relatively less, its rate of growth and its energy of action will be less. If a protozoon, the weaker progeny arising by division of it will originate an inferior stirp, unable to compete successfully with that arising from the sister-cell endowed with a larger portion of chromatin. By continual elimination of the varieties which produce unequal halving, necessarily at a disadvantage if a moiety of their members tend continually to disappear, there will be established a variety in which the halving is exact: the character of this variety being such that all its members aid the permanent multiplication of the species. If, again, the case is that of a metazoon, there will be the same eventual result. An animal or plant in which the chromatin is unequally divided among the cells, must have tissues of uncertain formation. Assume that an organ has, by survival of the fittest, been adjusted in the proportions and qualities of its parts to a given function. If the multiplying protoplasts, instead of taking equal portions of chromatin, have some of them smaller portions, the parts of the organ formed of these, developing less rapidly and having inferior energies, will throw the organ out of adjustment, and the individual will suffer in the struggle for life. That is to say, irregular division of the chromatin will introduce a deranging factor and natural selection will weed out individuals in which it occurs. Of course no interpretation is thus yielded of the special process known as karyokinesis. Probably other modes of equal division might have arisen. Here the argument implies merely that the tendency of evolution is to establish some mode. In verification of the view that equal division arises from the cause named, it is pointed out to me that amitosis, which is a negation of mitosis or karyokinesis, occurs in transitory tissues or diseased tissues or where degeneracy is going on.
But how does all this consist with the conclusion that the chromatin conveys hereditary traits--that it is the vehicle in which the constitutional structure, primarily of the species and secondarily of recent ancestors and parents, is represented? To this question there seems to be no definite answer. We may say only that this second function is not necessarily in conflict with the first. While the unstable units of chromatin, ever undergoing changes, diffuse energy around, they may also be units which, under the conditions furnished by fertilization, gravitate towards the organization of the species. Possibly it may be that the complex combination of proteids, common to chromatin and cytoplasm, is that part in which the constitutional characters inhere; while the phosphorized component, falling from its unstable union and decomposing, evolves the energy which, ordinarily the cause of changes, now excites the more active changes following fertilization. This suggestion harmonizes with the fact that the fertilizing substance which in animals constitutes the head of the spermatozoon, and in plants that of the spermatozoid or antherozoid, is distinguished from the other agents concerned by having the highest proportion of the phosphorized element; and it also harmonizes with the fact that the extremely active changes set up by fertilization are accompanied by decrease of this phosphorized element. Speculation aside, however, we may say that the two functions of the chromatin do not exclude one another, but that the general activity which originates from it may be but a lower phase of that special activity caused by fertilization.[27]
§ 74g. Here we come unawares to the remaining topic embraced under the title Cell-Life and Cell-Multiplication. We pass naturally from asexual multiplication of cells to sexual multiplication--from cell-reproduction to cell-generation. The phenomena are so numerous and so varied that a large part of them must be passed over. Conjugation among the Protophyta and Protozoa, beginning with cases in which there is a mingling of the contents of two cells in no visible respect different from one another, and developing into a great variety of processes in which they differ, must be left aside, and attention limited to the terminal process of fertilization as displayed in higher types of organisms.
Before fertilization there occurs in the ovum an incidental process of a strange kind--"strange" because it is a collateral change taking no part in subsequent changes. I refer to the production and extrusion of the "polar bodies." It is recognized that the formation of each is analogous to cell-formation in general; though process and product are both dwarfed. Apart from any ascribed meaning, the fact itself is clear. There is an abortive cell-formation. Abortiveness is seen firstly in the diminutive size of the separated body or cell, and secondly in the deficient number of its chromosomes: a corresponding deficiency being displayed in the group of chromosomes remaining in the egg--remaining, that is (on the hypothesis here to be suggested), in the sister-cell, supposing the polar body to be an aborted cell. It is currently assumed that the end to be achieved by thus extruding part of the chromosomes, is to reduce the remainder to half the number characterizing the species; so that when, to this group in the germ-cell, the sperm-cell brings a similarly-reduced group, union of the two shall bring the chromosomes to the normal number. I venture to suggest another interpretation. In doing this, however, I must forestall a conclusion contained in the next chapter; namely, the conclusion that gamogenesis begins when agamogenesis is being arrested by unfavourable conditions, and that the failing agamogenesis initiates the gamogenesis. Of numerous illustrations to be presently given I will, to make clear the conception, name only one--the formation of fructifying organs in plants at times when, and in places where, shoots are falling off in vigour and leaves in size. Here the successive foliar organs, decreasingly fitted alike in quality and dimensions for carrying on their normal lives, show us an approaching cessation of asexual multiplication, ending in the aborted individuals we call stamens; and the fact that sudden increase of nutrition while gamogenesis is being thus initiated, causes resumption of agamogenesis, shows that the gamogenesis is consequent upon the failing agamogenesis. See then the parallel. On going back from multicellular organisms to unicellular organisms (or those homologues of them which form the reproductive agents in multicellular organisms), we find the same law hold. The polar bodies are aborted cells, indicating that asexual multiplication can no longer go on, and that the conditions leading to sexual multiplication have arisen. If this be so, decrease in the chromatin becomes an initial cause of the change instead of an accompanying incident; and we need no longer assume that a quantity of precious matter is lost, not by passive incapacity, but by active expulsion. Another anomaly disappears. If from the germ-cell there takes place this extrusion of superfluous chromatin, the implication would seem to be that a parallel extrusion takes place from the sperm-cell. But this is not true. In the sperm-cell there occurs just that failure in the production of chromatin which, according to the hypothesis above sketched out, is to be expected; for, in the process of cell-multiplication, the cells which become spermatozoa are left with half the number of chromosomes possessed by preceding cells: there is actually that impoverishment and declining vigour here suggested as the antecedent of fertilization. It needs only to imagine the ovum and the polar body to be alike in size, to see the parallelism; and to see that obscuration of it arises from the accumulation of cytoplasm in the ovum.
A test fact remains. Sometimes the first polar body extruded undergoes fission while the second is being formed. This can have nothing to do with reducing the number of chromosomes in the ovum. Unquestionably, however, this change is included with the preceding changes in one transaction, effected by one influence. If, then, it is irrelevant to the decrease of chromosomes, so must the preceding changes be irrelevant: the hypothesis lapses. Contrariwise this fact supports the view suggested above. That extrusion of a polar body is a process of cell-fission is congruous with the fact that another fission occurs after extrusion. And that this occurs irregularly shows that the vital activities, seen in cell-growth and cell-multiplication, now succeed in producing further fission of the dwarfed cell and now fail: the energies causing asexual multiplication are exhausted and there arises the state which initiates sexual multiplication.
Maturation of the ovum having been completed, entrance of the spermatozoon, sometimes through the limiting membrane and sometimes through a micropyle or opening in it, takes place. This instantly initiates a series of complicated changes: not many seconds passing before there begins the formation of an aster around one end of the spermatozoon-head. The growth of this aster, apparently by linear rangings of the granules composing the reticulum of the germ-cell, progresses rapidly; while the whole structure hence arising moves inward. Soon there takes place the fusion of this sperm-nucleus with the germ-nucleus to form the cleavage-nucleus, which, after a pause, begins to divide and subdivide in the same manner as cells at large: so presently forming a cluster of cells out of which arise the layers originating the embyro. The details of this process do not concern us. It suffices to indicate thus briefly its general nature.
And now ending thus the account of genesis under its histological aspect, we pass to the account of genesis under its wider and more significant aspects.
VII. GENESIS
§ 75. Having, in the last chapter but one, concluded what constitutes an individual, and having, in the last chapter, contemplated the histological process which initiates a new individual, we are in a position to deal with the multiplication of individuals. For this, the title Genesis is here chosen as being the most comprehensive title--the least specialized in its meaning. By some biologists Generation has been used to signify one method of multiplication, and Reproduction to signify another method; and each of these words has been thus rendered in some degree unfit to signify multiplication in general.
Here the reader is indirectly introduced to the fact that the production of new organisms is carried on in fundamentally unlike ways. Up to quite recent times it was believed, even by naturalists, that all the various processes of multiplication observable in different kinds of organisms, have one essential character in common: it was supposed that in every species the successive generations are alike. It has now been proved, however, that in many plants and in numerous animals, the successive generations are not alike; that from one generation there proceeds another whose members differ more or less in structure from their parents; that these produce others like themselves, or like their parents, or like neither; but that eventually, the original form re-appears. Instead of there being, as in the cases most familiar to us, a constant recurrence of the same form, there is a cyclical recurrence of the same form. These two distinct processes of multiplication, may be aptly termed homogenesis and heterogenesis.[28] Under these heads let us consider them.
There are two kinds of homogenesis, the simplest of them, probably once universal but now exceptional, being that in which there is no other form of multiplication than one resulting from perpetual spontaneous fission. The rise of distinct sexes was doubtless a step in evolution, and before it took place the formation of new individuals could have arisen only by division of the old, either into two or into many. At present this process survives, so far as appears, among Bacteria, certain Algæ, and sundry Protozoa; though it is possible that a rarely-occurring conjugation has in these cases not yet been observed. It is a probable conclusion, however, that in the Bacteria at any rate, the once universal mode of multiplication still survives as an exceptional mode. But now passing over these cases, we have to note that the kind of genesis (once supposed to be the sole kind), in which the successive generations are alike, is sexual genesis, or, as it has been otherwise called--gamogenesis. In every species which multiplies by this kind of homogenesis, each generation consists of males and females; and from the fertilized germs they produce the next generation of similar males and females arises: the only needful qualification of this statement being that in many Protophyta and Protozoa the conjugating cells or protoplasts are not distinguishable in character. This mode of propagation has the further trait, that each fertilized germ usually gives rise to but one individual--the product of development is organized round one axis and not round several axes, Homogenesis in contrast with heterogenesis as exhibited in species which display distinct sexuality, has also the characteristic that each new individual begins as an egg detached from the maternal tissues, instead of being a portion of protoplasm continuous with them, and that its development proceeds independently. This development may be carried on either internally or externally; whence results the division into the oviparous and the viviparous. The oviparous kind is that in which the fertilized germ is extruded from the parent before it has undergone any considerable development. The viviparous kind is that in which development is considerably advanced, or almost completed, before extrusion takes place. This distinction is, however, not a sharply-defined one: there are transitions between the oviparous and the viviparous processes. In ovo-viviparous genesis there is an internal incubation; and though the young are in this case finally extruded from the parent in the shape of eggs, they do not leave the parent's body until after they have assumed something like the parental form. Looking around, we find that homogenesis is universal among the Vertebrata. Every vertebrate animal arises from a fertilized germ, and unites into its single individuality the whole product of this fertilized germ. In the mammals or highest Vertebrata, this homogenesis is in every case viviparous; in birds it is uniformly oviparous; and in reptiles and fishes it is always essentially oviparous, though there are cases of the kind above referred to, in which viviparity is simulated. Passing to the Invertebrata, we find oviparous homogenesis universal among the Arachnida (except the Scorpions, which are ovo-viviparous); universal among the higher Crustacea, but not among the lower; extremely general, though not universal, among Insects; and universal among the higher Mollusca though not among the lower. Along with extreme inferiority among animals, we find homogenesis to be the exception rather than the rule; and in the vegetal kingdom there appear to be no cases, except among the Algæ and a few aberrant parasites like the Rafflesiaceæ, in which the centre or axis which arises from a fertilized germ becomes the immediate producer of fertilized germs.
In propagation characterized by unlikeness of the successive generations, there is asexual genesis with occasionally-recurring sexual genesis; in other words--agamogenesis interrupted more or less frequently by gamogenesis. If we set out with a generation of perfect males and females, then, from their ova arise individuals which are neither males nor females, but which produce the next generation from buds. By this method of multiplication many individuals originate from a single fertilized germ. The product of development is organized round more than one centre or axis. The simplest form of heterogenesis is that seen in most uniaxial plants. If, as we find ourselves obliged to do, we regard each separate shoot or axis of growth as a distinct individual, homogenesis is seen in those which have absolutely terminal flowers; but in all other uniaxial plants, the successive individuals are not represented by the series A, A, A, A, &c., but they are represented by the series A, B, A, B, A, B, &c. For in the majority of plants which were classed as uniaxial (§ 50), and which may be conveniently so distinguished from other plants, the axis which shoots up from the seed, and substantially constitutes the plant, does not itself flower but gives lateral origin to flowering axes. Though in ordinary uniaxial plants the fructifying apparatus appears to be at the end of the primary, vertical axis; yet dissection shows that, morphologically considered, each fructifying axis is an offspring from the primary axis. There arises from the seed a sexless individual, from which spring by gemmation individuals having reproductive organs; and from these there result fertilized germs or seeds that give rise to sexless individuals. That is to say, gamogenesis and agamogenesis alternate: the peculiarity being that the sexual individuals arise from the sexless ones by continuous development. The Salpæ show us an allied form of heterogenesis in the animal kingdom. Individuals developed from fertilized ova, instead of themselves producing fertilized ova, produce, by gemmation, strings of individuals from which fertilized ova again originate. In multiaxial plants, we have a succession of generations represented by the series A, B, B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis or sexual individual, then, from any fertilized germ it casts off, there grows up a sexless individual, B; from this there bud-out other sexless individuals, B, and so on for generations more or less numerous, until at length, from some of these sexless individuals, there bud-out seed-bearing individuals of the original form A. Branched herbs, shrubs, and trees, exhibit this form of heterogenesis: the successive generations of sexless individuals thus produced being, in most cases, continuously developed, or aggregated into a compound individual, but being in some cases discontinuously developed. Among animals a kind of heterogenesis represented by the same succession of letters, occurs in such compound polypes as the Sertularia, and in those of the Hydrozoa which assume alternately the polypoid form and the form of the Medusa. The chief differences presented by these groups arise from the fact that the successive generations of sexless individuals produced by budding, are in some cases continuously developed, and in others discontinuously developed; and from the fact that, in some cases, the sexual individuals give off their fertilized germs while still growing on the parent-polypedom, but in other cases not until after leaving the parent-polypedom and undergoing further development. Where, as in all the foregoing kinds of agamogenesis, the new individuals bud out, not from any specialized reproductive organs but from unspecialized parts of the parent, the process has been named, by Prof. Owen, metagenesis. In most instances the individuals thus produced grow from the outsides of the parents--the metagenesis is external. But there is also a kind of metagenesis which we may distinguish as internal. Certain entozoa of the genus Distoma exhibit it. From the egg of a Distoma there results a rudely-formed creature known as a sporocyst and from this a redia. Gradually, as this divides and buds, the greater part of the inner substance is transformed into young animals called Cercariæ (which are the larvæ of Distomata); until at length it becomes little more than a living sac full of living offspring. In the Distoma pacifica, the brood of young animals thus arising by internal gemmation are not Cercariæ, but are like their parent: themselves becoming the producers of Cercariæ, after the same manner, at a subsequent period. So that now the succession of forms is represented by the series A, B, A, B, &c., now by the series A, B, B, A, B, B, &c., and now by A, B, B, C, A. Both cases, however, exemplify internal metagenesis in contrast with the several kinds of external metagenesis described above. That agamogenesis which is carried on in a reproductive organ--either an ovarium or the homologue of one--has been called, by Prof. Owen, parthenogenesis. It is the process familiarly exemplified in the Aphides. Here, from the fertilized eggs laid by perfect females there grow up imperfect females, in the ovaria of which are developed ova that though unfertilized, rapidly assume the organization of other imperfect females, and are born viviparously. From this second generation of imperfect females, there by-and-by arises, in the same manner, a third generation of the same kind; and so on for many generations: the series being thus symbolized by the letters A, B, B, B, B, B, &c., A. Respecting this kind of heterogenesis it should be added that, in animals as in plants, the number of generations of sexless individuals produced before the re-appearance of sexual ones, is indefinite; both in the sense that in the same species it may go on to a greater or less extent according to circumstances, and in the sense that among the generations of individuals proceeding from the same fertilized germ, a recurrence of sexual individuals takes place earlier in some of the diverging lines of multiplication than in others. In trees we see that on some branches flower-bearing axes arise while other branches are still producing only leaf-bearing axes; and in the successive generations of Aphides a parallel fact has been observed. Lastly has to be set down that kind of heterogenesis in which, along with gamogenesis, there occurs a form of agamogenesis exactly like it, save in the absence of fecundation. This is called true parthenogenesis--reproduction carried on by virgin mothers which are in all respects like other mothers. Among silk-worm-moths this parthenogenesis is exceptional rather than ordinary. Usually the eggs of these insects are fertilized; but if they are not they are still laid, and some of them produce larvæ. In certain Lepidoptera, however, of the groups Psychidæ and Tineidæ, parthenogenesis appears to be a normal process--indeed, so far as is known, the only process; for of some species the males have never been found.
