VII. THE ARGUMENTS FROM DISTRIBUTION
§ 137. In §§ 105 and 106, we contemplated the phenomena of distribution in Space. The general conclusions reached, in great part based on the evidence brought together by Mr. Darwin, were that, "on the one hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by quite different Faunas. On the other hand, we have areas remote from each other in latitude, and contrasted in soil as well as climate, which are occupied by closely-allied Faunas." Whence it was inferred that "as like organisms are not universally, or even generally, found in like habitats; nor very unlike organisms, in very unlike habitats; there is no manifest pre-determined adaptation of the organisms to the habitats." In other words, the facts of distribution in Space do not conform to the hypothesis of design. At the same time we saw that "the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers;" and these generalizations appeared to harmonize with the abundantly-illustrated truth, "that each species of organism tends ever to expand its sphere of existence--to intrude on other areas, other modes of life, other media."
By way of showing still more clearly the effects of competition among races of organisms, let me here add some recently-published instances of the usurpations of areas, and changes of distribution hence resulting. In the Natural History Review for January, 1864, Dr. Hooker quotes as follows from some New Zealand naturalists:--"You would be surprised at the rapid spread of European and other foreign plants in this country. All along the sides of the main lines of road through the plains, a Polygonum (aviculare), called 'Cow Grass,' grows most luxuriantly, the roots sometimes two feet in depth, and the plants spreading over an area from four to five feet in diameter. The dock (Rumex obtusifolius or R. crispus) is to be found in every river bed, extending into the valleys of the mountain rivers, until these become mere torrents. The sow-thistle is spread all over the country, growing luxuriantly nearly up to 6000 feet. The water-cress increases in our still rivers to such an extent, as to threaten to choke them altogether ... I have measured stems twelve feet long and three-quarters of an inch in diameter. In some of the mountain districts, where the soil is loose, the white clover is completely displacing the native grasses, forming a close sward.... In fact, the young native vegetation appears to shrink from competition with these more vigorous intruders." "The native (Maori) saying is 'as the white man's rat has driven away the native rat, so the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself.'"
Given this universal tendency of the superior to overrun the habitats of the inferior,[50] let us consider what, on the hypothesis of evolution, will be the effects on the geographical relationships of species.
§ 138. A race of organisms cannot expand its sphere of existence without subjecting itself to new external conditions. Those of its members which spread over adjacent areas, inevitably come in contact with circumstances partially different from their previous circumstances; and such of them as adopt the habits of other organisms, necessarily experience re-actions more or less contrasted with the re-actions before experienced. Now if changes of organic structure are caused, directly or indirectly, by changes in the incidence of forces; there must result unlikenesses of structure between the divisions of a race which colonizes new habitats. Hence, in the absence of obstacles to migration, we may anticipate manifest kinships between the animals and plants of one area, and those of areas adjoining it. This inference corresponds with an induction before set down (§ 106). In addition to illustrations of it already quoted from Mr. Darwin, his pages furnish others. One is that species which inhabit islands are allied to species which inhabit neighbouring main lands; and another is that the faunas of clustered islands show marked similarities. "Thus the several islands of the Galapagos Archipelago are tenanted," says Mr. Darwin, "in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world." Mr. Wallace has traced "variation as specially influenced by locality" among the Papilionidæ inhabiting the East Indian Archipelago: showing how "the species and varieties of Celebes possess a striking character in the form of the anterior wings, different from that of the allied species and varieties of all the surrounding islands;" and how "tailed species in India and the western islands lose their tails as they spread eastward through the archipelago." During his travels on the Upper Amazons, Mr. Bates found that "the greater part of the species of Ithomiæ changed from one locality to another, not further removed than 100 to 200 miles;" that "many of these local species have the appearance of being geographical varieties;" and that in some species "most of the local varieties are connected with their parent form by individuals exhibiting all the shades of variation."
Further general relationships are to be inferred. If races of organisms, ever being thrust by pressure of population into new habitats, undergo modifications of structure as they diverge more and more widely in Space, it follows that, speaking generally, the widest divergences in Space will indicate the longest periods during which the descendants from a common stock have been subject to modifying conditions; and hence that, among organisms of the same group, the smaller contrasts of structure will be limited to the smaller areas. This we find: "varieties being," as Dr. Hooker says in his Flora of Tasmania, "more restricted in locality than species, and these again than genera." Again, if races of organisms spread, and as they spread are altered by changing incident forces; it follows that where the incident forces vary greatly within given areas, the alterations will be more numerous than in equal areas which are less-variously conditioned. This, too, proves to be the fact. Dr. Hooker points out that the relatively uniform regions have the fewest species; while in the most multiform regions the species are the most numerous.
§ 139. Let us consider next, how the hypothesis of evolution corresponds with the facts of distribution, not over different areas but through different media. If all forms of organisms have descended from some primordial form, it follows that since this primordial form must have inhabited some one medium out of the several media now inhabited, the peopling of other media by its descendants implies migration from one medium to others--implies adaptations to media quite unlike the original medium. To speak specifically--water being the medium in which the lowest living forms exist, the implication is that the earth and the air have been colonized from the water. Great difficulties appear to stand in the way of this assumption. Ridiculing those who alleged the uniserial development of organic forms, who, indeed, laid themselves open to ridicule by their many untenable propositions, Von Baer writes--"A fish, swimming towards the shore desires to take a walk, but finds his fins useless. They diminish in breadth for want of use, and at the same time elongate. This goes on with children and grandchildren for a few millions of years, and at last who can be astonished that the fins become feet? It is still more natural that the fish in the meadow, finding no water, should gape after air, thereby, in a like period of time developing lungs; the only difficulty being that in the meanwhile, a few generations must manage without breathing at all." Though, as thus presented, the belief in a transition looks laughable; and though such derivation of terrestrial vertebrates by direct modification of piscine vertebrates, is untenable; yet we must not conclude that no migrations of the kind alleged can have taken place. The adage that "truth is stranger than fiction," applies quite as much to Nature in general as to human life. Besides the fact that certain fish actually do "take a walk" without any obvious reason; and besides the fact that sundry kinds of fish ramble about on land when prompted by the drying-up of the waters they inhabit; there is the still more astounding fact that one kind of fish climbs trees. Few things seem more manifestly impossible, than that a water-breathing creature without efficient limbs, should ascend eight or ten feet up the trunk of a palm; and yet the Anabas scandens does as much. To previous testimonies on this point Capt. Mitchell has recently added others. Such remarkable cases of temporary changes of media, will prepare us for conceiving how, under special conditions, permanent changes of media may have taken place; and for considering how the doctrine of evolution is elucidated by them.