A general conception of the relations among the different modes of Genesis, thus briefly described, will be best given by the following tabular statement.
GENESIS is { Oviparous { or Homogenesis, which is usually Gamogenesis { Ovo-viviparous { or { Viviparous or { Gamogenesis { alternating Heterogenesis, which is { with { Parthenogenesis { Agamogenesis { or { Internal { Metagenesis { or { External
This, like all other classifications of such phenomena, presents anomalies. It may be justly objected that the processes here grouped under the head agamogenesis, are the same as those before grouped under the head of discontinuous development (§ 50): thus making development and genesis partially coincident. Doubtless it seems awkward that what are from one point of view considered as structural changes are from another point of view considered as modes of multiplication.[29] There is, however, nothing for us but a choice of imperfections. We cannot by any logical dichotomies accurately express relations which, in Nature, graduate into one another insensibly. Neither the above, nor any other scheme, can do more than give an approximate idea of the truth.
§ 76. Genesis under every form is a process of negative or positive disintegration; and is thus essentially opposed to that process of integration which is the primary process in individual evolution. Negative disintegration occurs in those cases where, as among the compound Hydrozoa, there is a continuous development of new individuals by budding from the bodies of older individuals; and where the older individuals are thus prevented from growing to a greater size, or reaching a higher degree of integration. Positive disintegration occurs in those forms of agamogenesis where the production of new individuals is discontinuous, as well as in all cases of gamogenesis. The degrees of disintegration are various. At the one extreme the parent organism is completely broken up, or dissolved into new individuals; and at the other extreme each new individual forms but a small deduction from the parent organism. Protozoa and Protophyta show us that form of disintegration called spontaneous fission: two or more individuals being produced by the splitting-up of the original one. The Volvox and the Hydrodictyon are plants which, having developed broods within themselves, give them exit by bursting; and among animals the one lately referred to which arises from the Distoma egg, entirely loses its individuality in the individualities of the numerous Distoma-larvæ with which it becomes filled. Speaking generally, the degree of disintegration becomes less marked as we approach the higher organic forms. Plants of superior types throw off from themselves, whether by gamogenesis or agamogenesis, parts that are relatively small; and among superior animals there is no case in which the parent individuality is habitually lost in the production of new individuals. To the last, however, there is of necessity a greater or less disintegration. The seeds and pollen-grains of a flowering plant are disintegrated portions of tissue; as are also the ova and spermatozoa of animals. And whether the fertilized germs carry away from their parents small or large quantities of nutriment, these quantities in all cases involve further negative or positive disintegrations of the parents.
Except in spore-producing plants, new individuals which result from agamogenesis usually do not separate from the parent-individuals until they have undergone considerable development, if not complete development. The agamogenetic offspring of those lowest organisms which develop centrally, do not, of course, pass beyond central structure; but the agamogenetic offspring of organisms which develop axially, commonly assume an axial structure before they become independent. The vegetal kingdom shows us this in the advanced organization of detached bulbils, and of buds that root themselves before separating. Of animals, the Hydrozoa, the Trematoda, and the Salpæ, present us with different kinds of agamogenesis, in all of which the new individuals are organized to a considerable extent before being cast off. This rule is not without exceptions, however. The statoblasts of the Plumatella (which play the part of winter eggs), developed in an unspecialized part of the body, furnish a case of metagenesis in which centres of development, instead of axes, are detached; and in the above-described parthenogenesis of moths and bees, such centres are detached from an ovarium.
When produced by gamogenesis, the new individuals become (in a morphological sense) independent of the parents while still in the shape of centres of development, rather than axes of development; and this even where the reverse is apparently the case. The fertilized germs of those inferior plants which are central, or multicentral, in their development, are of course thrown off as centres; and the same is usually the case even in those which are uniaxial or multiaxial. In the higher plants, of the two elements that go to the formation of the fertilized germ, the pollen-cell is absolutely separated from the parent-plant under the shape of a centre, and the egg-cell, though not absolutely separated from the parent, is still no longer subordinate to the organizing forces of the parent. So that when, after the egg-cell has been fertilized by matter from the pollen-tube, the development commences, it proceeds without parental control: the new individual, though remaining physically united with the old individual, becomes structurally and functionally separate: the old individual doing no more than supply materials. Throughout the animal kingdom, the new individuals produced by gamogenesis are obviously separated in the shape of centres of development wherever the reproduction is oviparous: the only conspicuous variation being in the quantity of nutritive matter bequeathed by the parent at the time of separation. And though, where the reproduction is viviparous, the process appears to be different, and in one sense is so, yet, intrinsically, it is the same. For in these cases the new individual really detaches itself from the parent while still only a centre of development; but instead of being finally cast off in this state it is re-attached, and supplied with nutriment until it assumes a more or less complete axial structure.
§ 77. As we have lately seen, the essential act in gamogenesis is the union of two cell-nuclei, produced in the great majority of cases by different parent organisms. Nearly always the containing cells, often called gametes, are unlike: the sperm-cell being the male product, and the germ-cell the female. But among some Protozoa and many of the lower Algæ and Fungi, the uniting cells show no differentiation. Sexuality is only nascent.
There are very many modes and modifications of modes in which these cells are produced; very many modes and modifications of modes by which they are brought into contact; and very many modes and modifications of modes by which the resulting fertilized germs have secured to them the fit conditions for their development. But passing over these divergent and re-divergent kinds of sexual multiplication, which it would take too much space here to specify, the one universal trait is this coalescence of a detached portion of one organism with a more or less detached portion of another.
Such simple Algæ as the Desmidieæ, which are sometimes called unicellular plants, show us a coalescence, not of detached portions of two organisms, but of two entire organisms: the entire contents of the individuals uniting to form the germ-mass. Where, as among the Confervoideæ, we have aggregated cells whose individualities are scarcely at all subordinate to that of the aggregate, the gamogenetic act is often effected by the union "of separate motile protoplasmic masses produced by the division of the contents of any cell of the aggregate. These free-swimming masses of protoplasm, which are quite similar to (but generally smaller than) the agamogenetic 'zoospores' of the same plants, and to the free-swimming individuals of many Protophyta, are apparently the primitive type of gametes (conjugating cells); but it is noteworthy that such a gamete nearly always unites with one derived from another cell or from another individual. The same fact holds with regard to the gametes of the Protophytes themselves, which are formed in the same way from the single cell of the mother individual. In the higher types of Confervoideæ, and in Vaucheria, we find these equivalent, free-swimming, gametes replaced by sexually differentiated sperm- and germ-cells, in some cases arising in different organs set apart for their production, and essentially representing those found in the higher plants. Transitional forms, intermediate between these and the cases where equivalent gametes are formed from any cell of the plant are also known."
Recent investigations concerning the conjugation of Protozoa have shown that there is not, as was at one time thought, a fusion of two individualities, but a fusion of parts of their nuclei. The macro-nucleus having disappeared, and the micro-nucleus having broken up into portions, each individual receives from the other one of these portions, which becomes fused with its own nuclear matter. So that even in these humble forms, where there is no differentiation of sexes, the union is not between elements that have arisen in the same individual but between those which have arisen in different individuals: the parts being in this case alike.
The marvellous phenomena initiated by the meeting of sperm-cell and germ-cell, or rather of their nuclei, naturally suggest the conception of some quite special and peculiar properties possessed by these cells. It seems obvious that this mysterious power which they display of originating a new and complex organism, distinguishes them in the broadest way from portions of organic substance in general. Nevertheless, the more we study the evidence the more are we led towards the conclusion that these cells are not fundamentally different from other cells. The first fact which points to this conclusion is the fact recently dwelt upon (§ 63), that in many plants and inferior animals, a small fragment of tissue which is but little differentiated, is capable of developing into an organism like that from which it was taken. This implies that the component units of tissues have inherent powers of arranging themselves into the forms of the organisms which originated them. And if in these component units, which we distinguished as physiological, such powers exist,--if, under fit conditions, and when not much specialized, they manifest such powers in a way as marked as that in which the contents of sperm-cells and germ-cells manifest them; then, it becomes clear that the properties of sperm-cells and germ-cells are not so peculiar as we are apt to assume. Again, the organs emitting sperm-cells and germ-cells have none of the specialities of structure which might be looked for, did sperm-cells and germ-cells need endowing with properties unlike those of all other organic agents. On the contrary, these reproductive centres proceed from tissues characterized by their low organization. In plants, for example, it is not appendages that have acquired considerable structure which produce the fructifying particles: these arise at the extremities of the axes where the degree of structure is the least. The cells out of which come the egg and the pollen-grains, are formed from undifferentiated tissue in the interior of the ovule and of the stamen. Among many inferior animals devoid of special reproductive organs, such as the Hydra, the ova and spermatozoa originate from the interstitial cells of the ectoderm, which lie among the bases of the functional cells--have not been differentiated for function; and in the Medusæ, according to Weismann, they arise in the homologous layer, save where the medusoid form remains attached, and then they arise in the endoderm and migrate to the ectoderm: lack of specialization being in all cases implied. Then in the higher animals these same generative agents appear to be merely modified epithelium-cells--cells not remarkable for their complexity of structure but rather for their simplicity. If, by way of demurrer to this view, it be asked why other epithelium-cells do not exhibit like properties; there are two replies. The first is that other epithelium-cells are usually so far changed to fit them to their special functions that they are unfitted for assuming the reproductive function. The second is that in some cases, where they are but little specialized, they do exhibit the like properties: not, indeed, by uniting with other cells to produce new germs but by producing new germs without such union. I learn from Dr. Hooker that the Begonia phyllomaniaca habitually develops young plants from the scales of its stem and leaves--nay, that many young plants are developed by a single scale. The epidermal cells composing one of these scales swell, here and there, into large globular cells; form chlorophyll in their interiors; shoot out rudimentary axes; and then, by spontaneous constrictions, cut themselves off; drop to the ground; and grow into Begonias. Moreover, in a succulent English plant, the Malaxis paludosa, a like process occurs: the self-detached cells being, in this case, produced by the surfaces of the leaves.[30] Thus, there is no warrant for the assumption that sperm-cells and germ-cells possess powers fundamentally unlike those of other cells. The inference to which the facts point, is, that they differ from the rest mainly in not having undergone functional adaptations. They are cells which have departed but little from the original and most general type: such specializations as some of them exhibit in the shape of locomotive appliances, being interpretable as extrinsic modifications which have reference to nothing beyond certain mechanical requirements. Sundry facts tend likewise to show that there does not exist the profound distinction we are apt to assume between the male and female reproductive elements. In the common polype sperm-cells and germ-cells are developed in the same layer of indifferent tissue; and in Tethya, one of the sponges, Prof. Huxley has observed that they occur mingled together in the general parenchyma. The pollen-grains and embryo-cells of plants arise in adjacent parts of the meristematic tissue of the flower-bud; and from the description of a monstrosity in the Passion-flower, recently given by Mr. Salter to the Linnæan Society, it appears both that ovules may, in their general structure, graduate into anthers, and that they may produce pollen in their interiors. Moreover, among the lower Algæ, which show the beginning of sexual differentiation, the smaller gametes, which we must regard as incipient sperm-cells, are sometimes able to fuse inter se, and give rise to a zygote which will produce a new plant. All which evidence is in perfect harmony with the foregoing conclusion; since, if sperm-cells and germ-cells have natures not essentially unlike those of unspecialized cells in general, their natures cannot be essentially unlike each other.
The next general fact to be noted is that these cells whose union constitutes the essential act of gamogenesis, are cells in which the developmental changes have come to a close--cells which are incapable of further evolution. Though they are not, as many cells are, unfitted for growth and metamorphosis by being highly specialized, yet they have lost the power of growth and metamorphosis. They have severally reached a state of equilibrium. And while the internal balance of forces prevents a continuance of constructive changes, it is readily overthrown by external destructive forces. For it almost uniformly happens that sperm-cells and germ-cells which are not brought in contact disappear. In a plant, the egg-cell, if not fertilized, is absorbed or dissipated, while the ovule aborts; and the unimpregnated ovum eventually decomposes: save, indeed, in those types in which parthenogenesis is a part of the normal cycle.
Such being the characters of these cells, and such being their fates if kept apart, we have now to observe what happens when they are united. In plants the extremity of the elongated pollen-cell applies itself to the surface of the embryo-sac, and one of its nuclei having, with some protoplasm, passed into the egg-cell, there becomes fused with the nucleus of the egg-cell. Similarly in animals, the spermatozoon passes through the limiting membrane of the ovum, and a mixture takes place between the substance of its nucleus and the substance of the nucleus of the ovum. But the important fact which it chiefly concerns us to notice, is that on the union of these reproductive elements there begins, either at once or on the return of favourable conditions, a new series of developmental changes. The state of equilibrium at which each had arrived is destroyed by their mutual influence, and the constructive changes, which had come to a close, recommence. A process of cell-multiplication is set up; and the resulting cells presently begin to aggregate into the rudiment of a new organism.
Thus, passing over the variable concomitants of gamogenesis, and confining our attention to what is constant in it, we see:--that there is habitually, if not universally, a fusion of two portions of organic substance which are either themselves distinct individuals, or are thrown off by distinct individuals; that these portions of organic substance, which are severally distinguished by their low degree of specialization, have arrived at states of structural quiescence or equilibrium; that if they are not united this equilibrium ends in dissolution; but that by the mixture of them this equilibrium is destroyed and a new evolution initiated.
§ 78. What are the conditions under which Genesis takes place? How does it happen that some organisms multiply by homogenesis and others by heterogenesis? Why is it that where agamogenesis prevails it is usually from time to time interrupted by gamogenesis? A survey of the facts discloses certain correlations which, if not universal, are too general to be without significance.
Where multiplication is carried on by heterogenesis we find, in numerous cases, that agamogenesis continues as long as the forces which result in growth are greatly in excess of the antagonist forces. Conversely, we find that the recurrence of gamogenesis takes place when the conditions are no longer so favourable to growth. In like manner where there is homogenetic multiplication, new individuals are usually not formed while the preceding individuals are still rapidly growing--that is, while the forces producing growth exceed the opposing forces to a great extent; but the formation of new individuals begins when nutrition is nearly equalled by expenditure. A few out of the many facts which seem to warrant these inductions must suffice.
The relation in plants between fructification and innutrition (or rather, between fructification and such diminished nutrition as makes growth relatively slow) was long ago asserted by a German biologist--Wolff, I am told. Since meeting with this assertion I have examined into the facts for myself. The result has been a conviction, strengthened by every inquiry, that some such relation exists. Uniaxial plants begin to produce their lateral, flowering axes, only after the main axis has developed the great mass of its leaves, and is showing its diminished nutrition by smaller leaves, or shorter internodes, or both. In multiaxial plants two, three, or more generations of leaf-bearing axes, or sexless individuals, are produced before any seed-bearing individuals show themselves. When, after this first stage of rapid growth and agamogenetic multiplication, some gamogenetic individuals arise, they do so where the nutrition is least;--not on the main axis, or on secondary axes, or even on tertiary axes, but on axes that are the most removed from the channels which supply nutriment. Again, a flowering axis is commonly less bulky than the others: either much shorter or, if long, much thinner. And further, it is an axis of which the terminal internodes are undeveloped: the foliar organs, which instead of becoming leaves become sepals, and petals, and stamens, follow each other in close succession, instead of being separated by portions of the still-growing axis. Another group of evidences meets us when we observe the variations of fruit-bearing which accompany variations of nutrition in the plant regarded as a whole. Besides finding, as above, that gamogenesis commences only when growth has been checked by extension of the remoter parts to some distance from the roots, we find that gamogenesis is induced at an earlier stage than usual by checking the nutrition. Trees are made to fruit while still quite small by cutting their roots or putting them into pots; and luxuriant branches which have had the flow of sap into them diminished, by what gardeners call "ringing," begin to produce flower-shoots instead of leaf-shoots. Moreover, it is to be remarked that trees which, by flowering early in the year, seem to show a direct relation between gamogenesis and increasing nutrition, really do the reverse; for in such trees the flower-buds are formed in the autumn. That structure which determines these buds into sexual individuals is given when the nutrition is declining. Conversely, very high nutrition in plants prevents, or arrests, gamogenesis. It is notorious that unusual richness of soil, or too large a quantity of manure, results in a continuous production of leaf-bearing or sexless shoots; and a like result happens when the cutting down of a tree, or of a large part of it, is followed by the sending out of new shoots: these, supplied with excess of sap, are luxuriant and sexless. Besides being prevented from producing sexual individuals by excessive nutrition, plants are, by excessive nutrition, made to change the sexual individuals they were about to produce, into sexless ones. This arrest of gamogenesis may be seen in various stages. The familiar instance of flowers made barren by the transformation of their stamens into petals, shows us the lowest degree of this reversed metamorphosis. Where the petals and stamens are partially changed into green leaves, the return towards the agamogenetic structure is more marked; and it is still more marked when, as occasionally happens in luxuriantly-growing plants, new flowering axes, and even leaf-bearing axes, grow out of the centres of flowers.[31] The anatomical structure of the sexual axis affords corroborative evidence: giving the impression, as it does, of an aborted sexless axis. Besides lacking those internodes which the leaf-bearing axis commonly possesses, the flowering axis differs by the absence of rudimentary lateral axes. In a leaf-bearing shoot the axil of every leaf usually contains a small bud, which may or may not develop into a lateral shoot; but though the petals of a flower are homologous with leaves, they do not bear homologous buds at their bases. Ordinarily, too, the foliar appendages of sexual axes are much smaller than those of sexless ones--the stamens and pistils especially, which are the last formed, being extremely dwarfed; and it may be that the absence of chlorophyll from the parts of fructification is a fact of like meaning. Moreover, the formation of the seed-vessel appears to be a direct consequence of arrested nutrition. If a gloved-finger be taken to represent a growing shoot, (the finger standing for the pith of the shoot and the glove for the peripheral layers of meristem and young tissue, in which the process of growth takes place); and if it be supposed that there is a diminished supply of material for growth; then, it seems a fair inference that growth will first cease at the apex of the axis, represented by the end of the glove-finger; and supposing growth to continue in those parts of the peripheral layers of young tissue that are nearer to the supply of nutriment, their further longitudinal extension will lead to the formation of a cavity at the extremity of the shoot, like that which results in a glove-finger when the finger is partially withdrawn and the glove sticks to its end. Whence it seems, both that this introversion of the apical meristem may be considered as due to failing nutrition, and that the ovules growing from its introverted surface (which would have been its outer surface but for the defective nutrition) are extremely aborted homologues of external appendages: both they and the pollen-grains being either morphologically or literally quite terminal, and the last showing by their dehiscence the exhaustion of the organizing power.[32]
Those kinds of animals which multiply by heterogenesis, present us with a parallel relation between the recurrence of gamogenesis and the recurrence of conditions checking rapid growth: at least, this is shown where experiments have thrown light on the connexion of cause and effect; namely, among the Aphides. These creatures, hatched from eggs in the spring, multiply by agamogenesis, which in this case is parthenogenesis, throughout the summer. When the weather becomes cold and plants no longer afford abundant sap, perfect males and females are produced; and from gamogenesis result fertilized ova. But beyond this evidence we have much more conclusive evidence. For it has been shown, both that the rapidity of the agamogenesis is proportionate to the warmth and nutrition, and that if the temperature and supply of food be artificially maintained, the agamogenesis continues through the winter. Nay more--it not only, under these conditions, continues through one winter, but it has been known to continue for four successive years: some forty or fifty sexless generations being thus produced. And those who have investigated the matter see no reason to doubt the indefinite continuance of this agamogenetic multiplication, so long as the external requirements are duly met. Evidence of another kind, complicated by special influences, is furnished by the heterogenesis of the Daphnia--a small crustacean commonly known as the Water-flea, which inhabits ponds and ditches. From the nature of its habitat this little creature is exposed to very variable conditions. Besides being frozen in winter, the small bodies of water in which it lives are often unduly heated by the summer Sun, or dried up by continued drought. The circumstances favourable to the Daphnia's life and growth, being thus liable to interruptions which, in our climate, have a regular irregularity of recurrence; we may, in conformity with the hypothesis, expect to find both that the gamogenesis recurs along with declining physical prosperity and that its recurrence is very variable. I use the expression "declining physical prosperity" advisedly; since "declining nutrition," as measured by supply of food, does not cover all the conditions. This is shown by the experiments of Weismann (abstracted for me by Mr. Cunningham) who found that in various Daphnideæ which bring forth resting eggs, sexual and asexual reproduction go on simultaneously, as well as separately, in the spring and summer: these variable results being adapted to variable conditions. For not only are these creatures liable to die from lack of food, from the winter's cold, and from the drying up of their ditches, &c., as well as from the over-heating of them, but during this period of over-heating they are liable to die from that deoxygenation of the water which heat causes. Manifestly the favourable and unfavourable conditions recurring in combinations that are rarely twice alike, cannot be met by any regularly recurring form of heterogenesis; and it is interesting to see how survival of the fittest has established a mixed form. In the spring, as well as in the autumn, there is in some cases a formation of resting or winter eggs; and evidently these provide against the killing off of the whole population by summer drought. Meanwhile, by ordinary males and females there is a production of summer eggs adapted to meet the incident of drying up by drought and subsequent re-supply of water. And all along successive generations of parthenogenetic females effect a rapid multiplication as long as conditions permit. Since life and growth are impeded or arrested not by lack of food only, but by other unfavourable conditions, we may understand how change in one or more of these may set up one or other form of genesis, and how the mixture of them may cause a mixed mode of multiplication which, originally initiated by external causes, becomes by inheritance and selection a trait of the species.[33] And then in proof that external causes initiate these peculiarities, we have the fact that in certain Daphnideæ "which live in places where existence and parthenogenesis are possible throughout the year, the sexual period has disappeared:" there are no males.
Passing now to animals which multiply by homogenesis--animals in which the whole product of a fertilized germ aggregates round a single centre or axis instead of round many centres or axes--we see, as before, that so long as the conditions allow rapid increase in the mass of this germ-product, the formation of new individuals by gamogenesis does not take place. Only when growth is declining in relative rate, do perfect sperm-cells and germ-cells begin to appear; and the fullest activity of the reproductive function arises as growth ceases: speaking generally, at least; for though this relation is tolerably definite in the highest orders of animals which multiply by gamogenesis, it is less definite in the lower orders. This admission does not militate against the hypothesis, as it seems to do; for the indefiniteness of the relation occurs where the limit of growth is comparatively indefinite. We saw (§ 46) that among active, hot-blooded creatures, such as mammals and birds, the inevitable balancing of assimilation by expenditure establishes, for each species, an almost uniform adult size; and among creatures of these kinds (birds especially, in which this restrictive effect of expenditure is most conspicuous), the connexion between cessation of growth and commencement of reproduction is distinct. But we also saw (§ 46) that where, as in the Crocodile and the Pike, the conditions and habits of life are such that expenditure does not overtake assimilation as size increases, there is no precise limit of growth; and in creatures thus circumstanced we may naturally look for a comparatively indeterminate relation between declining growth and commencing reproduction.[34] There is, indeed, among fishes, at least one case which appears very anomalous. The male parr, or young of the male salmon, a fish of four or five inches in length, is said to produce milt. Having, at this early stage of its growth, not one-hundredth of the weight of a full-grown salmon, how does its production of milt consist with the alleged general law? The answer must be in great measure hypothetical. If the salmon is (as it appears to be in its young state) a species of fresh-water trout that has contracted the habit of annually migrating to the sea, where it finds a food on which it thrives--if the original size of this species was not much greater than that of the parr (which is nearly as large as some varieties of trout)--and if the limit of growth in the trout tribe is very indefinite, as we know it to be; then we may reasonably infer that the parr has nearly the adult form and size which this species of trout had before it acquired its migratory habit; and that this production of milt is, in such case, a concomitant of the incipient decline of growth naturally arising in the species when living under the conditions of the ancestral species. Should this be so, the immense subsequent growth of the parr into the salmon, consequent on a suddenly-increased facility in obtaining food, removes to a great distance the limit at which assimilation is balanced by expenditure; and has the effect, analogous to that produced in plants, of arresting the incipient reproductive process. A confirmation of this view may be drawn from the fact that when the parr, after its first migration to the sea, returns to fresh water, having increased in a few months from a couple of ounces to five or six pounds, it no longer shows any fitness for propagation: the grilse, or immature salmon, does not produce milt or spawn.
We conclude, then, that the products of a fertilized germ go on accumulating by simple growth, so long as the forces whence growth results are greatly in excess of the antagonist forces; but that when diminution of the one set of forces or increase of the other, causes a considerable decline in this excess and an approach towards equilibrium, fertilized germs are again produced. Whether the germ-product be organized round one axis or round the many axes that arise by agamogenesis, matters not. Whether, as in the higher animals, this approach to equilibrium results from that disproportionate increase of expenditure entailed by increase of size; or whether, as in most plants and many inferior animals, it results from absolute or relative decline of nutrition; matters not. In any case the recurrence of gamogenesis is associated with a decrease in the excess of tissue-producing power. We cannot say, indeed, that this decrease always results in gamogenesis: some organisms multiply for an indefinite period by agamogenesis only. The Weeping Willow, which has been propagated throughout Europe, does not seed in Europe; and yet, as the Weeping Willow, by its large size and the multiplication of generation upon generation of lateral axes, presents the same causes of local innutrition as other trees, we cannot ascribe the absence of sexual axes to the continued predominance of nutrition. Among animals, too, the anomalous case of the Tineidæ, a group of moths in which parthenogenetic multiplication goes on for generation after generation, seems to imply that gamogenesis does not necessarily result from an approximate balance of assimilation by expenditure. What we must say is that an approach towards equilibrium between the forces which cause growth and the forces which oppose growth, is the chief condition to the recurrence of gamogenesis; but that there appear to be other conditions, in the absence of which approach to equilibrium is not followed by gamogenesis.
§ 79. The above induction is an approximate answer to the question--When does gamogenesis recur? but not to the question which was propounded--Why does gamogenesis recur?--Why cannot multiplication be carried on in all cases, as it is in many cases, by agamogenesis? As already said, biologic science is not yet advanced enough to reply. Meanwhile, the evidence above brought together suggests a certain hypothetical answer.
Seeing, on the one hand, that gamogenesis recurs only in individuals which are approaching a state of organic equilibrium; and seeing, on the other hand, that the sperm-cells and germ-cells thrown off by such individuals are cells in which developmental changes have ended in quiescence, but in which, after their union, there arises a process of active cell-formation; we may suspect that the approach towards a state of general equilibrium in such gamogenetic individuals, is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell and germ-cell is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ--a result probably effected by mixing the slightly different physiological units of slightly different individuals. The several arguments which support this view, cannot be satisfactorily set forth until after the topics of Heredity and Variation have been dealt with. Leaving it for the present, I propose hereafter to re-consider it in connexion with sundry others raised by the phenomena of Genesis.
But before ending the chapter, it may be well to note the relations between these different modes of multiplication, and the conditions of existence under which they are respectively habitual. While the explanation of the teleologist is untrue, it is often an obverse to the truth; for though, on the hypothesis of Evolution, it is clear that things are not arranged thus or thus for the securing of special ends, it is also clear that arrangements which do secure these special ends tend to establish themselves--are established by their fulfilment of these ends. Besides insuring a structural fitness between each kind of organism and its circumstances, the working of "natural selection" also insures a fitness between the mode and rate of multiplication of each kind of organism and its circumstances. We may, therefore, without any teleological implication, consider the fitness of homogenesis and heterogenesis to the needs of the different classes of organisms which exhibit them.
Heterogenesis prevails among organisms of which the food, though abundant compared with their expenditure, is dispersed in such a way that it cannot be appropriated in a wholesale manner. Protophyta, subsisting on diffused gases and decaying organic matter in a state of minute subdivision, and Protozoa, to which food comes in the shape of extremely small floating particles, are enabled, by their rapid agamogenetic multiplication, to obtain materials for growth better than they would do did they not thus continually divide and disperse in pursuit of it. The higher plants, having for nutriment the carbonic acid of the air and certain mineral components of the soil, show us modes of multiplication adapted to the fullest utilization of these substances. A herb with but little power of forming the woody fibre requisite to make a stem that can support wide-spreading branches, after producing a few sexless axes produces sexual ones; and maintains its race better, by the consequent early dispersion of seeds, than by a further production of sexless axes. But a tree, able to lift its successive generations of sexless axes high into the air, where each gets carbonic acid and light almost as freely as if it grew by itself, may with advantage go on budding-out sexless axes year after year; since it thereby increases its subsequent power of budding-out sexual axes. Meanwhile it may advantageously transform into seed-bearers those axes which, in consequence of their less direct access to materials absorbed by the roots, are failing in their nutrition; for it thus throws off from a point at which sustenance is deficient, a migrating group of germs that may find sustenance elsewhere. The heterogenesis displayed by animals of the Coelenterate type has evidently a like utility. A polype, feeding on minute annelids and crustaceans which, flitting through the water, come in contact with its tentacles, and limited to that quantity of prey which chance brings within its grasp, buds out young polypes which, either as a colony or as dispersed individuals, spread their tentacles through a larger space of water than the parent alone can; and by producing them, the parent better insures the continuance of its species than it would do if it went on slowly growing until its nutrition was nearly balanced by its waste, and then multiplied by gamogenesis. Similarly with the Aphis. Living on sap sucked from tender shoots and leaves, and able thus to take in but a very small quantity in a given time, this creature's race is more likely to be preserved by a rapid asexual propagation of small individuals, which disperse themselves over a wide area of nutrition, than it would be did the individual growth continue so as to produce large individuals multiplying sexually. And then when autumnal cold and diminishing supply of sap put a check to growth, the recurrence of gamogenesis, or production of fertilized ova which remain dormant through the winter, is more favourable to the preservation of the race than would be a further continuance of agamogenesis. On the other hand, among the higher animals living on food which, though dispersed, is more or less aggregated into large masses, this alternation of gamic and agamic reproduction ceases to be useful. The development of the germ-product into a single organism of considerable bulk, is in many cases a condition without which these large masses of nutriment could not be appropriated; and here the formation of many individuals instead of one would be fatal. But we still see the beneficial results of the general law--the postponement of gamogenesis until the rate of growth begins to decline. For so long as the rate of growth continues rapid, there is proof that the organism gets food with facility--that expenditure does not seriously check accumulation; and that the size reached is as yet not disadvantageous: or rather, indeed, that it is advantageous. But when the rate of growth is much decreased by the increase of expenditure--when the excess of assimilative power is diminishing so fast as to indicate its approaching disappearance--it becomes needful, for the maintenance of the species, that this excess shall be turned to the production of new individuals; since, did growth continue until there was a complete balancing of assimilation and expenditure, the production of new individuals would be either impossible or fatal to the parent. And it is clear that "natural selection" will continually tend to determine the period at which gamogenesis commences, in such a way as most favours the maintenance of the race.