Inhabitants of the sea, of rivers, and of lakes, are many of them left from time to time partially or completely without water; and those which show the power to change their media temporarily or permanently, are in very many cases of the kinds most liable to be thus deserted by their medium. Let us consider what the sea-shore shows us. Twice a day the rise and the fall of the tide covers and uncovers plants and animals, fixed and moving; and through the alternation of spring and neap tides, it results that the exposure of the organisms living low down on the beach, varies both in frequency and duration: while some of them are left dry only once a fortnight for a very short time, others, a little higher up, are left dry during two or three hours at several ebb tides every fortnight. Then by small gradations we come to such as, living at the top of the beach, are bathed by salt-water only at long intervals; and still higher to some which are but occasionally splashed in stormy weather. What, now, do we find among the organisms thus subject to various regular and irregular alterations of media? Besides many plants and many fixed animals, we find moving animals of numerous kinds; some of which are confined to the lower zones of this littoral region, but others of which wander over the whole of it. Omitting the humbler types, it will suffice to observe that each of the two great sub-kingdoms, Mollusca and Arthropoda, supplies examples of creatures having a wide excursiveness within this region. We have gasteropods which, when the tide is down, habitually creep snail-like over sand and sea-weed, even up as far as high-water mark. We have several kinds of crustaceans, of which the crab is the most conspicuous, running about on the wet beach, and sometimes rambling beyond the reach of the water. And then note the striking fact that each of the forms thus habituated to changes of media, is allied to forms which are mainly or wholly terrestrial. On the West Coast of Ireland marine gasteropods are found on the rocks three hundred feet above the sea, where they are only at long intervals wetted by the spray; and though between gasteropods of this class and land-gasteropods the differences are considerable, yet the land-gasteropods are more closely allied to them than to any other Mollusca. Similarly, the two highest orders of crustaceans have their species which live occasionally, or almost entirely, out of the water: there is a kind of lobster in the Mauritius which climbs trees; and there is the land-crab of the West Indies, which deserts the sea when it reaches maturity and re-visits it only to spawn. Seeing, thus, how there are many kinds of marine creatures whose habitats expose them to frequent changes of media; how some of the higher kinds so circumstanced, show a considerable adaptation to both media; and how these amphibious kinds are allied to kinds that are mainly or wholly terrestrial; we shall see that the migrations from one medium to another, which evolution pre-supposes, are by no means impracticable. With such evidence before us, the assumption that the distribution of the Vertebrata through media so different as air and water, may have been gradually effected in some analogous manner, would not be altogether unwarranted even had we no clue to the process. We shall find, however, a tolerably distinct clue. Though rivers, and lakes, and pools, have no sensible tidal variations, they have their rises and falls, regular and irregular, moderate and extreme. Especially in tropical climates, we see them annually full for a certain number of months, and then dwindling away and drying up. The drying up may reach various degrees and last for various periods. It may go to the extent only of producing a liquid mud, or it may reduce the mud to a hardened, fissured solid. It may last for a few days or for months. That is to say, aquatic forms which are in one place annually subject to a slight want of water for a short time, are elsewhere subject to greater wants for longer times: we have gradations of transition, analogous to those which the tides furnish. Now it is well known that creatures inhabiting such waters have, in various degrees, powers of meeting these contingencies. The contained fish either bury themselves in the mud when the dry season comes, or ramble in search of other waters. This is proved by evidence from India, Guiana, Siam, Ceylon; and some of these fish, as the Anabas scandens, are known to survive for days out of the water. But the facts of greatest significance are furnished by an allied class of Vertebrata, almost peculiar to habitats of this kind. The Amphibia are not, like fish, usually found in waters that are never partially or wholly dried up; but they nearly all inhabit waters which, at certain seasons, evaporate, in great measure or completely--waters in which most kinds of fish cannot exist. And what are the leading structural traits of these Amphibia? They have two respiratory systems--pulmonic and branchial--variously developed in different orders; and they have two or four limbs, also variously developed. Further, the class Amphibia consists of two groups, in one of which this duality of the respiratory system is permanent, and the development of the limbs always incomplete; and in the other of which the branchiæ disappear as the lungs and limbs become fully developed. The lowest group, the Perennibranchiata, have internal organs for aerating the blood which approach in various degrees to lungs, until "in the Siren, the pulmonic respiration is more extensive and important than the branchial;" and to these creatures, having a habitat partially aërial and partially aquatic, there are at the same time supplied, in the shallow water covering soft mud, the mechanical conditions which render swimming difficult and rudimentary limbs useful. In the higher group, the Caducibranchiata, we find still more suggestive transformations. Having at first a structure resembling that which is permanent in the perennibranchiate amphibian, the larva of the caducibranchiate amphibian pursues for a time a similar life; but, eventually, while the branchial appendages dwindle the lungs grow: the respiration of air, originally supplementary to the respiration of water, predominates over it more and more, till it replaces it entirely; and an additional pair of legs is produced. This having been done, the creature either becomes, like the Triton, one which quits the water only occasionally; or, like the Frog, one which pursues a life mainly terrestrial, and returns to the water now and then. Finally, if we ask under what conditions this metamorphosis of a water-breather into an air-breather completes itself, the answer is--it completes itself at the time when the shallow pools inhabited by the larvæ are being dried up, or in danger of being dried up, by the summer's sun.[51]
See, then, how significant are the facts when thus brought together. There are particular habitats in which animals are subject to changes of media. In such habitats exist animals having, in various degrees, the power to live in both media, consequent on various phases of transitional organization. Near akin to these animals there are some that, after passing their early lives in the water, acquire more completely the structures fitting them to live on land, to which they then migrate. Lastly, we have closely-allied creatures, like the Surinam toad and the terrestrial salamander, which, though they belong by their structures to the class Amphibia, are not amphibious in their habits--creatures the larvæ of which do not pass their early lives in the water, and yet go through these same metamorphoses! Must we then think, like Von Baer, that the distribution of kindred organisms through different media presents an insurmountable difficulty? On the contrary, with facts like these before us, the evolution-hypothesis supplies possible interpretations of many phenomena that are else unaccountable. After seeing the ways in which such changes of media are in some cases gradually imposed by physical conditions, and in other cases voluntarily commenced and slowly increased in the search after food; we shall begin to understand how, in the course of evolution, there have arisen strange obscurations of one type by the externals of another type. When we see land-birds occasionally feeding by the water-side, and then learn that one of them, the water-ouzel, an "anomalous member of the strictly terrestrial thrush family, wholly subsists by diving--grasping the stones with its feet and using its wings under water"--we are enabled to comprehend how, under pressure of population, aquatic habits may be acquired by creatures organized for aërial life; and how there may eventually arise an ornithic type in which the traits of the bird are very much disguised. On finding among mammals some that, in search of prey or shelter, have taken to the water in various degrees, we shall cease to be perplexed on discovering the mammalian structure hidden under a fish-like form, as it is in the Cetacea and the Sirenia: especially on finding that in the sea-lion and the seals there are transitional forms. Grant that there has ever been going on that re-distribution of organisms which we see still resulting from their intrusions on one another's areas, media, and modes of life; and we have an explanation of those multitudinous cases in which homologies of structure are complicated with analogies. And while it accounts for the occurrence in one medium of organic types fundamentally organized for another medium, the doctrine of evolution accounts also for the accompanying unfitnesses. Either the seal has descended from some mammal which little by little became aquatic in its habits, in which case the structure of its hind limbs has a meaning; or else it was specially framed for its present habitat, in which case the structure of its hind limbs is incomprehensible.
§ 140. The facts respecting distribution in Time, which have more than any others been cited both in proof and in disproof of evolution, are too fragmentary to be conclusive either way. Were the geological record complete, or did it, as both Uniformitarians and Progressionists have commonly assumed, give us traces of the earliest organic forms; the evidence hence derived, for or against, would have had more weight than any other evidence. As it is, all we can do is to see whether such fragmentary evidence as remains, is congruous with the hypothesis.
Palæontology has shown that there is a "general relation between lapse of time and divergence of organic forms" (§ 107); and that "this divergence is comparatively slow and continuous where there is continuity in the geological formations, but is sudden and comparatively wide wherever there occurs a great break in the succession of strata." Now this is obviously what we should expect. The hypothesis implies structural changes that are not sudden but gradual. Hence, where conformable strata indicate a continuous record, we may anticipate successions of forms only slightly different from one another; while we may rationally look for marked contrasts between the groups of forms fossilized in adjacent strata, where there is evidence of a great blank in the record.
The permanent disappearances of species, of genera, and of orders, which we saw to be a fact tolerably-well established, is also a fact for which the belief in evolution prepares us. If later organic forms have in all cases descended from earlier organic forms, and have diverged during their descent, both from their prototypes and from one another; then it follows that such of them as become extinct at any epoch, will never re-appear at a subsequent epoch; since there can never again arise a concurrence and succession of conditions such as those under which each type was evolved.
Though comparisons of ancient and modern organic forms, prove that many types have persisted through enormous periods of time, without undergoing great changes; it was shown that such comparisons do not disprove the occurrence in other organic forms, of changes great enough to produce what are called different types. The result of inductive inquiry we saw to be, that while a few modern higher types yield signs of having been developed from ancient lower types; and that while there are many modern types which may have been thus developed, though we are without evidence that they have been so; yet that "any admissible hypothesis of progressive modification must be compatible with persistence without progression through indefinite periods." Now these results are quite congruous with the hypothesis of evolution. As rationally interpreted, evolution must in all cases be understood to result, directly or indirectly, from the incidence of forces. If there are no changes of conditions entailing organic changes, organic changes are not to be expected. Only in organisms which fall under conditions leading to additional modifications answering to additional needs, will there be that increased heterogeneity which characterizes higher forms. Hence, though the facts of palæontology cannot be held conclusive proof of evolution, yet they are congruous with it; and some of them yield it strong support.
§ 141. One general truth respecting distribution in Time, is profoundly significant. If, instead of contemplating the relations among past forms of life taken by themselves, we contemplate the relations between them and the forms now existing, we find a connexion which is in harmony with the belief in evolution but irreconcilable with any other belief.
Note, first, how full of meaning is the close kinship existing between the aggregate of organisms now living, and the aggregate of organisms which lived in the most recent geologic times. In the last-formed strata, nearly all the imbedded remains are those of species which still flourish. Strata a little older contain a few fossils of species now extinct, though, usually, species greatly resembling extant ones. Of the remains found in strata of still earlier date, the extinct species form a larger percentage; and the differences between them and the allied species now living are more marked. That is to say, the gradual change of organic types in Time, which we before saw is indicated by the geological record, is equally indicated by the relation between existing organic types and organic types of the epochs preceding our own. The evidence completely accords with the belief in a descent of present life from past life. Doubtless such a kinship is not incongruous with the doctrine of special creations. It may be argued that the introduction, from time to time, of new species better fitted to the somewhat changed conditions of the Earth's surface, would result in an apparent alliance between our living Flora and Fauna, and the Floras and Faunas that lately lived. No one can deny it. But on passing from the most general aspect of the alliance to its more special aspects, we shall find this interpretation completely negatived.
For besides a close kinship between the aggregate of surviving forms and the aggregate of forms which have died out in recent geologic times; there is a peculiar connexion of like nature between present and past forms in each great geographical region. The instructive fact, before cited from Mr. Darwin, is the "wonderful relationship in the same continent between the dead and the living." This relationship is not explained by the supposition that new species have been at intervals supernaturally placed in each habitat, as the habitat became modified; since, as we saw, species are by no means uniformly found in the habitats to which they are best adapted. It cannot be said that the marsupials imbedded in recent Australian strata, having become extinct because of unfitness to some new external condition, the existing marsupials were then specially created to fit the modified environment; since sundry animals found elsewhere are so much more in harmony with these new Australian conditions that, when taken to Australia, they rapidly extrude the marsupials. While, therefore, the similarity between the existing Australian Fauna and the Fauna which immediately preceded it over the same area, is just that which the belief in evolution leads us to expect; it is a similarity which cannot be otherwise accounted for. And so is it with parallel relations in New England, in South America, and in Europe.
§ 142. Given, then, that pressure which species exercise on one another, in consequence of the universal overfilling of their respective habitats--given the resulting tendency to thrust themselves into one another's areas, and media, and modes of life, along such lines of least resistance as from time to time are found--given besides the changes in modes of life, hence arising, those other changes which physical alterations of habitats necessitate--given the structural modifications directly or indirectly produced in organisms by modified conditions; and the facts of distribution in Space and Time are accounted for. That divergence and re-divergence of organic forms, which we saw to be shadowed forth by the truths of classification and the truths of embryology, we see to be also shadowed forth by the truths of distribution. If that aptitude to multiply, to spread, to separate, and to differentiate, which the human races have in all times shown, be a tendency common to races in general, as we have ample reason to assume; then there will result those kinds of spacial relations and chronological relations among the species, and genera, and orders, peopling the Earth's surface, which we find exist. The remarkable identities of type discovered between organisms inhabiting one medium, and strangely modified organisms inhabiting another medium, are at the same time rendered comprehensible. And the appearances and disappearances of species which the geological record shows us, as well as the connexions between successive groups of species from early eras down to our own, cease to be inexplicable.
VIII. HOW IS ORGANIC EVOLUTION CAUSED?
§ 143. Already it has been necessary to speak of the causes of organic evolution in general terms; and now we are prepared for considering them specifically. The task before us is to affiliate the leading facts of organic evolution, on those same first principles conformed to by evolution at large.
Before attempting this, however, it will be instructive to glance at the causes of organic evolution which have been from time to time alleged.
§ 144. The theory that plants and animals of all kinds were gradually evolved, seems to have been at first accompanied only by the vaguest conception of cause--or rather, by no conception of cause properly so called, but only by the blank form of a conception. One of the earliest who in modern times (1735) contended that organisms are indefinitely modifiable, and that through their modifications they have become adapted to various modes of existence, was De Maillet. But though De Maillet supposed all living beings to have arisen by a natural, continuous process, he does not appear to have had any definite idea of that which determines this process. In 1794, in his Zoonomia, Dr. Erasmus Darwin gave reasons (sundry of them valid ones) for believing that organized beings of every kind, have descended from one, or a few, primordial germs; and along with some observable causes of modification, which he points out as aiding the developmental process, he apparently ascribes it, in part, to a tendency given to such germ or germs when created. He suggests the possibility "that all warm-blooded animals have arisen from one living filament, which THE GREAT FIRST CAUSE endued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions, and associations; and thus possessing the faculty of continuing to improve by its own inherent activity." In this passage we see the idea to be, that evolution is pre-determined by some intrinsic proclivity. "It is curious," says Mr. Charles Darwin, "how largely my grandfather, Dr. Erasmus Darwin, anticipated the erroneous grounds of opinion, and the views of Lamarck." One of the anticipations was this ascription of development to some inherent tendency. To the "plan général de la nature, et sa marche uniforme dans ses opérations," Lamarck attributes "la progression évidente qui existe dans la composition de l'organisation des animaux;" and "la gradation régulière qu'ils devroient offrir dans la composition de leur organisation," he thinks is rendered irregular by secondary causes. Essentially the same in kind, though somewhat different in form, is the conception put forth in the Vestiges of Creation; the author of which contends "that the several series of animated beings, from the simplest and oldest up to the highest and most recent, are, under the providence of God, the results, first, of an impulse which has been imparted to the forms of life, advancing them, in definite times, by generation, through grades of organization terminating in the highest dicotyledons and vertebrata;" and that the progression resulting from these impulses, is modified by certain other causes. The broad contrasts between lower and higher forms of life, are regarded by him as implying an innate aptitude to give birth to forms of more perfect structures. The last to re-enunciate this doctrine has been Prof. Owen; who asserts "the axiom of the continuous operation of creative power, or of the ordained becoming of living things." Though these words do not suggest a very definite idea, yet they indicate the belief that organic progress is a result of some in-dwelling tendency to develop, supernaturally impressed on living matter at the outset--some ever-acting constructive force which, independently of other forces, moulds organisms into higher and higher forms.
In whatever way it is formulated, or by whatever language it is obscured, this ascription of organic evolution to some aptitude naturally possessed by organisms, or miraculously imposed on them, is unphilosophical. It is one of those explanations which explain nothing--a shaping of ignorance into the semblance of knowledge. The cause assigned is not a true cause--not a cause assimilable to known causes--not a cause that can be anywhere shown to produce analogous effects. It is a cause unrepresentable in thought: one of those illegitimate symbolic conceptions which cannot by any mental process be elaborated into a real conception. In brief, this assumption of a persistent formative power inherent in organisms, and making them unfold into higher types, is an assumption no more tenable than the assumption of special creations: of which, indeed, it is but a modification; differing only by the fusion of separate unknown processes into a continuous unknown process.
§ 145. Besides this intrinsic tendency to progress which Dr. Darwin ascribes to animals, he says they have a capacity for being modified by processes which their own desires initiate. He speaks of powers as "excited into action by the necessities of the creatures which possess them, and on which their existence depends;" and more specifically he says that "from their first rudiment or primordium, to the termination of their lives, all animals undergo perpetual transformations; which are in part produced by their own exertions, in consequence of their desires and aversions, of their pleasures and their pains, or of irritations, or of associations; and many of these acquired forms or properties are transmitted to their posterity." While it embodies a belief for which much may be said, this passage involves the assumption that desires and aversions, existing before experiences of the actions to which they are related, were the originators of the actions, and therefore of the structural modifications caused by them. In his Philosophie Zoologique, Lamarck much more specifically asserts "le sentiment intérieur," to be in all creatures that have developed nervous systems, an independent cause of those changes of form which are due to the exercise of organs: distinguishing it from that simple irritability possessed by inferior animals, which cannot produce what we call a desire or emotion; and holding that these last, along with all "qui manquent de système nerveux, ne vivent qu'à l'aide des excitations qu'ils reçoivent de l'extérieur." Afterwards he says--"je reconnus que la nature, obligée d'abord d'emprunter des milieux environnants la puissance excitatrice des mouvements vitaux et des actions des animaux imparfaits, sut, en composant de plus en plus l'organisation animale, transporter cette puissance dans l'intérieur même de ces êtres, et qu'à la fin, elle parvint à mettre cette même puissance à la disposition de l'individu." And still more definitely he contends that if one considers "la progression qui se montre dans la composition de l'organisation," ... "alors on eût pu apercevoir comment les besoins, d'abord réduits à nullité, et dont le nombre ensuite s'est accru graduellement, ont amené le penchant aux actions propres à y satisfaire: comment les actions devenues habituelles et énergiques, ont occasionné le développement des organes qui les exécutent."
Now though this conception of Lamarck is more precisely stated, and worked out with much greater elaboration and wider knowledge of the facts, it is essentially the same as that of Dr. Darwin; and along with the truth it contains, contains also the same error more distinctly pronounced. Merely noting that desires or wants, acting directly only on the nervo-muscular system, can have no immediate influence on very many organs, as the viscera, or such external appendages as hair and feathers; and observing, further, that even some parts which belong to the apparatus of external action, such as the bones of the skull, cannot be made to grow by increase of function called forth by desire; it will suffice to point out that the difficulty is not solved, but simply slurred over, when needs or wants are introduced as independent causes of evolution. True though it is, as Dr. Darwin and Lamarck contend, that desires, by leading to increased actions of motor organs, may induce further developments of such organs; and true, as it probably is, that the modifications hence arising are transmissible to offspring; yet there remains the unanswered question--Whence do these desires originate? The transference of the exciting power from the exterior to the interior, as described by Lamarck, begs the question. How comes there a wish to perform an action not before performed? Until some beneficial result has been felt from going through certain movements, what can suggest the execution of such movements? Every desire consists primarily of a mental representation of that which is desired, and secondarily excites a mental representation of the actions by which it is attained; and any such mental representations of the end and the means, imply antecedent experience of the end and antecedent use of the means. To assume that in the course of evolution there from time to time arise new kinds of actions dictated by new desires, is simply to remove the difficulty a step back.