Here, too, may fitly be pointed out the fact that, by "natural selection," there will in every case be produced the most advantageous proportion of males and females. If the conditions of life render numerical inequality of the sexes beneficial to the species, in respect either of the number of the offspring or the character of the offspring; then, those varieties of the species which approach more than other varieties towards this beneficial degree of inequality, will be apt to supplant other varieties. And conversely, where equality in the number of males and females is beneficial, the equilibrium will be maintained by the dying out of such varieties as produce offspring among which the sexes are not balanced.
NOTE.--Such alterations of statement in this chapter as have been made necessary by the advance of biological knowledge since 1864 have not, I think, tended to invalidate its main theses, but have tended to verify them. Some explanations to be here added may remove remaining difficulties.
Certain types, which are transitional between Protozoa and Metazoa, exhibit under its simplest form the relation between self-maintenance and race-maintenance--the integration primarily effecting the one and the disintegration primarily effecting the other. Among the Mycetozoa a number of amoeba-like individuals aggregate into what is called a plasmodium; and while, in some orders, they become fused into a mass of protoplasm through which their nuclei are dispersed, in other orders (Sorophora) they retain their individualities and simply form a coherent aggregate. These last, presumably the earliest in order of evolution, remain united so long as the plasmodium, having a small power of locomotion, furthers the general nutrition; but when this is impeded by drought or cold, there arise spores. Each spore contains an amoeboid individual; and this, escaping when favourable conditions return, establishes by fission and by union with others like itself a new colony or plasmodium. Reduced to its lowest terms, we here see the antagonism between that growth of the coherent mass of units which accompanies its physical prosperity, and that incoherence and dispersion of the units which follows unfavourable conditions and arrest of growth, and which presently initiates new plasmodia.
This antagonism, seen in these incipient Metazoa which show us none of that organization characterizing the Metazoa in general, is everywhere in more or less disguised forms exhibited by them--must necessarily be so if growth of the individual is a process of integration while formation of new individuals is a process of disintegration. And, primarily, it is an implication that whatever furthers the one impedes the other.
But now while recognizing the truth that nutrition and innutrition (using these words to cover not supply of nutriment only but the presence of other influences favourable or unfavourable to the vital processes) primarily determine the alternations of these; we have also to recognize the truth that from the beginning survival of the fittest has been shaping the forms and effects of their antagonism. By inheritance a physiological habit which modifies the form of the antagonism in a way favourable to the species, will become established. Especially will this be the case where the lives of the individuals have become relatively definite and where special organs have been evolved for casting off reproductive centres. The resulting physiological rhythm may in such cases become so pronounced as greatly to obscure the primitive relation. Among plants we see this in the fact that those which have been transferred from one habitat to another having widely different seasons, long continue their original time of flowering, though it is inappropriate to the new circumstances--the reproductive periodicity has become organic. Similarly in each species of higher animal, development of the reproductive organs and maturation of reproductive cells take place at a settled age, whether the conditions have been favourable or unfavourable to physical prosperity. The established constitutional tendency, adapted to the needs of the species, over-rides the constitutional needs of the individual.
Even here, however, the primitive antagonism, though greatly obscured, occasionally shows itself. Instance the fact that in plants where gamogenesis is commencing a sudden access of nutrition will cause resumption of agamogenesis; and I suspect that an illustration may be found among human beings in the earlier establishment of the reproductive function among the ill-fed poor than among the well-fed rich.
One other qualification has to be added. In plants and animals which have become so definitely constituted that at an approximately fixed stage, the proclivity towards the production of new individuals becomes pronounced, it naturally happens that good nutrition aids it. Surplus nutriment being turned into the reproductive channel, the reproduction is efficient in proportion as the surplus is great. Hence the fact that in fruit trees which have reached the flowering stage, manuring has the effect that though it does not increase the quantity of blossoms it increases the quantity of fruit; and hence the fact that well-fed and easy-living races of men are prolific.
VIII. HEREDITY
§ 80. Already, in the last two chapters, the law of hereditary transmission has been tacitly assumed; as, indeed, it unavoidably is in all such discussions. Understood in its entirety, the law is that each plant or animal, if it reproduces, gives origin to others like itself: the likeness consisting, not so much in the repetition of individual traits as in the assumption of the same general structure. This truth has been rendered so familiar by daily illustration as almost to have lost its significance. That wheat produces wheat--that existing oxen have descended from ancestral oxen--that every unfolding organism eventually takes the form of the class, order, genus, and species from which it sprang; is a fact which, by force of repetition, has acquired in our minds almost the aspect of a necessity. It is in this, however, that Heredity is principally displayed: the manifestations of it commonly referred to being quite subordinate. And, as thus understood, Heredity is universal. The various instances of heterogenesis lately contemplated seem, indeed, to be at variance with this assertion. But they are not really so. Though the recurrence of like forms is, in these instances, not direct but cyclical, still, the like forms do recur; and, when taken together, the group of forms produced during one of the cycles is as much like the groups produced in preceding cycles, as the single individual arising by homogenesis is like ancestral individuals.
While, however, the general truth that organisms of a given type uniformly descend from organisms of the same type, is so well established by infinite illustrations as to have assumed the character of an axiom; it is not universally admitted that non-typical peculiarities are inherited. Many entertain a vague belief that the law of Heredity applies only to main characters of structure and not to details; or, at any rate, that though it applies to such details as constitute differences of species, it does not apply to smaller details. The circumstance that the tendency to repetition is in a slight degree qualified by the tendency to variation (which, as we shall hereafter see, is but an indirect result of the tendency to repetition), leads some to doubt whether Heredity is unlimited. A careful weighing of the evidence, however, and a due allowance for the influences by which the minuter manifestations of Heredity are obscured, may remove this scepticism.
First in order of importance comes the fact that not only are there uniformly transmitted from an organism to its offspring, those traits of structure which distinguish the class, order, genus, and species; but also those which distinguish the variety. We have numerous cases, among both plants and animals, where, by natural or artificial conditions, there have been produced divergent modifications of the same species; and abundant proof exists that the members of any one sub-species habitually transmit their distinctive peculiarities to their descendants. Agriculturists and gardeners can furnish unquestionable illustrations. Several varieties of wheat are known, of which each reproduces itself. Since the potato was introduced into England there have been formed from it a number of sub-species; some of them differing greatly in their forms, sizes, qualities, and periods of ripening. Of peas, also, the like may be said. And the case of the cabbage-tribe is often cited as showing the permanent establishment of races which have diverged widely from a common stock. Among fruits and flowers the multiplication of kinds, and the continuance of each kind with certainty by agamogenesis, and to some extent by gamogenesis, might be exemplified without end. From all sides evidence may be gathered showing a like persistence of varieties among animals. We have our distinct breeds of sheep, our distinct breeds of cattle, our distinct breeds of horses: each breed maintaining its characteristics. The many sorts of dogs which, if we accept the physiological test, we must consider as all of one species, show us in a marked manner the hereditary transmission of small differences--each sort, when kept pure, reproducing itself not only in size, form, colour, and quality of hair, but also in disposition and speciality of intelligence. Poultry, too, have their permanently-established races. And the Isle of Man sends us a tail-less kind of cat. Even in the absence of other evidence, that which ethnology furnishes would suffice. Grant them to be derived from one stock, and the varieties of man yield proof upon proof that non-specific traits of structure are bequeathed from generation to generation. Or grant only their derivation from several stocks, and we still have, between races descended from a common stock, distinctions which prove the inheritance of minor peculiarities. Besides seeing the Negroes continue to produce Negroes, copper-coloured men to produce men of a copper colour, and the fair-skinned races to perpetuate their fair skins--besides seeing that the broad-faced and flat-nosed Calmuck begets children with broad faces and flat noses, while the Jew bequeaths to his offspring the features which have so long characterized Jews; we see that those small unlikenesses which distinguish more nearly-allied varieties of men, are maintained from generation to generation. In Germany, the ordinary shape of skull is appreciably different from that common in Britain: near akin though the Germans are to the British. The average Italian face continues to be unlike the faces of northern nations. The French character is now, as it was centuries ago, contrasted in sundry respects with the characters of neighbouring peoples. Nay, even between races so closely allied as the Scotch Celts, the Welsh Celts, and the Irish Celts, appreciable differences of form and nature have become established.
The fact that sub-species and sub-sub-species thus exemplify the general law of inheritance which shows itself in the perpetuation of ordinal, generic, and species peculiarities, is strong reason for the belief that this general lay is unlimited in its application. This has the support of still more special evidences. They are divisible into two classes. In the one come cases where congenital peculiarities, not traceable to any obvious causes, are bequeathed to descendants. In the other come cases where the peculiarities thus bequeathed are not congenital, but have resulted from changes of functions during the lives of the individuals bequeathing them. We will consider first the cases that come in the first class.
§ 81. Note at the outset the character of the chief testimony. Excluding those inductions that have been so fully verified as to rank with exact science, there are no inductions so trustworthy as those which have undergone the mercantile test. When we have thousands of men whose profit or loss depends on the truth of their inferences from perpetually-repeated observations; and when we find that their inferences, handed down from generation to generation, have generated an unshakable conviction; we may accept it without hesitation. In breeders of animals we have such a class, led by such experiences, and entertaining such a conviction--the conviction that minor peculiarities of organization are inherited as well as major peculiarities. Hence the immense prices given for successful racers, bulls of superior forms, sheep that have certain desired peculiarities. Hence the careful record of pedigrees of high-bred horses and sporting dogs. Hence the care taken to avoid intermixture with inferior stocks. As quoted by Mr. Darwin, Youatt says the principle of selection "enables the agriculturist not only to modify the character of his flock but to change it altogether." Lord Somerville, speaking of what breeders have done for sheep, says:--"It would seem that they have chalked upon a wall a form perfect in itself and then given it existence." That most skilful breeder, Sir John Sebright, used to say, with respect to pigeons, that "he would produce any given feather in three years, but it would take him six years to obtain head and beak." In all which statements the tacit assertion is, that individual traits are bequeathed from generation to generation, and may be so perpetuated and increased as to become permanent distinctions.
Of special instances there are many besides that of the often-cited Otto-breed of sheep, descended from a single short-legged lamb, and that of the six-fingered Gratio Kelleia, who transmitted his peculiarity, in different degrees, to several of his children and to some of his grandchildren. In a paper contributed to the Edinburgh New Philosophical Journal for July, 1863, Dr. (now Sir John) Struthers gives cases of hereditary digital variations. Esther P----, who had six fingers on one hand, bequeathed this malformation along some lines of her descendants for two, three, and four generations. A---- S---- inherited an extra digit on each hand and each foot from his father; and C---- G----, who also had six fingers and six toes, had an aunt and a grandmother similarly formed. A collection of evidence published by Mr. Sedgwick in the Medico-Chirurgical Review for April and for July, 1863, in two articles on "The Influence of Sex in limiting Hereditary Transmission," includes the following cases:--Augustin Duforet, a pastry-cook of Douai, who had but two instead of three phalanges to all his fingers and toes, inherited this malformation from his grandfather and father, and had it in common with an uncle and numerous cousins. An account has been given by Dr. Lepine, of a man with only three fingers on each hand and four toes on each foot, and whose grandfather and son exhibited the like anomaly. Béchet describes Victoire Barré as a woman who, like her father and sister, had but one developed finger on each hand and but two toes on each foot, and whose monstrosity re-appeared in two daughters. And there is a case where the absence of two distal phalanges on the hands was traced for two generations. The various recorded instances in which there has been transmission from one generation to another, of webbed-fingers, of webbed-toes, of hare-lip, of congenital luxation of the thigh, of absent patellæ, of club-foot, &c., would occupy more space than can here be spared. Defects in the organs of sense are also not unfrequently inherited. Four sisters, their mother, and grandmother, are described by Duval as similarly affected by cataract. Prosper Lucas details an example of amaurosis affecting the females of a family for three generations. Duval, Graffe, Dufon, and others testify to like cases coming under their observation.[35] Deafness, too, is occasionally transmitted from parent to child. There are deaf-mutes whose imperfections have been derived from ancestors; and malformations of the external ears have also been perpetuated in offspring. Of transmitted peculiarities of the skin and its appendages, many cases have been noted. One is that of a family remarkable for enormous black eyebrows; another that of a family in which every member had a lock of hair of a lighter colour than the rest on the top of the head; and there are also instances of congenital baldness being hereditary. From one of our leading sculptors I learn that his wife has a flat mole under the foot near the little toe, and one of her sons has the same. Entire absence of teeth, absence of particular teeth, and anomalous arrangements of teeth, are recorded as traits that have descended to children. And we have evidence that soundness and unsoundness of teeth are transmissible.
The inheritance of tendencies to such diseases as gout, consumption, and insanity is universally admitted. Among the less-common diseases of which the descent has been observed, are ichthyosis, leprosy, pityriasis, sebaceous tumours, plica polonica, dipsomania, somnambulism, catalepsy, epilepsy, asthma, apoplexy, elephantiasis. General nervousness displayed by parents almost always re-appears in their children. Even a bias towards suicide appears to be sometimes hereditary.
§ 82. To prove the transmission of those structural peculiarities which have resulted from functional peculiarities, is, for several reasons, comparatively difficult. Changes produced in the sizes of parts by changes in their amounts of action, are mostly unobtrusive. A muscle which has increased in bulk is usually so obscured by natural or artificial clothing, that unless the alteration is extreme it passes without remark. Such nervous developments as are possible in the course of a single life, cannot be seen externally. Visceral modifications of a normal kind are observable but obscurely, or not at all. And if the changes of structure worked in individuals by changes in their habits are thus difficult to trace, still more difficult to trace must be the transmission of them: further hidden, as this is, by the influences of other individuals who are often otherwise modified by other habits. Moreover, such specialities of structure as are due to specialities of function, are usually entangled with specialities of structure which are, or may be, due to selection, natural or artificial. In most cases it is impossible to say that a structural peculiarity which seems to have arisen in offspring from a functional peculiarity in a parent, is wholly independent of some congenital peculiarity of structure in the parent, whence this functional peculiarity arose. We are restricted to cases with which natural or artificial selection can have had nothing to do, and such cases are difficult to find. Some, however, may be noted.
A species of plant that has been transferred from one soil or climate to another, frequently undergoes what botanists call "change of habit"--a change which, without affecting its specific characters, is yet conspicuous. In its new locality the species is distinguished by leaves that are much larger or much smaller, or differently shaped, or more fleshy; or instead of being as before comparatively smooth, it becomes hairy; or its stem becomes woody instead of being herbaceous; or its branches, no longer growing upwards, assume a drooping character. Now these "changes of habit" are clearly determined by functional changes. Occurring, as they do, in many individuals which have undergone the same transportation, they cannot be classed as "spontaneous variations." They are modifications of structure consequent on modifications of function that have been produced by modifications in the actions of external forces. And as these modifications re-appear in succeeding generations, we have, in them, examples of functionally-established variations that are hereditarily transmitted.
Evidence of analogous changes in animals is difficult to disentangle. Only among domesticated kinds have we any opportunity of tracing the results of altered habits; and here, in nearly all cases, artificial selection has obscured them. Still, there are some facts which seem to the point. Mr. Darwin, while ascribing almost wholly to "natural selection" the production of those modifications which eventuate in differences of species, nevertheless admits the effects of use and disuse. He says--"I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild duck; and I presume that this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parent. The great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with the state of these organs in other countries, is another instance of the effect of use. Not a single domestic animal can be named which has not in some country drooping ears; and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable." Again--"The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection." ... "It is well known that several animals belonging to the most different classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs the footstalk of the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse."[36] The direct inheritance of an acquired peculiarity is sometimes observable. Mr. Lewes gives a case. He "had a puppy taken from its mother at six weeks old, who, although never taught 'to beg' (an accomplishment his mother had been taught), spontaneously took to begging for everything he wanted when about seven or eight months old: he would beg for food, beg to be let out of the room, and one day was found opposite a rabbit hutch begging for rabbits." Instances are on record, too, of sporting dogs which spontaneously adopted in the field, certain modes of behaviour which their parents had learnt.