§ 146. Changes of external conditions are named, by Dr. Darwin, as causes of modifications in organisms. Assigning as evidence of original kinship, that marked similarity of type which exists among animals, he regards their deviations from one another, as caused by differences in their modes of life: such deviations being directly adaptive. After enumerating various appliances for procuring food, he says they all "seem to have been gradually produced during many generations by the perpetual endeavour of the creatures to supply the want of food, and to have been delivered to their posterity with constant improvement of them for the purposes required." And the creatures possessing these various appliances are considered as having been rendered unlike by seeking for food in unlike ways. As illustrating the alterations wrought by changed circumstances, he names the acquired characters of domestic animals. Lamarck has elaborated the same view in detail: using for the purpose, with great ingenuity, his extensive knowledge of the animal kingdom. From a passage in the Avertissement it would at first sight seem that he looks upon direct adaptation to new conditions as the chief cause of evolution. He says--"Je regardai comme certain que le mouvement des fluides dans l'intérieur des animaux, mouvement qui c'est progressivement accéléré avec la composition plus grande de l'organisation; et que l'influence des circonstances nouvelles, à mesure que les animaux s'y exposèrent en se répandant dans tous les lieux habitables, furent les deux causes générales qui ont amené les différents animaux à l'état où nous les voyons actuellement." But elsewhere the view he expresses appears decidedly different from this. He asserts that "dans sa marche, la nature a commencé, et recommence encore tous les jours, par former les corps organisés les plus simples;" and that "les premières ébauches de l'animal et du végétal étant formées dans les lieux et les circonstances convenables, les facultés d'une vie commençante et d'un mouvement organique établi, ont nécessairement développé peu à peu les organes, et qu'avec le temps elles les ont diversifies ainsi que les parties." And then, further on, he puts in italics this proposition:--"La progression dans la composition de l'organisation subit, çà et là, dans la série générale des animaux, des anomalies opérées par l'influence des circonstances d'habitation, et par celle des habitudes contractées." These, and sundry other passages, joined with his general scheme of classification, make it clear that Lamarck conceived adaptive modification to be, not the cause of progression, but the cause of irregularities in progression. The inherent tendency which organisms have to develop into more perfect forms, would, according to him, result in a uniform series of forms; but varieties in their conditions work divergences of structure, which break up the series into groups: groups which he nevertheless places in uni-serial order, and regards as still substantially composing an ascending succession.
§ 147. These speculations, crude as they may be considered, show much sagacity in their respective authors, and have done good service. Without embodying the truth in definite shapes, they contain adumbrations of it. Not directly, but by successive approximations, do mankind reach correct conclusions; and those who first think in the right direction, loose as may be their reasonings, and wide of the mark as their inferences may be, yield indispensable aid by framing provisional conceptions and giving a bent to inquiry.
Contrasted with the dogmas of his age, the idea of De Maillet was a great advance. Before it can be ascertained how organized beings have been gradually evolved, there must be reached the conviction that they have been gradually evolved; and this conviction he reached. His wild notions about the way in which natural causes acted in the production of plants and animals, must not make us forget the merit of his intuition that animals and plants were produced by natural causes. In Dr. Darwin's brief exposition, the belief in a progressive genesis of organisms is joined with an interpretation having considerable definiteness and coherence. In the space of ten pages he not only indicates several of the leading classes of facts which support the hypothesis of development, but he does something towards suggesting the process of development. His reasonings show an unconscious mingling of the belief in a supernaturally-impressed tendency to develop, with the belief in a development arising from the changing incidence of conditions. Probably had he pursued the inquiry further, this last belief would have grown at the expense of the first. Lamarck, in elaborating this general conception, has given greater precision both to its truth and to its error. Asserting the same imaginary factors and the same real factors, he has traced out their supposed actions in detail; and has, in consequence, committed himself to a greater number of untenable positions. But while, in trying to reconcile the facts with a theory which is only an adumbration of the truth, he laid himself open to the criticisms of his contemporaries; he proved himself profounder than his contemporaries by seeing that natural genesis, however caused, has been going on. If they were wise in not indorsing a theory which fails to account for a great part of the facts; they were unwise in ignoring that degree of congruity with the facts, which shows the theory to contain some fundamental verity.
Leaving out, however, the imaginary factors of evolution which these speculations allege, and looking only at the one actual factor which Dr. Darwin and Lamarck assign as accounting for some of the phenomena; it is manifest, from our present stand-point, that this, so far as it is a cause of evolution, is a proximate cause and not an ultimate cause. To say that functionally-produced adaptation to conditions originates either evolution in general, or the irregularities of evolution, is to raise the further question--why is there a functionally-produced adaptation to conditions?--why do use and disuse generate appropriate changes of structure? Neither this nor any other interpretation of biologic evolution which rests simply on the basis of biologic induction, is an ultimate interpretation. The biologic induction must itself be interpreted. Only when the process of evolution of organisms is affiliated on the process of evolution in general, can it be truly said to be explained. The thing required is to show that its various results are corollaries from first principles. We have to reconcile the facts with the universal laws of the re-distribution of matter and motion.
IX. EXTERNAL FACTORS
§ 148. When illustrating the rhythm of motion (First Principles, § 83) it was pointed out that besides the daily and annual alternations in the quantities of light and heat which any portion of the Earth's surface receives from the Sun, there are alternations which require immensely-greater periods to complete. Reference was made to the fact that "every planet, during a certain long period, presents more of its northern than of its southern hemisphere to the Sun at the time of its nearest approach to him; and then again, during a like period, presents more of its southern hemisphere than of its northern--a recurring coincidence which, though it causes in some planets no sensible alterations of climate, involves, in the case of the Earth, an epoch of 21,000 years during which each hemisphere goes through a cycle of temperate seasons, and seasons that are extreme in their heat and cold." Further, we saw that there is a variation of this variation. The slow rhythm of temperate and intemperate climates, which takes 21,000 years to complete itself, undergoes exaggeration and mitigation during epochs that are far longer. The Earth's orbit slowly alters in form: now approximating to a circle, and now becoming more eccentric. During the period in which the Earth's orbit has least eccentricity, the temperate and intemperate climates which repeat their cycle in 21,000 years, are severally less temperate and less intemperate, than when, some one or two millions of years later, the Earth's orbit has reached its extreme of eccentricity.
Thus, besides those daily variations in the quantities of light and heat received by organisms, and responded to by variations in their functions; and besides the annual variations in the quantities of light and heat which organisms receive, and similarly respond to by variations in their functions; there are variations that severally complete themselves in 21,000 years and in some millions of years--variations to which there must also be responses in the changed functions of organisms. The whole vegetal and animal kingdoms, are subject to quadruply-compounded rhythms in the incidence of the forces on which life primarily depends--rhythms so involved in their slow working round that at no time during one of these vast epochs, can the incidence of these various forces be exactly the same as at any other time. To the direct effects so produced on organisms, have to be added much more important indirect effects. Changes of distribution must result. Certain redistributions are occasioned even by the annual variations in the quantities of the solar rays received by each part of the Earth's surface. The migrations of birds thus caused are familiar. So, too, are the migrations of certain fishes: in some cases from one part of the sea to another; in some cases from salt water to fresh water; and in some cases from fresh water to salt water. Now just as the yearly changes in the amounts of light and heat falling on each locality, yearly extend and restrict the habitats of many organisms which are able to move about with some rapidity; so must the alterations of temperate and intemperate climates produce extensions and restrictions of habitats. These, though slow, must be universal--must affect the habitats of stationary organisms as well as those of locomotive ones. For if, during an astronomic era, there is going on at any limit to a plant's habitat, a diminution of the winter's cold or summer's heat, which had before stopped its spread at that limit; then, though the individual plants are fixed, yet the species will move: the seeds of plants living at the limit, will produce individuals which survive beyond the limit. The gradual spread so effected, having gone on for some ten thousand years, the opposite change of climate will begin to cause retreat. The tide of each species will, during one half of a long epoch, slowly flow into new regions, and then will slowly ebb away from them. Further, this rise and fall in the tide of each species will, during far longer intervals, undergo increasing rises and falls and then decreasing rises and falls. There will be an alteration of spring tides and neap tides, answering to the changing eccentricity of the Earth's orbit.
These astronomical rhythms, therefore, entail on organisms unceasing changes in the incidence of forces in two ways. They directly subject them to variations of solar influences, in such a manner that each generation is somewhat differently affected in its functions; and they indirectly bring about complicated alterations in the environing agencies, by carrying each species into the presence of new physical conditions, new soil and surface.
§ 149. The power of geological actions to modify everywhere the circumstances in which plants and animals are placed, is conspicuous. In each locality denudation slowly uncovers different deposits, and slowly changes the exposed areas of deposits already uncovered. Simultaneously, the alluvial beds in course of formation, are qualitatively affected by these progressive changes in the natures and proportions of the strata denuded. The inclinations of surfaces and their directions with respect to the Sun, are at the same time modified; and the organisms existing on them are thus having their thermal conditions continually altered, as well as their drainage. Igneous action, too, complicates these gradual modifications. A flat region cannot be step by step thrust up into a protuberance without unlike climatic changes being produced in its several parts, by their exposures to different aspects. Extrusions of trap, wherever they take place, revolutionize the localities; both over the areas covered and over the areas on to which their detritus is carried. And where volcanoes are formed, the ashes they occasionally send out modify the character of the soil throughout large surrounding tracts.
In like manner alterations in the Earth's crust cause the ocean to be ever subjecting the organisms it contains to new combinations of conditions. Here the water is being deepened by subsidence, and there shallowed by upheaval. While the falling upon it of sediment brought down by neighbouring large rivers, is raising the sea-bottom in one place, in another the habitual rush of the tide is carrying away the sediment deposited in past times. The mineral character of the submerged surface on which sea-weeds grow and molluscs crawl, is everywhere occasionally changed; now by the bringing away from an adjacent shore some previously untouched strata; and now by the accumulation of organic remains, such as the shells of pteropods or of foraminifera. A further series of alterations in the circumstances of marine organisms, is entailed by changes in the movements of the water. Each modification in the outlines of neighbouring shores makes the tidal streams vary their directions or velocities or both. And the local temperature is from time to time raised or lowered, because some far-distant change of form in the Earth's crust has wrought a divergence in those circulating currents of warm and cold water which pervade the ocean.
These geologically-caused changes in the physical characters of each environment, occur in ever-new combinations, and with ever-increasing complexity. As already shown (First Principles, § 158), it follows from the law of the multiplication of effects, that during long periods each tract of the Earth's surface increases in heterogeneity of both form and substance. So that plants and animals of all kinds are, in the course of generations, subjected by alterations in the crust of the Earth, to sets of incident forces differing from previous sets, both by changes in the proportions of the factors and, occasionally, by the addition of new factors.
§ 150. Variations in the astronomical conditions joined with variations in the geological conditions, bring about variations in the meteorological conditions. Those slow alternations of elevation and subsidence which take place over immense areas, here producing a continent where once there was a fathomless ocean, and there causing wide seas to spread where in a long past epoch there stood snow-capped mountains, gradually work great atmospheric changes. While the highest parts of an emerging surface of the Earth's crust exist as a cluster of islands, the plants and animals which in course of time migrate to them have climates that are peculiar to small tracts of land surrounded by large tracts of water. As, by successive upheavals, greater areas are exposed, there begin to arise sensible contrasts between the states of their peripheral parts and their central parts. The breezes which daily moderate the extremes of temperature near the shores, cease to affect the interiors; and the interiors, less qualified too in their heat and cold by such ocean-currents as approach the coast, acquire more decidedly the characters due to their latitudes. Along with the further elevations which unite the members of the archipelago into a continent, there come new meteorologic changes, as well as exacerbations of the old. The winds, which were comparatively uniform in their directions and periods when only islands existed, grow involved in their distribution, and widely-different in different parts of the continent. The quantities of rain which they discharge and of moisture which they absorb, vary everywhere according to the proximity to the sea and to surfaces of land having special characters.
Other complications result from variations of height above the sea: elevation producing a decrease of heat and consequently an increase in the precipitation of water--a precipitation which takes the shape of snow where the elevation is very great, and of rain where it is not so great. The gatherings of clouds and descents of showers around mountain tops, are familiar to every tourist. Inquiries in the neighbouring valleys prove that within distances of a mile or two the recurring storms differ in their frequency and violence. Nay, even a few yards off, the meteorological conditions vary in such regions: as witness the way in which the condensing vapour keeps eddying round on one side of some high crag, while the other side is clear; or the way in which the snowline runs irregularly to different heights, in all the hollows and ravines of each mountain side.
As climatic variations thus geologically produced, are compounded with those which result from slow astronomical changes; and as no correspondence exists between the geologic and the astronomic rhythms; it results that the same plexus of actions never recurs. Hence the incident forces to which the organisms of every locality are exposed by atmospheric agencies, are ever passing into unparalleled combinations; and these are on the average ever becoming more complex.
§ 151. Besides changes in the incidence of inorganic forces, there are equally continuous, and still more involved, changes in the incidence of forces which organisms exercise on one another. As before pointed out (§ 105), the plants and animals inhabiting each locality are held together in so entangled a web of relations, that any considerable modification which one species undergoes, acts indirectly on many other species, and eventually changes, in some degree, the circumstances of nearly all the rest. If an increase of heat, or modification of soil, or decrease of humidity, causes a particular kind of plant either to thrive or to dwindle, an unfavourable or favourable effect is wrought on all such competing kinds of plants as are not immediately influenced in the same way. The animals which eat the seeds or browse on the leaves, either of the plant primarily affected or those of its competitors, are severally altered in their states of nutrition and in their numbers; and this change presently tells on various predatory animals and parasites. And since each of these secondary and tertiary changes becomes itself a centre of others, the increase or decrease of each species produces waves of influence which spread and reverberate and re-reverberate throughout the whole Flora and Fauna of the locality.
More marked and multiplied still, are the ultimate effects of those causes which make possible the colonization of neighbouring areas. Each intruding plant or animal, besides the new inorganic conditions to which it is subject, is subject to organic conditions different from those to which it has been accustomed. It has to compete with some organisms unlike those of its preceding habitat. It must preserve itself from enemies not before encountered. Or it may meet with a species over which it has some advantage greater than any it had over the species it was previously in contact with. Even where migration does not bring it face to face with new competitors or new enemies or new prey, it inevitably experiences new proportions among these. Further, an expanding species is almost certain to invade more than one adjacent region. Spreading both north and south, or east and west, it will come among the plants and animals, here of a level district and there of a hilly one--here of an inland tract and there of a tract bordered by the sea. And while different groups of its members will thus expose themselves to the actions and reactions of different Floras and Faunas, these different Floras and Faunas will simultaneously have their organic conditions changed by the intruders.
This process becomes gradually more active and more complicated. Though, in particular cases, a plant or animal may fall into simpler relations with the living things around than those it was before placed in, yet it is manifest that, on the average, the organic environments of organisms have been advancing in heterogeneity. As the number of species with which each species is directly or indirectly implicated, multiplies, each species is oftener subject to changes in the organic actions which influence it. These more frequent changes severally grow more involved. And the corresponding reactions affect larger Floras and Faunas, in ways increasingly complex and varied.
§ 152. When the astronomic, geologic, meteorologic, and organic agencies which are at work on each species of plant and animal are contemplated as becoming severally more complicated in themselves, and as co-operating in ways that are always partially new; it will be seen that throughout all time there has been an exposure of organisms to endless successions of modifying causes which gradually acquire an intricacy scarcely conceivable. Every kind of plant and animal may be regarded as for ever passing into a new environment--as perpetually having its relations to external circumstances altered, either by their changes with respect to it when it remains stationary, or by its changes with respect to them when it migrates, or by both.
Yet a further cause of progressive alteration and complication in the incident forces, exists. All other things continuing the same, every additional faculty by which an organism is brought into relation with external objects, as well as every improvement in such faculty, becomes a means of subjecting the organism to a greater number and variety of external stimuli, and to new combinations of external stimuli. So that each advance in complexity of organization, itself becomes an added source of complexity in the incidence of external forces.
Once more, every increase in the locomotive powers of animals, increases both the multiplicity and the multiformity of the actions of things upon them, and of their reactions upon things. Doubling a creature's activity quadruples the area that comes within the range of its excursions; thus augmenting in number and heterogeneity, the external agencies which act on it during any given interval.
By compounding the actions of these several orders of factors, there is produced a geometric progression of changes, increasing with immense rapidity. And there goes on an equally rapid increase in the frequency with which the combinations of the actions are altered, and the intricacies of their co-operations enhanced.
X. INTERNAL FACTORS
§ 153. We saw at the outset (§§ 10-16), that organic matter is built up of molecules so unstable, that the slightest variation in their conditions destroys their equilibrium, and causes them either to assume altered structures or to decompose. But a substance which is beyond all others changeable by the actions and reactions of the forces liberated from instant to instant within its own mass, must be a substance which is beyond all others changeable by the forces acting on it from without. If their composition fits organic aggregates for undergoing with special facility and rapidity those re-distributions of matter and motion whence result individual organization and life; then their composition must make them similarly apt to undergo those permanent re-distributions of matter and motion which are expressed by changes of structure, in correspondence with permanent re-distributions of matter and motion in their environments.
In First Principles, when considering the phenomena of Evolution at large, the leading characters and causes of those changes which constitute organic evolution were briefly traced. Under each of the derivative laws of force to which the passage from an incoherent, indefinite homogeneity to a coherent, definite heterogeneity, conforms, were given illustrations drawn from the metamorphoses of living bodies. Here it will be needful to contemplate the several resulting processes as going on at once, in both individuals and species.