But the best examples of inherited modifications produced by modifications of function, occur in mankind. To no other cause can be ascribed the rapid metamorphoses undergone by the British races when placed in new conditions. In the United States the descendants of the immigrant Irish lose their Celtic aspect, and become Americanized. This cannot be ascribed to mixture, since the feeling with which Irish are regarded by Americans prevents any considerable amount of intermarriage. Equally marked is the case of the immigrant Germans who, though they keep very much apart, rapidly assume the prevailing type. To say that "spontaneous variation" increased by natural selection, can have produced this effect, is going too far. Peoples so numerous cannot have been supplanted in the course of two or three generations by varieties springing from them. Hence the implication is that physical and social conditions have wrought modifications of function and structure, which offspring have inherited and increased. Similarly with special cases. In the Cyclopædia of Practical Medicine, Vol. II., p. 419, Dr. Brown states that he "has in many instances observed in the case of individuals whose complexion and general appearance has been modified by residence in hot climates, that children born to them subsequently to such residence, have resembled them rather in their acquired than primary mien."
Some visible modifications of organs caused by changes in their functions, may be noted. That large hands are inherited by those whose ancestors led laborious lives, and that those descended from ancestors unused to manual labour commonly have small hands, are established opinions. It seems very unlikely that in the absence of any such connexion, the size of the hand should have come to be generally regarded as some index of extraction. That there exists a like relation between habitual use of the feet and largeness of the feet, we have strong evidence in the customs of the Chinese. The torturing practice of artificially arresting the growth of the feet, could never have become established among the ladies of China, had they not seen that a small foot was significant of superior rank--that is of a luxurious life--that is of a life without bodily labour. There is evidence, too, that modifications of the eyes, caused by particular uses of the eyes, are inherited. Short sight appears to be uncommon among peasants; but it is frequent among classes who use their eyes much for reading and writing, and is often congenital. Still more marked is this relation in Germany. There, the educated are notoriously studious, and judging from the numbers of young Germans who wear spectacles, there is reason to think that congenital myopia is very frequent among them.
Some of the best illustrations of functional heredity, are furnished by mental characteristics. Certain powers which mankind have gained in the course of civilization cannot, I think, be accounted for without admitting the inheritance of acquired modifications. The musical faculty is one of these. To say that "natural selection" has developed it by preserving the most musically endowed, seems an inadequate explanation. Even now that the development and prevalence of the faculty have made music an occupation by which the most musical can get sustenance and bring up families; it is very questionable whether, taking the musical career as a whole, it has any advantage over other careers in the struggle for existence and multiplication. Still more if we look back to those early stages through which the faculty must have passed before definite perception of melody was arrived at, we fail to see how those possessing the rudimentary faculty in a somewhat greater degree than the rest, would thereby be enabled the better to maintain themselves and their children. There is no explanation but that the habitual association of certain cadences of speech with certain emotions, has slowly established in the race an organized and inherited connection between such cadences and such emotions; that the combination of such cadences, more or less idealized, which constitutes melody, has all along had a meaning in the average mind, only because of the meaning which cadences had acquired in the average mind; and that by the continual hearing and practice of melody there has been gained and transmitted an increasing musical sensibility. Confirmation of this view may be drawn from individual cases. Grant that among a people endowed with musical faculty to a certain degree, spontaneous variation will occasionally produce men possessing it in a higher degree; it cannot be granted that spontaneous variation accounts for the frequent production, by such highly-endowed men, of men still more highly endowed. On the average, the children of marriages with others not similarly endowed, will be less distinguished rather than more distinguished. The most that can be expected is that this unusual amount of faculty shall re-appear in the next generation undiminished. How then shall we explain cases like those of Bach, Mozart, and Beethoven, all of them sons of men having unusual musical powers who were constantly exercising those powers, and who greatly excelled their fathers in their musical powers? What shall we say to the facts that Haydn was the son of an organist, that Hummel was born to a music master, and that Weber's father was a distinguished violinist? The occurrence of so many cases in one nation within a short period of time, cannot rationally be ascribed to the coincidence of "spontaneous variations." It can be ascribed to nothing but inherited developments of structure caused by augmentations of function.
But the clearest proof that structural alterations caused by alterations of function are inherited, occurs when the alterations are morbid. I had originally named in this place the results of M. Brown-Sequard's experiments on guinea-pigs, showing that those which had been artificially made epileptic had offspring which were epileptic; and I name them again though his inference is by many rejected. For, as exemplified a few pages back, strong evidence is often disregarded for trivial reasons by those who dislike the conclusion drawn. Just naming this evidence and its possible invalidity, let me pass to some results of experiences recently set forth by Dr. Savage, President of the Neurological Society. In an essay on "Heredity and Neurosis" published in Brain, Parts LXXVII, LXXVIII, 1897, he says:--"We recognise the transmission of a tendency to develop gout, and we recognise that the disease produced by the individual himself differs little from that which may have been inherited." [That is, acquired gout may be transmitted as constitutional gout.] "I have seen several patients whose history I have been able to examine carefully, in whom mental tricks have been transmitted from one generation to another." In the "musical prodigies" descending from musical parents, "there seemed to be a transmission of a greatly increased aptitude or tendency which is all one is contending for." "Though there is, in my opinion, power to transmit acquired peculiarities, yet the tendency is to transmit a predisposition." (pp. 19-21.) And an authority on nervous diseases who is second to none--Dr. Hughlings Jackson--takes the same view. The liability to consumption shown by children of consumptive parents, which no one doubts, shows us the same thing. It is admitted that consumption may be produced by conditions very unfavourable to life; and unless it is held that the disease so produced differs from the disease when inherited, the conclusion must be that here, too, there is a transmission of functionally-produced organic changes. This holds true whether the production of tubercle is due to innate defect or whether it is due to the invasion of a bacillus. For in this last case the consumptive diathesis must be regarded as a state of body more than usually liable to invasion by the bacillus, and this is the same when acquired as when transmitted.
§ 83. Two modified manifestations of Heredity remain to be noticed. The one is the re-appearance in offspring of traits not borne by the parents, but borne by the grandparents or by remoter ancestors. The other is the limitation of Heredity by sex--the restriction of transmitted peculiarities to offspring of the same sex as the parent possessing them.
Atavism, which is the name given to the recurrence of ancestral traits, is proved by many and varied facts. In the picture-galleries of old families, and on the monumental brasses in the adjacent churches, are often seen types of feature which are still, from time to time, repeated in members of these families. It is a matter of common remark that some constitutional diseases, such as gout and insanity, after missing a generation, will show themselves in the next. Dr. Struthers, in his above-quoted paper "On Variation in the Number of Fingers and Toes, and in the Phalanges in Man," gives cases of malformations common to grandparent and grandchild, but of which the parent had no trace. M. Girou (as quoted by Mr. Sedgwick) says--"One is often surprised to see lambs black, or spotted with black, born of ewes and rams with white wool, but if one takes the trouble to go back to the origin of this phenomena, it is found in the ancestors." Instances still more remarkable, in which the remoteness of the ancestors copied is very great, are given by Mr. Darwin. He points out that in crosses between varieties of the pigeon, there will sometimes re-appear the plumage of the original rock-pigeon, from which these varieties descended; and he thinks the faint zebra-like markings occasionally traceable in horses have probably a like meaning.
The other modified manifestation of heredity above referred to is the limitation of heredity by sex. In Mr. Sedgwick's essays, already named, will be found evidence implying that there exists some such tendency to limitation, which does or does not show itself distinctly according to the nature of the organic modification to be conveyed. On joining to the evidence he gives certain bodies of allied evidence we shall, I think, find the inconsistences comprehensible.
Beyond the familiar facts that in ourselves, along with the essential organs of sex there go minor structures and traits distinctive of sex, such as the beard and the voice in man, we have numerous cases in which, along with different sex-organs there go general differences, sometimes immense and often conspicuous. We have those in which (as in sundry parasites) the male is extremely small compared with the female; we have those in which the male is winged and the female wingless; we have those, as among birds, in which the plumage of males contrasts strongly with that of females; and among butterflies we have kindred instances in which the wings of the two sexes are wholly unlike--some, indeed, in which there is not simply dimorphism but polymorphism: two kinds of females both differing from the male. How shall we range these facts with the ordinary facts of inheritance? Without difficulty if heredity results from the proclivity which the component units contained in a germ-cell or a sperm-cell have to arrange themselves into a structure like that of the structure from which they were derived. For the obvious corollary is that where there is gamogenesis there will result partly concurring and partly conflicting proclivities. In the fertilized germ we have two groups of physiological units, slightly different in their structures. These slightly-different units severally multiply at the expense of the nutriment supplied to the unfolding germ--each kind moulding this nutriment into units of its own type. Throughout the process of development the two kinds of units, mainly agreeing in their proclivities and in the form which they tend to build themselves into, but having minor differences, work in unison to produce an organism of the species from which they were derived, but work in antagonism to produce copies of their respective parent-organisms. And hence ultimately results an organism in which traits of the one are mixed with traits of the other; and in which, according to the predominance of one or other group of units, one or other sex with all its concomitants is produced.
If so, it becomes comprehensible that with the predominance of either group, and the production of the same sex as that of the parent whence it was derived, there will go the repetition not only of the minor sex-traits of that parent but also of any peculiarities he or she possessed, such as monstrosities. Since the two groups are nearly balanced, and since inheritance is never an average of the two parents but a mixture of traits of the one with traits of the other, it is not difficult to see why there should be some irregularity in the transmission of these monstrosities and constitutional tendencies, though they are most frequently transmitted only to those of the same sex.[37]
§ 84. Unawares in the last paragraph there has been taken for granted the truth of that suggestion concerning Heredity ventured in § 66. Anything like a positive explanation is not to be expected in the present stage of Biology, if at all. We can look for nothing beyond a simplification of the problem; and a reduction of it to the same category with certain other problems which also admit of hypothetical solutions only. If an hypothesis which sundry widespread phenomena have already thrust upon us, can be shown to render the phenomena of Heredity more intelligible than they at present seem, we shall have reason to entertain it. The applicability of any method of interpretation to two different but allied classes of facts, is evidence of its truth.
The power which many animals display of reproducing lost parts, we saw to be inexplicable except on the assumption that the units of which any organism is built have a tendency to arrange themselves into the shape of that organism (§ 65). This power is sufficiently remarkable in cases where a lost limb or tail is replaced, but it is still more remarkable in cases where, as among some annelids, the pieces into which an individual is cut severally complete themselves by developing heads and tails, or in cases like that of the Holothuria, which having, when alarmed, ejected its viscera, reproduces them. Such facts compel us to admit that the components of an organism have a proclivity towards a special structure--that the adult organism when mutilated exhibits that same proclivity which is exhibited by the young organism in the course of its normal development. As before said, we may, for want of a better name, figuratively call this power organic polarity: meaning by this phrase nothing more than the observed tendency towards a special arrangement. And such facts as those presented by the fragments of a Hydra, and by fragments of leaves from which complete plants are produced, oblige us to recognize this proclivity as existing throughout the tissues in general--nay, in the case of the Begonia phyllomaniaca, obliges us to recognize this proclivity as existing in the physiological units contained in each undifferentiated cell. Quite in harmony with this conclusion, are certain implications since noticed, respecting the characters of sperm-cells and germ-cells. We saw sundry reasons for rejecting the supposition that these are highly-specialized cells and for accepting the opposite supposition, that they are cells differing from others rather in being unspecialized. And here the assumption to which we seem driven by the ensemble of the evidence, is, that sperm-cells and germ-cells are essentially nothing more than vehicles in which are contained small groups of the physiological units in a fit state for obeying their proclivity towards the structural arrangement of the species they belong to.
If the likeness of offspring to parents is thus determined, it becomes manifest, à priori, that besides the transmission of generic and specific peculiarities, there will be a transmission of those individual peculiarities which, arising without assignable causes, are classed as "spontaneous." For if the assumption of a special arrangement of parts by an organism, is due to the proclivity of its physiological units towards that arrangement; then the assumption of an arrangement of parts slightly different from that of the species, implies physiological units slightly unlike those of the species; and these slightly-unlike physiological units, communicated through the medium of sperm-cell or germ-cell, will tend, in the offspring, to build themselves into a structure similarly diverging from the average of the species.
But it is not equally manifest that, on this hypothesis, alterations of structure caused by alterations of function must be transmitted to offspring. It is not obvious that change in the form of a part, caused by changed action, involves such change in the physiological units throughout the organism that these, when groups of them are thrown off in the shape of reproductive centres, will unfold into organisms that have this part similarly changed in form. Indeed, when treating of Adaptation (§ 69), we saw that an organ modified by increase or decrease of function, can but slowly re-act on the system at large, so as to bring about those correlative changes required to produce a new equilibrium; and yet only when such new equilibrium has been established, can we expect it to be fully expressed in the modified physiological units of which the organism is built--only then can we count on a complete transfer of the modification to descendants. Nevertheless, that changes of structure caused by changes of action must also be transmitted, however obscurely, appears to be a deduction from first principles--or if not a specific deduction, still, a general implication. For if an organism A, has, by any peculiar habit or condition of life, been modified into the form A', it follows that all the functions of A', reproductive function included, must be in some degree different from the functions of A. An organism being a combination of rhythmically-acting parts in moving equilibrium, the action and structure of any one part cannot be altered without causing alterations of action and structure in all the rest; just as no member of the Solar System could be modified in motion or mass, without producing rearrangements throughout the whole Solar System. And if the organism A, when changed to A', must be changed in all its functions; then the offspring of A' cannot be the same as they would have been had it retained the form A. That the change in the offspring must, other things equal, be in the same direction as the change in the parent, appears implied by the fact that the change propagated throughout the parental system is a change towards a new state of equilibrium--a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions. Or, bringing the question to its ultimate and simplest form, we may say that as, on the one hand, physiological units will, because of their special polarities, build themselves into an organism of a special structure; so, on the other hand, if the structure of this organism is modified by modified function, it will impress some corresponding modification on the structures and polarities of its units. The units and the aggregate must act and re-act on each other. If nothing prevents, the units will mould the aggregate into a form in equilibrium with their pre-existing polarities. If, contrariwise, the aggregate is made by incident actions to take a new form, its forces must tend to re-mould the units into harmony with this new form. And to say that the physiological units are in any degree so re-moulded as to bring their polar forces towards equilibrium with the forces of the modified aggregate, is to say that when separated in the shape of reproductive centres, these units will tend to build themselves up into an aggregate modified in the same direction.
NOTE.--A large amount of additional evidence supporting the belief that functionally produced modifications are inherited, will be found in Appendix B.
IX. VARIATION
§ 85. Equally conspicuous with the truth that every organism bears a general likeness to its parents, is the truth that no organism is exactly like either parent. Though similar to both in generic and specific traits, and usually, too, in those traits which distinguish the variety, it diverges in numerous traits of minor importance. No two plants are indistinguishable; and no two animals are without differences. Variation is co-extensive with Heredity.