§ 154. Our postulate being that organic evolution in general commenced with homogeneous organic matter, we have first to remember that the state of homogeneity is an unstable state (First Principles, § 149). In any aggregate "the relations of outside and inside, and of comparative nearness to neighbouring sources of influence, imply the reception of influences that are unlike in quantity, or quality, or both; and it follows that unlike changes will be produced in the parts thus dissimilarly acted upon." Further, "if any given whole, instead of being absolutely uniform throughout, consists of parts distinguishable from one another--if each of these parts, while somewhat unlike other parts, is uniform within itself; then, each of them being in unstable equilibrium, it follows that while the changes set up within it must render it multiform, they must at the same time render the whole more multiform than before;" and hence, "whether that state with which we commence be or be not one of perfect homogeneity, the process must equally be towards a relative heterogeneity." This loss of homogeneity which the special instability of organic aggregates fits them to display more promptly and variously than any other aggregates, must be shown in more numerous ways in proportion as the incident forces are more numerous. Every differentiation of structure being a result of some difference in the relations of the parts to the agencies acting on them, it follows that the more multiplied and more unlike the agencies, the more varied must be the differentiations wrought. Hence the change from a state of homogeneity to a state of heterogeneity, will be marked in proportion as the environing actions to which the organism is supposes it is only are complex. This transition from a uniform to a multiform state, must continue through successive individuals. Given a series of organisms, each of which is developed from a portion of a preceding organism, and the question is whether, after exposure of the series for a million years to changed incident forces, one of its members will be the same as though the incident forces had only just changed. To say that it will, is implicitly to deny the persistence of force. In relation to any cause of divergence, the whole series of such organisms may be considered as fused together into a continuously-existing organism; and when so considered, it becomes manifest that a continuously-acting cause will go on working a continuously-increasing effect, until some counteracting cause prevents any further effect.
But now if any primordial organic aggregate must, in itself and through its descendants, gravitate from uniformity to multiformity, in obedience to the more or less multiform forces acting on it; what must happen if these multiform forces are themselves undergoing slow variations and complications? Clearly the process, ever-advancing towards a temporary limit but ever having its limit removed, must go on unceasingly. On those structural changes wrought in the once homogeneous aggregate by an original set of incident forces, will be superposed further changes wrought by a modified set of incident forces; and so on throughout all time. Omitting for the present those circumstances which check and qualify its consequences, the instability of the homogeneous must be recognized as an ever-acting cause of organic evolution, as of all other evolution.
While it follows that every organism, considered as an individual and as one of a series, tends thus to pass into a more heterogeneous state; it also follows that every species, considered as an aggregate of individuals, tends to do the like. Throughout the area it inhabits, the conditions can never be absolutely uniform: its members must, in different parts of the area, be exposed to different sets of incident forces. Still more decided must this difference of exposure be when its members spread into other habitats. Those expansive and repressive energies which set to each species a limit that perpetually oscillates from side to side of a certain mean, are, as we lately saw, frequently changed by new combinations of the external factors--astronomic, geologic, meteorologic, and organic. Hence there from time to time arise lines of diminished resistance, along which the species flows into new localities. Such portions of the species as thus migrate, are subject to circumstances unlike its previous average circumstances. And from multiformity of the circumstances, must come multiformity of the species.
Thus the law of the instability of the homogeneous has here a three-fold corollary. As interpreted in connexion with the ever-progressing, ever-complicating changes in external factors, it involves the conclusion that there is a prevailing tendency towards greater heterogeneity in all kinds of organisms, considered both individually and in successive generations; as well as in each assemblage of organisms constituting a species; and, by consequence, in each genus, order, and class.
§ 155. When considering the causes of evolution in general, we further saw (First Principles, § 156), that the multiplication of effects aids continually to increase that heterogeneity into which homogeneity inevitably lapses. It was pointed out that since "the several parts of an aggregate are differently modified by any incident force;" and since "by the reactions of the differently modified parts the incident force itself must be divided into differently modified parts;" it follows that "each differentiated division of the aggregate thus becomes a centre from which a differentiated division of the original force is again diffused. And since unlike forces must produce unlike results, each of these differentiated forces must produce, throughout the aggregate, a further series of differentiations." To this it was added that, in proportion as the heterogeneity increases, the complications arising from this multiplication of effects grow more marked; because the more strongly contrasted the parts of an aggregate become, the more different must be their reactions on incident forces, and the more unlike must be the secondary effects which these initiate; and because every increase in the number of unlike parts adds to the number of such differentiated incident forces, and such secondary effects.
How this multiplication of effects conspires, with the instability of the homogeneous, to work an increasing multiformity of structure in an organism, was shown at the time; and the foregoing pages contain further incidental illustrations. In § 69 it was pointed out that a change in one function must produce ever-complicating perturbations in other functions; and that, eventually, all parts of the organism must be modified in their states. Suppose that the head of a bison becomes much heavier, what must be the indirect results? The muscles of the neck are put to greater exertions; and its vertebræ have to bear additional tensions and pressures, caused both by the increased weight of the head, and by the stronger contractions of the muscles that support and move it. These muscles also affect their special attachments: several of the dorsal spines suffer augmented strains; and the vertebræ to which they are fixed are more severely taxed. Further, this heavier head and the more massive neck it necessitates, require a stronger fulcrum: the whole thoracic arch, and the fore-limbs which support it, are subject to greater continuous stress and more violent occasional shocks. And the required strengthening of the fore-quarters cannot take place without the centre of gravity being changed, and the hind limbs being differently reacted upon during locomotion. Any one who compares the outline of the bison with that of its congener, the ox, will see how profoundly a heavier head affects the entire osseous and muscular systems. Besides this multiplication of mechanical effects, there is a multiplication of physiological effects. The vascular apparatus is modified throughout its whole structure by each considerable modification in the proportions of the body. Increase in the size of any organ implies a quantitative, and often a qualitative, reaction on the blood; and thus alters the nutrition of all other organs. Such physiological correlations are exemplified in the many differences which accompany difference of sex. That the minor sexual peculiarities are brought about by the physiological actions and reactions, is shown both by the fact that they are commonly but faintly marked until the fundamentally distinctive organs are developed, and that when the development of these is prevented, the minor sexual peculiarities do not arise. No further proof is, I think, needed, that in any individual organism or its descendants, a new external action must, besides the primary internal change which it works, work many secondary changes, as well as tertiary changes still more multiplied. That tendency towards greater heterogeneity which is given to an organism by disturbing its environment, is helped by the tendency which every modification has to produce other modifications--modifications which must become more numerous in proportion as the organism becomes more complex. Lastly, among the indirect and involved manifestations of this tendency, we must not omit the innumerable small irregularities of structure which result from the crossing of dissimilarly-modified individuals. It was shown (§§ 89, 90) that what are called "spontaneous variations," are interpretable as results of miscellaneously compounding the changes wrought in different lines of ancestors by different conditions of life. These still more complex and multitudinous effects so produced, are further illustrations of the multiplication of effects.
Equally in the aggregate of individuals constituting a species, does multiplication of effects become the continual cause of increasing multiformity. The lapse of a species into divergent varieties, initiates fresh combinations of forces tending to work further divergences. The new varieties compete with the parent species in new ways; and so add new elements to its circumstances. They modify somewhat the conditions of other species existing in their habitat, or in the habitat they have invaded; and the modifications wrought in such other species become additional sources of influence. The Flora and Fauna of every region are united by their entangled relations into a whole, of which no part can be affected without affecting the rest. Hence, each differentiation in a local assemblage of species, becomes the cause of further differentiations.
§ 156. One of the universal principles to which we saw that the re-distribution of matter and motion conforms, is that in any aggregate made up of mixed units, incident forces produce segregation--separate unlike units and bring together like units; and it was shown that the increasing integration and definiteness which characterizes each part of an evolving organic aggregate, as of every other aggregate, results from this (First Principles, § 166). It remains here to say that while the actions and reactions between organisms and their changing environments, add to the heterogeneity of organic structures, they also give to the heterogeneity this growing distinctness. At first sight the reverse might be inferred. It might be argued that any new set of effects wrought in an organism by some new set of external forces, must tend more or less to obliterate the effects previously wrought--must produce confusion or indefiniteness. A little consideration, however, will dissipate this impression.
Doubtless the condition under which alone increasing definiteness of structure can be acquired by any part of an organism, either in an individual or in successive generations, is that such part shall be exposed to some set of tolerably-constant forces; and doubtless, continual change of circumstances interferes with this. But the interference can never be considerable. For the pre-existing structure of an organism prevents it from living under any new conditions except such as are congruous with the fundamental characters of its organization--such as subject its essential organs to actions substantially the same as before. Great changes must kill it. Hence, it can continuously expose itself and its descendants, only to those moderate changes which do not destroy the general harmony between the aggregate of incident forces and the aggregate of its functions. That is, it must remain under influences calculated to make greater the definiteness of the chief differentiations already produced. If, for example, we set out with an animal in which a rudimentary vertebral column with its attached muscular system has been established; it is clear that the mechanical arrangements have become thereby so far determined, that subsequent modifications are extremely likely, if not certain, to be consistent with the production of movement by the actions of muscles on a flexible central axis. Hence, there will continue a general similarity in the play of forces to which the flexible central axis is subject; and so, notwithstanding the metamorphoses which the vertebrate type undergoes, there will be a maintenance of conditions favourable to increasing definiteness and integration of the vertebral column. Moreover, this maintenance of such conditions becomes secure in proportion as organization advances. Each further complexity of structure, implying some further complexity in the relations between an organism and its environment, must tend to specialize the actions and reactions between it and its environment--must tend to increase the stringency with which it is restrained within such environments as admit of those special actions and reactions for which its structure fits it; that is, must further guarantee the continuance of those actions and reactions to which its essential organs respond, and therefore the continuance of the segregating process.
How in each species, considered as an aggregate of individuals, there must arise stronger and stronger contrasts among those divergent varieties which result from the instability of the homogeneous and the multiplication of effects, need only be briefly indicated. It has already been shown (First Principles, § 166), that in conformity to the universal law that mixed units are segregated by like incident forces, there are produced increasingly-definite distinctions among varieties, wherever there occur definitely-distinguished sets of conditions to which the varieties are respectively subject.
§ 157. Probably in the minds of some, the reading of this chapter has been accompanied by a running commentary, to the effect that the argument proves too much. The apparent implication is, that the passage from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity in organic aggregates, must have been going on universally; whereas we find that in many cases there has been persistence without progression. This apparent implication, however, is not a real one.
For though every environment on the Earth's surface undergoes changes; and though usually the organisms which each environment contains, cannot escape certain resulting new influences; yet occasionally such new influences are escaped, by the survival of species in the unchanged parts of their habitats, or by their spread into neighbouring habitats which the change has rendered like their original habitats, or by both. Any alteration in the temperature of a climate or its degree of humidity, is unlikely to affect simultaneously the whole area occupied by a species; and further, it can scarcely fail to happen that the addition or subtraction of heat or moisture, will give to a part of some adjacent area, a climate like that to which the species has been habituated. If, again, the circumstances of a species are modified by the intrusion of some foreign kind of plant or animal, it follows that since the intruders will probably not spread throughout its whole habitat, the species will, in one or more localities, remain unaffected by them. Especially among marine creatures, must there frequently occur cases in which modifying causes are continually eluded. Comparatively uniform as are the physical conditions to which the sea exposes its inhabitants, it becomes possible for such of them as live on widely-diffused food, to be widely distributed; and wide distribution generally prevents the members of a species from being all subject to the same cause. Our commonest cirriped, for instance, subsisting on minute creatures everywhere dispersed through the water; needing only to have some firm surface on which to build up its shell; and in scarcely any danger from surrounding animals; is able to exist on shores so widely remote from one another, that nearly every change in the incident forces must fall within narrower areas than that which the species occupies. Nearly always, therefore, a portion of the species will survive unmodified. Its easily-transported germs will take possession of such new habitats as have been rendered fitter by the change that has unfitted some parts of its original habitat. Hence, on successive occasions, while some parts of the species are slightly transformed, another part may continually escape transformation by migrating hither and thither, where the simple conditions needed for its existence recur in nearly the same combinations as before. And it will so become possible for it to survive, with insignificant structural changes, throughout long geologic periods.
§ 158. The results to which we find ourselves led, are these.
In subordination to the different amounts and kinds of forces to which its different parts are exposed, every individual organic aggregate, like all other aggregates, tends to pass from its original indistinct simplicity towards a more distinct complexity. Unless we deny the persistence of force, we must admit that the lapse of an organism's structure from an indefinitely homogeneous to a definitely heterogeneous state, must be cumulative in successive generations, if the forces causing it continue to act. And for the like reasons, the increasing assemblage of individuals arising from a common stock, is also liable to lose its original uniformity; and, in successive generations, to grow more pronounced in its multiformity.
These changes, which would go to but a comparatively small extent were organisms exposed to constant external conditions, are kept up by the continual changes in external conditions, produced by astronomic, geologic, meteorologic, and organic agencies: the average result being, that on previous complications wrought by previous incident forces, new complications are continually superposed by new incident forces. And hence simultaneously arises increasing heterogeneity in the structures of individuals, in the structures of species, and in the structures of the Earth's Flora and Fauna.
But while, in very many or in most cases, the ever-changing incidence of forces is ever adding to the complexity of organisms, and to the complexity of the organic world as a whole; it does this only where its action cannot be eluded. And since, by migration, it is possible for a species to keep itself under conditions that are tolerably constant, there must be a proportion of cases in which greater heterogeneity of structure is not to be expected.
To show, however, that there must arise a certain average tendency to the production of greater heterogeneity is not sufficient. Aggregates might be rendered more heterogeneous by changing incident forces, without having given to them that kind of heterogeneity required for carrying on life. Hence it remains now to inquire how the production and maintenance of this kind of heterogeneity is insured.
XI. DIRECT EQUILIBRATION
§ 159. Every change is towards a balance of forces; and of necessity can never cease until a balance of forces is reached. When treating of equilibration under its general aspects (First Principles, Part II., Chap. xxii.), we saw that every aggregate having compound movements tends continually towards a moving equilibrium; since any unequilibrated force to which such an aggregate is subject, if not of a kind to overthrow it altogether, must continue modifying its state until an equilibrium is brought about. And we saw that the structure simultaneously reached must be "one presenting an arrangement of forces that counterbalance all the forces to which the aggregate is subject;" since, "so long as there remains a residual force in any direction--be it excess of a force exercised by an aggregate on its environment, or of a force exercised by its environment on the aggregate, equilibrium does not exist; and therefore the re-distribution of matter must continue."
It is essential that this truth should here be fully comprehended; and to the end of insuring clear comprehension of it, some re-illustration is desirable. The case of the Solar System will best serve our purpose. An assemblage of bodies, each of which has its simple and compound motions that severally alternate between two extremes, and the whole of which has its involved perturbations, that now increase and now decrease, is here presented to us. Suppose a new factor were brought to bear on this moving equilibrium--say by the arrival of some wandering mass, or by an additional momentum given to one of the existing masses--what would be the result? If the strange body or the extra energy were very large, it might so derange the entire system as to cause its collapse. But what if the incident energy, falling on the system from without, proved insufficient to overthrow it? There would then arise a set of perturbations which would, in the course of an enormous period, slowly work round into a modified moving equilibrium. The effects primarily impressed on the adjacent masses, and in a smaller degree on the remoter masses, would presently become complicated with the secondary effects impressed by the disturbed masses on one another; and these again with tertiary effects. Waves of perturbation would continue to be propagated throughout the entire system; until, around a new centre of gravity, there had been established a set of planetary motions different from the preceding ones. The new energy must gradually be used up in overcoming the energies resisting the divergence it generates; which antagonizing energies, when no longer opposed, set up a counter-action, ending in a compensating divergence in the opposite direction, followed by a re-compensating divergence, and so on. Now though instead of being, like the Solar System, in a state of independent moving equilibrium, an organism is in a state of dependent moving equilibrium (First Principles, § 170); yet this does not prevent the manifestation of the same law. Every animal daily obtains from without, a supply of energy to replace the energy it expends; but this continual giving to its parts a new momentum, to make up for the momentum continually lost, does not interfere with the carrying on of actions and reactions like those just described. Here, as before, we have a definitely-arranged aggregate of parts, called organs, having their definitely-established actions and reactions, called functions. These rhythmical actions or functions, and the various compound rhythms resulting from their combinations, are so adjusted as to balance the actions to which the organism is subject: there is a constant or periodic genesis of energies which, in their kinds, amounts, and directions, suffice to antagonize the energies the organism has constantly or periodically to bear. If, then, there exists this moving equilibrium among a set of internal actions, exposed to a set of external actions, what must result if any of the external actions are changed? Of course there is no longer an equilibrium. Some energy which the organism habitually generates, is too great or too small to balance some incident energy; and there arises a residual energy exerted by the environment on the organism, or by the organism on the environment. This residual or unbalanced energy, of necessity expends itself in producing some change of state in the organism. Acting directly on some organ and modifying its function, it indirectly modifies dependent functions and remotely influences all the functions. As we have already seen (§§ 68, 69), if this new energy is permanent, its effects must be gradually diffused throughout the entire system; until it has come to be equilibrated in producing those structural rearrangements whence result a counter-balancing energy.
The bearing of this general truth on the question we are now dealing with is obvious. Those modifications upon modifications, which the unceasing mutations of their environments have been all along generating in organisms, have been in each case modifications involved by the establishment of a new balance with the new combination of actions. In every species throughout all geologic time, there has been perpetually going on a rectification of the equilibrium, which has been perpetually disturbed by the alteration of its circumstances; and every further heterogeneity has been the addition of a structural change entailed by a new equilibration, to the structural changes entailed by previous equilibrations. There can be no other ultimate interpretation of the matter, since change can have no other goal.