The degrees of variation have a wide range. There are deviations so small as to be not easily detected; and there are deviations great enough to be called monstrosities. In plants we may pass from cases of slight alteration in the shape of a leaf, to cases where, instead of a flower with its calyx above the seed-vessel, there is produced a flower with its calyx below the seed-vessel; and while in one animal there arises a scarcely noticeable unlikeness in the length or colour of the hair, in another an organ is absent or a supernumerary organ appears. Though small variations are by far the most general, yet variations of considerable magnitude are not uncommon; and even those variations constituted by additions or suppressions of parts, are not so rare as to be excluded from the list of causes by which organic forms are changed. Cattle without horns are frequent. Of sheep there are horned breeds and breeds that have lost their horns. At one time there existed in Scotland a race of pigs with solid feet instead of cleft feet. In pigeons, according to Mr. Darwin, "the number of the caudal and sacral vertebræ vary; as does the number of the ribs, together with their relative breadth and the presence of processes."
That variations, both small and large, which arise without any specific assignable cause, tend to become hereditary, was shown in the last chapter. Indeed the evidence which proves Heredity in its smaller manifestations is the same evidence which proves Variation; since it is only when there occur variations that the inheritance of anything beyond the structural peculiarities of the species can be proved. It remains here, however, to be observed that the transmission of variations is itself variable; and that it varies both in the direction of decrease and in the direction of increase. An individual trait of one parent may be so counteracted by the influence of the other parent, that it may not appear in the offspring; or, not being so counteracted, the offspring may possess it, perhaps in an equal degree or perhaps in a less degree; or the offspring may exhibit the trait in even a still higher degree. Among illustrations of this, one must suffice. I quote it from the essay by Sir J. Struthers referred to in the last chapter.
"The great-great-grandmother, Esther P---- (who married A---- L----), had a sixth little finger on one hand. Of their eighteen children (twelve daughters and six sons), only one (Charles) is known to have had digital variety. We have the history of the descendants of three of the sons, Andrew, Charles, and James.
"(1.) Andrew L---- had two sons, Thomas and Andrew; and Thomas had two sons all without digital variety. Here we have three successive generations without the variety possessed by the great-grandmother showing itself.
"(2.) James L----, who was normal, had two sons and seven daughters, also normal. One of the daughters became Mrs. J---- (one of the informants), and had three daughters and five sons, all normal except one of the sons, James J----, now æt. 17, who had six fingers on each hand....
"In this branch of the descendants of Esther, we see it passing over two generations and reappearing in one member of the third generation, and now on both hands.
"(3.) Charles L----, the only child of Esther who had digital variety, had six fingers on each hand. He had three sons, James, Thomas, and John, all of whom were born with six fingers on each hand, while John has also a sixth toe on one foot. He had also five other sons and four daughters, all of whom were normal.
"(a.) Of the normal children of this, the third generation, the five sons had twelve sons and twelve daughters, and the four daughters have had four sons and four daughters, being the fourth generation, all of whom were normal. A fifth generation in this sub-group consists as yet of only two boys and two girls who are also normal.
"In this sub-branch, we see the variety of the first generation present in the second, passing over the third and fourth, and also the fifth as far as it has yet gone.
"(b.) James had three sons and two daughters, who are normal.
"(c.) Thomas had four sons and five daughters, who are normal; and has two grandsons, also normal.
"In this sub-branch of the descent, we see the variety of the first generation, showing itself in the second and third, and passing over the fourth, and (as far as it yet exists) the fifth generation.
"(d.) John L---- (one of the informants) had six fingers, the additional finger being attached on the outer side, as in the case of his brothers James and Thomas. All of them had the additional digits removed. John has also a sixth toe on one foot, situated on the outer side. The fifth and sixth toes have a common proximal phalange, and a common integument invests the middle and distal phalanges, each having a separate nail.
"John L---- has a son who is normal, and a daughter, Jane, who was born with six fingers on each hand and six toes on each foot. The sixth fingers were removed. The sixth toes are not wrapped with the fifth as in her father's case, but are distinct from them. The son has a son and daughter, who, like himself, are normal.
"In this, the most interesting sub-branch of the descent, we see digital increase, which appeared in the first generation on one limb, appearing in the second on two limbs, the hands; in the third on three limbs, the hands and one foot; in the fourth on all the four limbs. There is as yet no fifth generation in uninterrupted transmission of the variety. The variety does not yet occur in any member of the fifth generation of Esther's descendants, which consists, as yet, only of three boys and one girl, whose parents were normal, and of two boys and two girls, whose grandparents were normal. It is not known whether in the case of the great-great-grandmother, Esther P----, the variety was original or inherited."[38]
§ 86. Where there is great uniformity among the members of a species, the divergences of offspring from the average type are usually small; but where, among the members of a species, considerable unlikenesses have once been established, unlikenesses among the offspring are frequent and great. Wild plants growing in their natural habitats are uniform over large areas, and maintain from generation to generation like structures; but when cultivation has caused appreciable differences among the members of any species of plant, extensive and numerous deviations are apt to arise. Similarly, between wild and domesticated animals of the same species, we see the contrast that though the homogeneous wild race maintains its type with great persistence, the comparatively heterogeneous domestic race frequently produces individuals more unlike the average type than the parents are.
Though unlikeness among progenitors is one antecedent of variation, it is by no means the sole antecedent. Were it so, the young ones successively born to the same parents would be alike. If any peculiarity in a new organism were a direct resultant of the structural differences between the two organisms which produced it; then all subsequent new organisms produced by these two would show the same peculiarity. But we know that the successive offspring have different peculiarities: no two of them are ever exactly alike.
One cause of such structural variation in progeny, is functional variation in parents. Proof of this is given by the fact that, among progeny of the same parents, there is more difference between those begotten under different constitutional states than between those begotten under the same constitutional state. It is notorious that twins are more nearly alike than children borne in succession. The functional conditions of the parents being the same for twins, but not the same for their brothers and sisters (all other antecedents being constant), we have no choice but to admit that variations in the functional conditions of the parents, are the antecedents of those greater unlikenesses which their brothers and sisters exhibit.
Some other antecedent remains, however. The parents being the same, and their constitutional states the same, variation, more or less marked, still manifests itself. Plants grown from seeds out of one pod, or animals produced at one birth, are not alike. Sometimes they differ considerably. In a litter of pigs or of kittens, we rarely see uniformity of markings; and occasionally there are important structural contrasts. I have myself recently been shown a litter of Newfoundland puppies, some of which had four digits to their feet, while in others there was present, on each hind-foot, what is called the "dew-claw"--a rudimentary fifth digit.
Thus, induction points to three causes of variation, all in action together. We have heterogeneity among progenitors, which, did it act uniformly and alone in generating, by composition of forces, new deviations, would impress such new deviations to the same extent on all offspring of the same parents; which it does not. We have functional variation in the parents, which, acting either alone or in combination with the preceding cause, would entail the same structural variations on all young ones simultaneously produced; which it does not. Consequently there is some third cause of variation, yet to be found, which acts along with the structural and functional variations of ancestors and parents.
§ 87. Already, in the last section, there has been implied some relation between variation and the action of external conditions. The above-cited contrast between the uniformity of a wild species and the multiformity of the same species when cultivated or domesticated, thrusts this truth upon us. Respecting the variations of plants, Mr. Darwin remarks that "'sports' are extremely rare under nature, but far from rare under cultivation." Others who have studied the matter assert that if a species of plant which, up to a certain time, has maintained great uniformity, once has its constitution thoroughly disturbed, it will go on varying indefinitely. Though, in consequence of the remoteness of the periods at which they were domesticated, there is a lack of positive proof that our extremely variable domestic animals have become variable under the changed conditions implied by domestication, having been previously constant; yet competent judges do not doubt that this has been the case.
Now the constitutional disturbance which precedes variation, can be nothing else than an overthrowing of the pre-established equilibrium of functions. Transferring a plant from forest lands to a ploughed field or a manured garden, is altering the balance of forces to which it has been hitherto subject, by supplying it with different proportions of the assimilable matters it requires, and taking away some of the positive impediments to its growth which competing wild plants before offered. An animal taken from woods or plains, where it lived on wild food of its own procuring, and placed under restraint while artificially supplied with food not quite like what it had before, is an animal subject to new outer actions to which its inner actions must be adjusted. From the general law of equilibration we found it to follow that "the maintenance of such a moving equilibrium" as an organism displays, "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" (First Principles, § 173); and more recently (§ 27), we have seen that Life itself is "the definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences." Necessarily, therefore, an organism exposed to a permanent change in the arrangement of outer forces must undergo a permanent change in the arrangement of inner forces. The old equilibrium has been destroyed; and a new equilibrium must be established. There must be functional perturbations, ending in a re-adjusted balance of functions.
If, then, change of conditions is the only known cause by which the original homogeneity of a species is destroyed; and if change of conditions can affect an organism only by altering its functions; it follows that alteration of functions is the only known internal cause to which the commencement of variation can be ascribed. That such minor functional changes as parents undergo from year to year are influential on the offspring, we have seen is proved by the greater unlikeness that exists between children born to the same parents at different times, than exists between twins. And here we seem forced to conclude that the larger functional variations produced by greater external changes, are the initiators of those structural variations which, when once commenced in a species, lead by their combinations and antagonisms to multiform results. Whether they are or are not the direct initiators, they must still be the indirect initiators.
§ 87a. In the foregoing sentence those pronounced structural variations from which may presently arise new varieties and eventually species, are ascribed to "the larger functional variations produced by greater external changes"; and this limitation is a needful one, since there is a constant cause of minor variations of a wholly different kind.
There are the variations arising from differences in the conditions to which the germ is subject, both before detachment from the parent and after. At first sight it seems that plants grown from seeds out of the same seed-vessel and animals belonging to the same litter, ought, in the absence of any differences of ancestral antecedents, to be entirely alike. But this is not so. Inevitably they are subject from the very outset to slightly different sets of agencies. The seeds in a seed-vessel do not stand in exactly the same relations to the sources of nutriment: some are nearer than others. They are somewhat differently exposed to the heat and light penetrating their envelope; and some are more impeded in their growth by neighbours than others are. Similarly with young animals belonging to the same litter. Their uterine lives are made to some extent unlike by unlike connexions with the blood-supply, by mutual interferences not all the same, and even by different relations to the disturbances caused by the mother's movements. So, too, is it after separation from the parent plant or animal. Even the biblical parable reminds us that seeds fall into places here favourable and there unfavourable in various degrees. In respect of soil, in respect of space for growth, in respect of shares of light, none of them are circumstanced in quite the same ways. With animals the like holds. In a litter of pigs some, weaker than others, do not succeed as often in getting possession of teats. And then in both cases the differences thus initiated become increasingly pronounced. Among young plants the smaller, outgrown by their better-placed neighbours, are continually more shaded and more left behind; and among the litter the weakly ones, continually thrust aside by the stronger, become relatively more weakly from deficient nutrition.
Differentiations thus arising, both before and after separation from parents, though primarily differences of growth, entail structural differences; for it is a general law of nutrition that when there is deficiency of food the non-essential organs suffer more than the essential ones, and the unlikenesses of proportion hence arising constitute unlikenesses of structure. It may be concluded, however, that variations generated in this manner usually have no permanent results. In the first place, the individuals which, primarily in growth and secondarily in smaller developments of less-important organs, are by implication inferior, are likely to be eliminated from the species. In the second place, differences of structure produced in the way shown do not express differences of constitution--are not the effects of somewhat divergent physiological units; and consequently are not likely to be repeated in posterity.
§ 88. We have still, therefore, to explain those variations which have no manifest causes of the kinds thus far considered. These are the variations termed "spontaneous." Not that those who apply to them this word, or some equivalent, mean to imply that they are uncaused. Mr. Darwin expressly guards himself against such an interpretation. He says:--"I have hitherto sometimes spoken as if the variations--so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature--had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation." Not only, however, do I hold, in common with Mr. Darwin, that there must be some cause for these apparently-spontaneous variations, but it seems to me that a definite cause is assignable. I think it may be shown that unlikenesses must necessarily arise even between the new individuals simultaneously produced by the same parents. Instead of the occurrence of such variations being inexplicable, the absence of them would be inexplicable.
In any series of dependent changes a small initial difference often works a marked difference in the results. The mode in which a particular breaker bursts on the beach, may determine whether the seed of some foreign plant which it bears is or is not stranded--may cause the presence or absence of this plant from the Flora of the land; and may so affect, for millions of years, in countless ways, the living creatures throughout the land. A single touch, by introducing into the body some morbid matter, may set up an immensely involved set of functional disturbances and structural alterations. The whole tenor of a life may be changed by a word of advice; or a glance may determine an action which alters thoughts, feelings, and deeds throughout a long series of years. In those still more involved combinations of changes which societies exhibit, this truth is still more conspicuous. A hair's-breadth difference in the direction of some soldier's musket at the battle of Arcola, by killing Napoleon, might have changed events throughout Europe; and though the type of social organization in each European country would have been now very much what it is, yet in countless details it would have been different.
Illustrations like these, with which pages might be filled, prepare us for the conclusion that organisms produced by the same parents at the same time, must be more or less differentiated, both by insensible initial differences and by slight differences in the conditions to which they are subject during their evolution. We need not, however, rest with assuming such initial differences: the necessity of them is demonstrable. The individual germ-cells which, in succession or simultaneously, are separated from the same parent, can never be exactly alike; nor can the sperm-cells which fertilize them. When treating of the instability of the homogeneous (First Principles, § 149), we saw that no two parts of any aggregate can be similarly conditioned with respect to incident forces; and that being subject to forces that are more or less unlike, they must become more or less unlike. Hence, no two ova in an ovarium or ovules in a seed-vessel--no two spermatozoa or pollen-cells, can be identical. Whether or not there arise other contrasts, there are certain to arise quantitative contrasts; since the process of nutrition cannot be absolutely alike for all. The reproductive centres must begin to differentiate from the very outset. Such being the necessities of the case, what will happen on any successive or simultaneous fertilizations? Inevitably unlikenesses between the respective parental influences must result. Quantitative differences among the sperm-cells and among the germ-cells, will insure this. Grant that the number of physiological units contained in any one reproductive cell, can rarely if ever be exactly equal to the number contained in any other, ripened at the same time or at a different time; and it follows that among the fertilized germs produced by the same parents, the physiological units derived from them respectively will bear a different numerical ratio to each other in every case. If the parents are constitutionally quite alike, the variation in the ratio between the units they severally bequeath, cannot cause unlikenesses among the offspring. But if otherwise, no two of the offspring can be alike. In every case the small initial difference in the proportions of the slightly-unlike units, will lead, during evolution, to a continual multiplication of differences. The insensible divergence at the outset will generate sensible divergences at the conclusion. Possibly some may hence infer that though, in such case, the offspring must differ somewhat from each other and from both parents, yet that in every one of them there must result a homogeneous mixture of the traits of the two parents. A little consideration shows that the reverse is inferable. If, throughout the process of development, the physiological units derived from each parent preserved the same ratio in all parts of the growing organism, each organ would show as much as every other, the influence of either parent. But no such uniform distribution is possible. It has been shown (First Principles, § 163), that in any aggregate of mixed units segregation must inevitably go on. Incident forces will tend ever to cause separation of the two orders of units from each other--will tend to integrate groups of the one order in one place and groups of the other order in another place. Hence there must arise not a homogeneous mean between the two parents, but a mixture of organs, some of which mainly follow the one and some the other. And this is the kind of mixture which observation shows us.
Still it may be fairly objected that however the attributes of the two parents are variously mingled in their offspring, they must in all of them fall between the extremes displayed in the parents. In no characteristic could one of the young exceed both parents, were there no cause of "spontaneous variation" but the one alleged. Evidently, then, there is a cause yet unfound.
§ 89. Thus far we have contemplated the process under its simplest aspect. While we have assumed the two parents to be somewhat unlike, we have assumed that each parent has a homogeneous constitution--is built up of physiological units which are exactly alike. But in no case can such a homogeneity exist. Each parent had parents who were more or less contrasted--each parent inherited at least two orders of physiological units not quite identical. Here then we have a further cause of variation. The sperm-cells or germ-cells which any organism produces, will differ from each other not quantitatively only but qualitatively. Of the slightly-unlike physiological units bequeathed to it, the reproductive cells it casts off cannot habitually contain the same proportions; and we may expect the proportions to vary not slightly but greatly. Just as, during the evolution of an organism, the physiological units derived from the two parents tend to segregate, and produce likeness to the male parent in this part and to the female parent in that; so, during the formation of reproductive cells, there will arise in one a predominance of the physiological units derived from the father, and in another a predominance of the physiological units derived from the mother. Thus, then, every fertilized germ, besides containing different amounts of the two parental influences, will contain different kinds of influences--this having received a marked impress from one grandparent, and that from another. Without further exposition the reader will see how this cause of complication, running back through each line of ancestry, must produce in every germ numerous minute differences among the units.
Here, then, we have a clue to the multiplied variations, and sometimes extreme variations, that arise in races which have once begun to vary. Amid countless different combinations of units derived from parents, and through them from ancestors, immediate and remote--amid the various conflicts in their slightly-different organic polarities, opposing and conspiring with one another in all ways and degrees; there will from time to time arise special proportions causing special deviations. From the general law of probabilities it may be concluded that while these involved influences, derived from many progenitors, must, on the average of cases, obscure and partially neutralize one another; there must occasionally result such combinations of them as will produce considerable divergences from average structures; and, at rare intervals, such combinations as will produce very marked divergences. There is thus a correspondence between the inferable results and the results as habitually witnessed.
§ 90. Still there remains a difficulty. It may be said that admitting functional change to be the initiator of variation--granting that the physiological units of an organism long subject to new conditions, will tend to become modified in such way as to cause change of structure in offspring; yet there will still be no cause of the supposed heterogeneity among the physiological units of different individuals. There seems validity in the objection, that as all the members of a species whose circumstances have been altered will be affected in the same manner, the results, when they begin to show themselves in descendants, will show themselves in the same manner: not multiform variations will arise, but deviations all in one direction.
The reply is simple. The members of a species thus circumstanced will not be similarly affected. In the absence of absolute uniformity among them, the functional changes caused in them will be more or less dissimilar. Just as men of slightly-unlike dispositions behave in quite opposite ways under the same circumstances; or just as men of slightly-unlike constitutions get diverse disorders from the same cause, and are diversely acted on by the same medicine; so, the insensibly-differentiated members of a species whose conditions have been changed, may at once begin to undergo various kinds of functional changes. As we have already seen, small initial contrasts may lead to large terminal contrasts. The intenser cold of the climate into which a species has migrated, may cause in one individual increased consumption of food to balance the greater loss of heat; while in another individual the requirement may be met by a thicker growth of fur. Or, when meeting with the new foods which a new region furnishes, accident may determine one member of the species to begin with one kind and another member with another kind; and hence may arise established habits in these respective members and their descendants. Now when the functional divergences thus set up in sundry families of a species have lasted long enough to affect their constitutions, and to modify somewhat the physiological units thrown off in their reproductive cells, the divergences produced by these in offspring will be of divers kinds. And the original homogeneity of constitution having been thus destroyed, variation may go on with increasing facility. There will result a heterogeneous mixture of modifications of structure caused by modifications of function; and of still more numerous correlated modifications, indirectly so caused. By natural selection of the most divergent forms, the unlikenesses of parents will be rendered more marked, and the limits of variation wider. Until at length the divergences of constitutions and modes of life, become great enough to lead to segregation of the varieties.
§ 91. That variations must occur, and that they must ever tend, both directly and indirectly, towards adaptive modifications, are conclusions deducible from first principles; apart from any detailed interpretations like the above. That the state of homogeneity is an unstable state we have found to be a universal truth. Each species must pass from the uniform into the more or less multiform, unless the incidence of external forces is exactly the same for all its members, which it never can be. Through the process of differentiation and integration, which of necessity brings together, or keeps together, like individuals, and separates unlike ones from them, there must nevertheless be maintained a tolerably uniform species, so long as there continues a tolerably uniform set of conditions in which it may exist. But if the conditions change, either absolutely by some disturbance of the habitat or relatively by spread of the species into other habitats, then the divergent individuals that result must be segregated by the divergent sets of conditions into distinct varieties (First Principles, § 166). When, instead of contemplating a species in the aggregate, we confine our attention to a single member and its descendants, we see it to be a corollary from the general law of equilibration that the moving equilibrium constituted by the vital actions in each member of this family, must remain constant so long as the external actions to which they correspond remain constant; and that if the external actions are changed, the disturbed balance of internal changes, if not overthrown, cannot cease undergoing modification until the internal changes are again in equilibrium with the external actions: corresponding structural alterations having arisen.
On passing from these derivative laws to the ultimate law, we see that Variation is necessitated by the persistence of force. The members of a species inhabiting any area cannot be subject to like sets of forces over the whole of that area. And if, in different parts of the area, different kinds or amounts or combinations of forces act on them, they cannot but become different in themselves and in their progeny. To say otherwise, is to say that differences in the forces will not produce differences in the effects; which is to deny the persistence of force.
X. GENESIS, HEREDITY, AND VARIATION
§ 92. A question raised, and hypothetically answered, in §§ 78 and 79, was there postponed until we had dealt with the topics of Heredity and Variation. Let us now resume the consideration of this question, in connexion with sundry others which the facts suggest.
After contemplating the several methods by which the multiplication of organisms is carried on--after ranging them under the two heads of Homogenesis, in which the successive generations are similarly produced, and Heterogenesis, in which they are dissimilarly produced--after observing that Homogenesis is nearly always sexual genesis, while Heterogenesis is asexual genesis with occasionally-recurring sexual genesis; we came to the questions--why is it that some organisms multiply in the one way and some in the other? and why is it that where agamogenesis prevails it is usually, from time to time, interrupted by gamogenesis? In seeking answers to these questions, we inquired whether there are common to both Homogenesis and Heterogenesis, any conditions under which alone sperm-cells and germ-cells arise and are united for the production of new organisms; and we reached the conclusion that, in all cases, they arise only when there is an approach to equilibrium between the forces which produce growth and the forces which oppose growth. This answer to the question--when does gamogenesis recur? still left unanswered the question--why does gamogenesis recur? And to this the reply suggested was, that the approach towards general equilibrium in organisms, "is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell with germ-cell is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ--a result probably effected by mixing the slightly-different physiological units of slightly-different individuals." This is the hypothesis which we have now to consider. Let us first look at the evidences which certain inorganic phenomena furnish.
The molecules of any aggregate which have not a balanced arrangement, inevitably tend towards a balanced arrangement. As before mentioned (First Principles, § 100), amorphous wrought iron, when subject to continuous jar, begins to arrange itself into crystals--its atoms assume a condition of polar equilibrium. The particles of unannealed glass, which are so unstably arranged that slight disturbing forces make them separate into small groups, take advantage of that greater freedom of movement given by a raised temperature, to adjust themselves into a state of relative rest. During any such re-arrangement the aggregate exercises a coercive force over its units. Just as in a growing crystal the atoms successively assimilated from the solution, are made by the already crystallized atoms to take a certain form, and even to re-complete that form when it is broken; so in any mass of unstably-arranged atoms which passes into a stable arrangement, each atom conforms to the forces exercised on it by all the other atoms. This is a corollary from the general law of equilibration. We saw (First Principles, § 170) that every change is towards equilibrium; and that change can never cease until equilibrium is reached. Organisms, above all other aggregates, conspicuously display this progressive equilibration; because their units are of such kinds, and so conditioned, as to admit of easy re-arrangement. Those extremely active changes which go on during the early stages of evolution, imply an immense excess of the molecular forces over those antagonist forces which the aggregate exercises on the molecules. While this excess continues, it is expended in growth, development, and function: expenditure for any of these purposes being proof that part of the force constituting molecular tensions remains unbalanced. Eventually, however, this excess diminishes. Either, as in organisms which do not expend much energy, decrease of assimilation leads to its decline; or, as in organisms which expend much energy, it is counterbalanced by the rapidly-increasing reactions of the aggregate (§ 46). The cessation of growth when followed, as in some organisms, by death, implies the arrival at an equilibrium between the molecular forces and those forces which the aggregate opposes to them. When, as in other organisms, growth ends in the establishment of a moving equilibrium, there is implied such a decreased preponderance of the molecular forces, as leaves no surplus beyond that which is used up in functions. The declining functional activity characteristic of advancing life, expresses a further decline in this surplus. And when all vital movements come to an end, the implication is that the actions of the units on the aggregate and the reactions of the aggregate on the units are completely balanced. Hence, while a state of rapid growth indicates such a play of forces among the units of an aggregate as will produce active re-distribution, the diminution and arrest of growth shows that the units have fallen into such relative positions that re-distribution is no longer so facile. When, therefore, we see that gamogenesis recurs only when growth is decreasing, or has come to an end, we must say that it recurs only when the organic units are approximating to equilibrium--only when their mutual restraints prevent them from readily changing their arrangements in obedience to incident forces.
That units of like forms can be built up into a more stable aggregate than units of slightly unlike forms, is tolerably manifest à priori. And we have facts which prove that mixing allied but somewhat different units, does lead to comparative instability. Most metallic alloys exemplify this truth. Common solder, which is a mixture of lead and tin, melts at a much lower temperature than either lead or tin. The compound of lead, tin, and bismuth, called "fusible metal," becomes fluid at the temperature of boiling water; while the temperatures at which lead, tin, and bismuth become fluid are, respectively, 612°, 442°, and 497° F. Still more remarkable is the illustration furnished by potassium and sodium. These metals are very near akin in all respects--in their specific gravities, their atomic weights, their chemical affinities, and the properties of their compounds. That is to say, all the evidences unite to show that their units, though not identical, have a close resemblance. What now happens when they are mixed? Potassium alone melts at 136°, sodium alone melts at 190°, but the alloy of potassium and sodium is liquid at the ordinary temperature of the air. Observe the meaning of these facts, expressed in general terms. The maintenance of a solid form by any group of units implies among them an arrangement so stable that it is not overthrown by the incident forces. Whereas the assumption of a liquid form implies that the incident forces suffice to destroy the arrangement of the units. In the one case the thermal undulations fail to dislocate the parts; while in the other case the parts are so dislocated by the thermal undulations that they fall into total disorder--a disorder admitting of easy re-arrangement into any other order. For the liquid state is a state in which the units become so far free from mutual restraints, that incident forces can change their relative positions very readily. Thus we have reason to conclude that an aggregate of units which, though in the main similar to one another, have minor differences, must be more unstable than an aggregate of homogeneous units. The one will yield to disturbing forces which the other successfully resists.
Now though the colloidal molecules of which organisms are mainly built, are themselves highly composite; and though the physiological units compounded out of these colloidal molecules must have structures far more involved; yet it must happen with such units, as with simple units, that those which have exactly like forms will admit of arrangement into a more stable aggregate than those which have slightly-unlike forms. Among units of this order, as among units of a simpler order, imperfect similarity must entail imperfect balance in anything formed of them, and consequent diminished ability to withstand disturbing forces. Hence, given two organisms which, by diminished nutrition or increased expenditure, are being arrested in their growths--given in each an approaching equilibrium between the forces of the units and the forces of the aggregate--given, that is, such a comparatively balanced state among the units that re-arrangement of them by incident forces is no longer so easy; and it will follow that by uniting a group of units from the one organism with a group of slightly-different units from the other, the tendency towards equilibrium will be diminished, and the mixed units will be rendered more modifiable in their arrangements by the forces acting on them: they will be so far freed as to become again capable of that re-distribution which constitutes evolution.
And now let us test this hypothesis by seeing what power it gives us of interpreting established inductions.
§ 93. The majority of plants being hermaphrodites, it has, until quite recently, been supposed that the ovules of each flower are fertilized by pollen from the anthers of the same flower. Mr. Darwin, however, has shown that the arrangements are generally such as to prevent this. Either the ovules and the pollen are not ripe simultaneously, or obstacles prevent access of the one to the other. At the same time he has shown that there exist arrangements, often of a remarkable kind, which facilitate the transfer of pollen by insects from the stamens of one flower to the pistil of another. Similarly, it has been found that among the lower animals, hermaphrodism does not usually involve the production of fertile ova by the union of sperm-cells and germ-cells developed in the same individual; but that the reproductive centres of one individual are united with those of another to produce fertile ova. Either, as in Pyrosoma, Perophora, and in many higher molluscs, the ova and spermatozoa are matured at different times; or, as in annelids, they are prevented by their relative positions from coming in contact.
Remembering the fact that among the higher classes of organisms, fertilization is always effected by combining the sperm-cell of one individual with the germ-cell of another; and joining with it the above fact that among hermaphrodite organisms, the germ-cells developed in any individual are usually not fertilized by sperm-cells developed in the same individual; we see reason for thinking that the essential thing in fertilization, is the union of specially-fitted portions of different organisms. If fertilization depended on the peculiar properties of sperm-cell and germ-cell, as such; then, in hermaphrodite organisms, it would be a matter of indifference whether the united sperm-cells and germ-cells were those of the same individual or those of different individuals. But the circumstance that there exist in such organisms elaborate appliances for mutual fertilization, shows that unlikeness of derivation in the united reproductive centres, is the desideratum. Now this is just what the foregoing hypothesis implies. If, as was concluded, fertilization has for its object the disturbance of that approaching equilibrium existing among the physiological units separated from an adult organism; and if, as we saw reason to think, this object is effected by mixture with the slightly-different physiological units of another organism; then, we at the same time see that this object will not be effected by mixture with physiological units belonging to the same organism. Thus, the hypothesis leads us to expect such provisions as we find.
§ 94. But here a difficulty presents itself. These propositions seem to involve the conclusion that self-fertilization is impossible. It apparently follows from them, that a group of physiological units from one part of an organism ought to have no power of altering the state of approaching balance in a group from another part of it. Yet self-fertilization does occur. Though the ovules of one plant are generally fertilized by pollen from another plant of the same kind, yet they may be, some of them, fertilized by pollen of the same plant; and, indeed, there are plants in which self-fertilization is the rule: even provision being in some cases made to prevent fertilization by pollen from other individuals. And though, among hermaphrodite animals, self-fertilization is usually negatived by structural or functional arrangements, yet in certain Entozoa there appear to be special provisions by which the sperm-cells and the germ-cells of the same individual may be united, when not previously united with those of another individual. Nay, it has even been shown that in certain Ascidians the contents of oviduct and spermiduct of the same individual produce, when united, fertile ova whence evolve perfect individuals. Certainly, at first sight, these facts do not consist with the above supposition. Nevertheless there is something like a solution.
In the last chapter, when considering the variations caused in offspring from uniting elements representing unlike parental constitutions, it was pointed out that in an unfolding organism, composed of slightly-different physiological units derived from slightly-different parents, there cannot be maintained an even distribution of the two orders of units. We saw that the instability of the homogeneous negatives the uniform blending of them; and that, by the process of differentiation and integration, they must be more or less separated; so that in one part of the body the influence of one parent will predominate, and in another part of the body the influence of the other parent: an inference which harmonizes with daily observation. We also saw that the sperm-cells or germ-cells produced by such an organism must, in virtue of these same laws, be more or less unlike one another. It was shown that through segregation, some of the sperm-cells or germ-cells will get an excess of the physiological units derived from one side, and some of them an excess of those derived from the other side: a cause which accounts for the unlikenesses among offspring simultaneously produced. Now from this segregation of the different orders of physiological units, inherited from different parents and lines of ancestry, there arises the possibility of self-fertilization in hermaphrodite organisms. If the physiological units contained in the sperm-cells and germ-cells of the same flower, are not quite homogeneous--if in some of the ovules the physiological units derived from the one parent greatly predominate, and in some of the ovules those derived from the other parent; and if the like is true of the pollen-cells; then, some of the ovules may be nearly as much contrasted with some of the pollen-cells in the characters of their contained units, as were the ovules and pollen-cells of the parents from which the plant proceeded. Between part of the sperm-cells and part of the germ-cells, the community of nature will be such that fertilization will not result from their union; but between some of them, the differences of constitution will be such that their union will produce the requisite molecular instability. The facts, so far as they are known, seem in harmony with this deduction. Self-fertilization in flowers, when it takes place, is not so efficient as mutual fertilization. Though some of the ovules produce seeds, yet more of them than usual are abortive. From which, indeed, results the establishment of varieties that have structures favourable to mutual fertilization; since, being more prolific, these have, other things equal, greater chances in the "struggle for existence."
Further evidence is at hand supporting this interpretation. There is reason to believe that self-fertilization, which at the best is comparatively inefficient, loses all efficiency in course of time. After giving an account of the provisions for an occasional, or a frequent, or a constant crossing between flowers; and after quoting Prof. Huxley to the effect that among hermaphrodite animals, there is no case in which "the occasional influence of a distinct individual can be shown to be physically impossible;" Mr. Darwin writes--"from these several considerations and from the many special facts which I have collected, but which I am not here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct individual is a law of nature ... in none, as I suspect, can self-fertilization go on for perpetuity." This conclusion, based wholly on observed facts, is just the conclusion to which the foregoing argument points. That necessary action and the re-action between the parts of an organism and the organism as a whole--that power of an aggregate to re-mould the units, which is the correlative of the power of the units to build up into such an aggregate; implies that any differences existing among the units inherited by an organism, must gradually diminish. Being subject in common to the total forces of the organism, they will in common be modified towards congruity with these forces, and therefore towards likeness with one another. If, then, in a self-fertilizing organism and its self-fertilizing descendants, such contrasts as originally existed among the physiological units are progressively obliterated--if, consequently, there can no longer be a segregation of different physiological units in different sperm-cells and germ-cells; self-fertilization will become impossible. Step by step the fertility will diminish, and the series will finally die out.
And now observe, in confirmation of this view, that self-fertilization is limited to organisms in which an approximate equilibrium among the organic forces is not long maintained. While growth is actively going on, and the physiological units are subject to a continually-changing distribution of forces, no decided assimilation of the units can be expected: like forces acting on the unlike units will tend to segregate them, so long as continuance of evolution permits further segregation; and only when further segregation cannot go on, will the like forces tend to assimilate the units. Hence, where there is no prolonged maintenance of an approximate organic balance, self-fertilization may be possible for some generations; but it will be impossible in organisms distinguished by a sustained moving equilibrium.
§ 95. The interpretation which it affords of sundry phenomena familiar to breeders of animals, adds probability to the hypothesis. Mr. Darwin has collected a large "body of facts, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility,"--a conclusion harmonizing with the current belief respecting family-intermarriages in the human race. Have we not here a solution of these facts? Relations must, on the average of cases, be individuals whose physiological units are more nearly alike than usual. Animals of different varieties must be those whose physiological units are more unlike than usual. In the one case, the unlikeness of the units may frequently be insufficient to produce fertilization; or, if sufficient to produce fertilization, not sufficient to produce that active molecular change required for vigorous development. In the other case, both fertilization and vigorous development will be made probable.
Nor are we without a cause for the irregular manifestations of these general tendencies. The mixed physiological units composing any organism being, as we have seen, more or less segregated in the reproductive centres it throws off; there may arise various results according to the degrees of difference among the units, and the degrees in which the units are segregated. Of two cousins who have married, the common grandparents may have had either similar or dissimilar constitutions; and if their constitutions were dissimilar, the probability that their married grandchildren will have offspring will be greater than if their constitutions were similar. Or the brothers and sisters from whom these cousins descended, instead of severally inheriting the constitutions of their parents in tolerably equal degrees, may have severally inherited them in very different degrees: in which last case, intermarriages among the cousins will be less likely to prove infertile. Or the brothers and sisters from whom these cousins descended, may severally have married persons very like, or very unlike, themselves; and from this cause there may have resulted, either an undue likeness, or a due unlikeness, between the married cousins.[39] These several causes, conspiring and conflicting in endless ways and degrees, will work multiform effects. Moreover, differences of segregation will make the reproductive centres produced by the same nearly-related organisms, vary considerably in their amounts of unlikeness; and therefore, supposing their amounts of unlikeness great enough to cause fertilization, this fertilization will be effective in various degrees. Hence it may happen that among offspring of nearly-related parents, there may be some in which the want of vigour is not marked, and others in which there is decided want of vigour. So that we are alike shown why in-and-in breeding tends to diminish both fertility and vigour: and why the effect cannot be a uniform effect, but only an average effect.
§ 96. While, if the foregoing arguments are valid, gamogenesis has for its main result the initiation of a new development by the overthrow of that approximate equilibrium arrived at among the molecules of the parent-organisms, a further result appears to be subserved by it. Those inferior organisms which habitually multiply by agamogenesis, have conditions of life that are simple and uniform; while those organisms which have highly-complex and variable conditions of life, habitually multiply by gamogenesis. Now if a species has complex and variable conditions of life, its members must be severally exposed to sets of conditions that are slightly different: the aggregates of incident forces cannot be alike for all the scattered individuals. Hence, as functional deviation must ever be inducing structural deviation, each individual throughout the area occupied tends to become fitted for the particular habits which its particular conditions necessitate; and in so far, _un_fitted for the average habits proper to the species. But these undue specializations are continually checked by gamogenesis. As Mr. Darwin remarks, "intercrossing plays a very important part in nature in keeping the individuals of the same species, or of the variety, true and uniform in character:" the idiosyncratic divergences obliterate one another. Gamogenesis, then, is a means of turning to positive advantage the individual differentiations which, in its absence, would result in positive disadvantage. Were it not that individuals are ever being made unlike one another by their unlike conditions, there would not arise in them those contrasts of molecular constitution, which we have seen to be needful for producing the fertilized germs of new individuals. And were not these individual differentiations ever being mutually cancelled, they would end in a fatal narrowness of adaptation.
This truth will be most clearly seen if we reduce it to its purely abstract form, thus:--Suppose a quite homogeneous species, placed in quite homogeneous conditions; and suppose the constitutions of all its members in complete concord with their absolutely-uniform and constant conditions; what must happen? The species, individually and collectively, is in a state of perfect moving equilibrium. All disturbing forces have been eliminated. There remains no force which can, in any way, change the state of this moving equilibrium; either in the species as a whole or in its members. But we have seen (First Principles, § 173) that a moving equilibrium is but a transition towards complete equilibration, or death. The absence of differential or un-equilibrated forces among the members of a species, is the absence of all forces which can cause changes in the conditions of its members--is the absence of all forces which can initiate new organisms. To say, as above, that complete molecular homogeneity existing among the members of a species, must render impossible that mutual molecular disturbance which constitutes fertilization, is but another way of saying that the actions and re-actions of each organism, being in perfect balance with the actions and re-actions of the environment upon it, there remains in each organism no force by which it differs from any other--no force which any other does not meet with an equal force--no force which can set up a new evolution among the units of any other.
And so we reach the remarkable conclusion that the life of a species, like the life of an individual, is maintained by the unequal and ever-varying actions of incident forces on its different parts.[40] An individual homogeneous throughout, and having its substance everywhere continuously subject to like actions, could undergo none of those changes which life consists of; and similarly, an absolutely-uniform species, having all its members exposed to identical influences, would be deprived of that initiator of change which maintains its existence as a species. Just as, in each organism, incident forces constantly produce divergences from the mean state in various directions, which are constantly balanced by opposite divergences indirectly produced by other incident forces; and just as the combination of rhythmical functions thus maintained, constitutes the life of the organism; so, in a species, there is, through gamogenesis, a perpetual neutralization of those contrary deviations from the mean state which are caused in its different parts by different sets of incident forces; and it is similarly by the rhythmical production and compensation of these contrary deviations, that the species continues to live. The moving equilibrium in a species, like the moving equilibrium in an individual, would rapidly end in complete equilibration, or death, were not its continually-dissipated forces continually re-supplied from without. Besides owing to the external world those energies which, from moment to moment, keep up the lives of its individual members, every species owes to certain more indirect actions of the external world, those energies which enable it to perpetuate itself in successive generations.
§ 97. What evidence still remains may be conveniently woven up along with a recapitulation of the argument pursued through the last three chapters. Let us contemplate the facts in their synthetic order.
That compounding and re-compounding through which we pass from the simplest inorganic substances to the most complex organic substances, has several concomitants. Each successive stage of composition presents us with molecules that are severally larger or more integrated, that are severally more heterogeneous, that are severally more unstable, and that are more numerous in their kinds (First Principles, § 151). And when we come to the substances of which living bodies are formed, we find ourselves among innumerable divergent groups and sub-groups of compounds, the units of which are large, heterogeneous, and unstable, in high degrees. There is no reason to assume that this process ends with the formation of those complex colloids which constitute organic matter. A more probable assumption is that out of the complex colloidal molecules there are evolved, by a still further integration, molecules which are still more heterogeneous, and of kinds which are still more multitudinous. What must be their properties? Already the colloidal molecules are extremely unstable--capable of being variously modified in their characters by very slight incident forces; and already the complexity of their polarities prevents them from readily falling into such positions of equilibrium as results in crystallization. Now the organic molecules composed of these colloidal molecules, must be similarly characterized in far higher degrees. Far more numerous must be the minute changes that can be wrought in them by minute external forces; far more free must they remain for a long time to obey forces tending to re-distribute them; and far greater must be the number of their kinds.
Setting out with these physiological units, the existence of which various organic phenomena compel us to recognize, and the production of which the general law of Evolution thus leads us to anticipate; we get an insight into the phenomena of Genesis, Heredity, and Variation. If each organism is built of certain of these highly-plastic units peculiar to its species--units which slowly work towards an equilibrium of their complex proclivities, in producing an aggregate of the specific structure, and which are at the same time slowly modifiable by the re-actions of this aggregate--we see why the multiplication of organisms proceeds in the several ways, and with the various results, which naturalists have observed.
Heredity, as shown not only in the repetition of the specific structure but in the repetition of ancestral deviations from it, becomes a matter of course; and it falls into unison with the fact that, in various inferior organisms, lost parts can be replaced, and that, in still lower organisms, a fragment can develop into a whole.
While an aggregate of physiological units continues to grow by the assimilation of matter which it moulds into other units of like type; and while it continues to undergo changes of structure; no equilibrium can be arrived at between the whole and its parts. Under these conditions, then, an un-differentiated portion of the aggregate--a group of physiological units not bound up into a specialized tissue--will be able to arrange itself into the structure peculiar to the species; and will so arrange itself, if freed from controlling forces and placed in fit conditions of nutrition and temperature. Hence the continuance of agamogenesis in little-differentiated organisms, so long as assimilation continues to be greatly in excess of expenditure.
But let growth be checked and development approach its completion--let the units of the aggregate be severally exposed to an almost constant distribution of forces; and they must begin to equilibrate themselves. Arranged, as they will gradually be, into comparatively stable attitudes in relation to one another, their mobility will diminish; and groups of them, partially or wholly detached, will no longer readily re-arrange themselves into the specific form. Agamogenesis will be no longer possible; or, if possible, will be no longer easy.
When we remember that the force which keeps the Earth in its orbit is the gravitation of each particle in the Earth towards every one of the group of particles existing 92,000,000 of miles off; we cannot reasonably doubt that each unit in an organism acts on all the other units, and is reacted on by them: not by gravitation only but chiefly by other energies. When, too, we learn that glass has its molecular constitution changed by light, and that substances so rigid and stable as metals have their atoms re-arranged by forces radiated in the dark from adjacent objects;[41] we are obliged to conclude that the excessively-unstable units of which organisms are built, must be sensitive in a transcendant degree to all the forces pervading the organisms composed of them--must be tending ever to re-adjust, not only their relative attitudes but their molecular structures, into equilibrium with these forces. Hence, if aggregates of the same species are differently conditioned, and re-act differently on their component units, their component units will be rendered somewhat different; and they will become the more different the more widely the re-actions of the aggregates upon them differ, and the greater the number of generations through which these different re-actions of the aggregates upon them are continued.
If, then, unlikenesses of function among individuals of the same species, produce unlikenesses between the physiological units of one individual and those of another, it becomes comprehensible that when groups of units derived from two individuals are united, the group formed will be more unstable than either of the groups was before their union. The mixed units will be less able to resist those re-distributing forces which cause evolution; and may thus have restored to them the capacity for development which they had lost.
This view harmonizes with the conclusion, which we saw reason to draw, that fertilization does not depend on any intrinsic peculiarities of sperm-cells and germ-cells, but depends on their derivation from different individuals. It explains the facts that nearly-related individuals are less likely to have offspring than others, and that their offspring, when they have them, are frequently feeble. And it gives us a key to the converse fact that the crossing of varieties results in unusual vigour.
Bearing in mind that the slightly-different orders of physiological units which an organism inherits from its parents, are subject to the same set of forces, and that when the organism is fully developed this set of forces, becoming constant, tends slowly to re-mould the two orders of units into the same form; we see how it happens that self-fertilization becomes impossible in the higher organisms, while it remains possible in the lower organisms. In long-lived creatures which have tolerably-definite limits of growth, this assimilation of the somewhat-unlike physiological units is liable to go on to an appreciable extent; whereas in organisms which do not continuously subject their component units to constant forces, there will be much less of this assimilation. And where the assimilation is not considerable, the segregation of mixed units may cause the sperm-cells and germ-cells developed in the same individual, to be sufficiently different to produce, by their union, fertile germs; and several generations of self-fertilizing descendants may succeed one another, before the two orders of units have had their unlikenesses so far diminished that they will no longer do this. The same principles explain for us the variable results of union between nearly-related organisms. According to the contrasts among the physiological units they inherit from parents and ancestors; according to the unlike proportions of the contrasted units which they severally inherit; and according to the degrees of segregation of such units in different sperm-cells and germ-cells; it may happen that two kindred individuals will produce the ordinary number of offspring or will produce none; or will at one time be fertile and at another not; or will at one time have offspring of tolerable strength and at another time feeble offspring.
To the like causes are also ascribable the phenomena of Variation. These are unobtrusive while the tolerably-uniform conditions of a species maintain tolerable uniformity among the physiological units of its members; but they become obtrusive when differences of conditions, entailing considerable functional differences, have entailed decided differences among the physiological units, and when the different physiological units, differently mingled in every individual, come to be variously segregated and variously combined.
Did space permit, it might be shown that this hypothesis is a key to many further facts--to the fact that mixed races are comparatively plastic under new conditions; to the fact that pure races show predominant influences in the offspring when crossed with mixed races; to the fact that while mixed breeds are often of larger growth, pure breeds are the more hardy--have functions less-easily thrown out of balance. But without further argument it will, I think, be admitted that the power of this hypothesis to explain so many phenomena, and to bring under a common bond phenomena which seem so little allied, is strong evidence of its truth. And such evidence gains greatly in strength on observing that this hypothesis brings the facts of Genesis, Heredity, and Variation into harmony with first principles. We see that these plastic physiological units, which we find ourselves obliged to assume, are just such more integrated, more heterogeneous, more unstable, and more multiform molecules, as would result from continuance of the steps through which organic matter is reached. We see that the differentiations of them assumed to occur in differently-conditioned aggregates, and the equilibrations of them assumed to occur in aggregates which maintain constant conditions, are but corollaries from those universal principles implied by the persistence of force. We see that the maintenance of life in the successive generations of a species, becomes a consequence of the continual incidence of new forces on the species, to replace the forces that are ever being rhythmically equilibrated in the propagation of the species. And we thus see that these apparently-exceptional phenomena displayed in the multiplication of organic beings, fall into their places as results of the general laws of Evolution. We have, therefore, weighty reasons for entertaining the hypothesis which affords us this interpretation.