This equilibration between the functions of an organism and the actions in its environment, may be either direct or indirect. The new incident force may either immediately call forth some counteracting force, and its concomitant structural change; or it may be eventually balanced by some otherwise-produced change of function and structure. These two processes of equilibration are quite distinct, and must be separately dealt with. We will devote this chapter to the first of them.
§ 160. Direct equilibration is that process currently known as adaptation. We have already seen (Part II., Chap, v.), that individual organisms become modified when placed in new conditions of life--so modified as to re-adjust the powers to the requirements; and though there is great difficulty in disentangling the evidence, we found reason for thinking (§ 82) that structural changes thus caused by functional changes are inherited. In the last chapter, it was argued that if, instead of the succession of individuals constituting a species, there were a continuously-existing individual, any functional and structural divergence produced by a new incident action, would increase until the new incident action was counterpoised; and that the replacing of a continuously-existing individual by a succession of individuals, each formed out of the modified substance of its predecessor, will not prevent the like effect from being produced. Here we further find that this limit towards which any such organic change advances, in the species as in the individual, is a new moving equilibrium adjusted to the new arrangement of external forces.
But now what are the conditions under which alone, direct equilibration can occur? Are all the modifications that serve to re-fit organisms to their environments, directly adaptive modifications? And if otherwise, which are the directly adaptive and which are not? How are we to distinguish between them?
There can be no direct equilibration with an external agency which, if it acts at all, acts fatally; since the organism to be adapted disappears. Conversely, some inaccessible benefit which a small modification in the organism would make accessible, cannot by its action tend to produce this modification: the modification and the benefit do not stand in dynamic relation. The only new incident forces which can work the changes of function and structure required to bring any animal or plant into equilibrium with them, are such incident forces as operate on this animal or plant, either continuously or frequently. They must be capable of appreciably changing that set of complex rhythmical actions and reactions constituting the life of the organism; and yet must not usually produce perturbations that are fatal. Let us see what are the limits to direct equilibration hence arising.
§ 161. In plants, organs engaged in nutrition, and exposed to variations in the amounts and proportions of matters and forces utilized in nutrition, may be expected to undergo corresponding variations. We find evidence that they do this. The "changes of habit" which are common in plants, when taken to places unlike in climate or soil to those before inhabited by them, are changes of parts in which the modified external actions directly produce modified internal actions. The characters of the stem and shoots as woody or succulent, erect or procumbent; of the leaves in respect of their sizes, thicknesses, and textures; of the roots in their degrees of development and modes of growth; are obviously in immediate relation to the characters of the environment. A permanent difference in the quantity of light or heat affects, day after day, the processes going on in the leaves. Habitual rain or drought alters all the assimilative actions, and appreciably influences the organs that carry them on. Some particular substance, by its presence in the soil, gives new qualities to some of the tissues; causing greater rigidity or flexibility, and so affecting the general aspect. Here then we have changes towards modified sets of functions and structures, in equilibrium with modified sets of external forces.
But now let us turn to other classes of organs possessed by plants--organs which are not at once affected in their actions by variations of incident forces. Take first the organs of defence. Many plants are shielded against animals that would else devour them, by formidable thorns; and others, like the nettle, by stinging hairs. These must be counted among the appliances by which equilibrium is maintained between the actions in the organism and the actions in its environment; seeing that were these defences absent, the destruction by herbivorous animals would be so much increased, that the number of young plants annually produced would not suffice, as it now does, to balance the mortality, and the species would disappear. But these defensive appliances, though they aid in maintaining the balance between inner and outer actions, cannot have been directly called forth by the outer actions which they serve to neutralize; for these outer actions do not continuously affect the functions of the plant even in a general way, still less in the special way required. Suppose a species of nettle bare of poison-hairs, to be habitually eaten by some mammal intruding on its habitat. The actions of this mammal would have no direct tendency to develop poison-hairs in the plant; since the individuals devoured could not bequeath changes of structure, even were the actions of a kind to produce fit ones; and since the individuals which perpetuated themselves would be those on which the new incident force had not fallen. Organs of another class, similarly circumstanced, are those of reproduction. Like the organs of defence these are not, during the life of the individual plant, variably exercised by variable external actions; and therefore do not fulfil those conditions under which structural changes may be directly caused by changes in the environment. The generative apparatus contained in every flower acts only once during its existence; and even then, the parts subserve their ends in a passive rather than an active way. Functionally-produced modifications are therefore out of the question. If a plant's anthers are so placed that the insect which most commonly frequents its flowers, must come in contact with the pollen, and fertilize with it other flowers of the same species; and if this insect, dwindling away or disappearing from the locality, leaves behind no insects having such shapes and habits as cause them to do the same thing efficiently, but only some which do it inefficiently; it is clear that this change of its conditions has no immediate tendency to work in the plant any such structural change as shall bring about a new balance with its conditions. For the anthers, which, even when they discharge their functions, do it simply by standing in the way of the insect, are, under the supposed circumstances, left untouched by the insect; and this remaining untouched cannot have the effect of so modifying the stamens as to bring the anthers into a position to be touched by some other insect. Only those individuals whose parts of fructification so far differed from the average form that some other insect could serve them as pollen-carrier, would have good chances of perpetuating themselves. And on their progeny, inheriting the deviation, there would act no external force directly tending to make the deviation greater; since the new circumstances to which re-adaptation is required, are such as do not in the least alter the equilibrium of functions constituting the life of the individual plant.
§ 162. Among animals, adaptation by direct equilibration is similarly traceable wherever, during the life of the individual, an external change generates some constant or repeated change of function. This is conspicuously the case with such parts of an animal as are immediately exposed to diffused influences, like those of climate, and with such parts of an animal as are occupied in its mechanical actions on the environment. Of the one class of cases, the darkening of the skin which follows exposure to one or other extreme of temperature, may be taken as an instance; and with the other class of cases we are made familiar by the increase and decrease which use and disuse cause in the organs of motion. It is needless here to exemplify these: they were treated of in the Second Part of this work.
But in animals, as in plants, there are many indispensable offices fulfilled by parts between which and the external conditions they respond to, there is no such action and reaction as can directly produce an equilibrium. This is especially manifest with dermal appendages. Some ground exists for the conclusion that the greater or less development of hairs, is in part immediately due to increase or decrease of demand on the passive function, as forming a non-conducting coat; but be this as it may, it is impossible that there can exist any such cause for those immense developments of hairs which we see in the quills of the porcupine, or those complex developments of them known as feathers. Such an enamelled armour as is worn by Lepidosteus, is inexplicable as a direct result of any functionally-worked change. For purposes of defence, such an armour is as needful, or more needful, for hosts of other fishes; and did it result from any direct reaction of the organism against any offensive actions it was subject to, there seems no reason why other fishes should not have developed similar protective coverings. Of sundry reproductive appliances the like may be said. The secretion of an egg-shell round the substance of an egg, in the oviduct of a bird, is quite inexplicable as a consequence of some functionally-wrought modification of structure, immediately caused by some modification of external conditions. The end fulfilled by the egg-shell, is that of protecting the contained mass against certain slight pressures and collisions, to which it is liable during incubation. How, by any process of direct equilibration, could it come to have the required thickness? or, indeed, how could it come to exist at all? Suppose this protective envelope to be too weak, so that some of the eggs a bird lays are broken or cracked. In the first place, the breakages or crackings are actions which cannot react on the maternal organism in such ways as to cause the secretion of thicker shells for the future: to suppose that they can, is to suppose that the bird understands the cause of the evil, and that the secretion of thicker shells can be effected by its will. In the second place, such developing chicks as are contained in the shells which crack or break, are almost certain to die; and cannot, therefore, acquire appropriately-modified constitutions: even supposing any relation could exist between the impression received and the change required. Meanwhile, such eggs as escape breakage are not influenced at all by the requirement; and hence, on the birds developed from them, there cannot have acted any force tending to work the needful adjustment of functions. In no way, therefore, can a direct equilibration between constitution and conditions be here produced. Even in organs that can be modified by certain incident actions into correspondence with such incident actions, there are some re-adjustments which cannot be effected by direct balancing. It is thus with the bones. The majority of the bones have to resist muscular strains; and variations in the muscular strains call forth, by reaction, variations in the strengths of the bones. Here there is direct equilibration. But though the greater massiveness acquired by bones subject to greater strains, may be ascribed to counter-acting forces evoked by forces brought into action; it is impossible that the acquirement of greater lengths by bones can be thus accounted for. It has been supposed that the elongation of the metatarsals in wading birds, has resulted from direct adaptation to conditions of life. To justify this supposition, however, it must be shown that the mechanical actions and reactions in the legs of a wading bird, differ from those in the legs of other birds; and that the differential actions are equilibrated by the extra lengths. There is not the slightest evidence of this. The metatarsals of a bird have to bear no appreciable strains but those due to the superincumbent weight. Standing in the water does not appreciably alter such strains; and even if it did, an increase in the lengths of these bones would not fit them any better to meet the altered strains.
§ 163. The conclusion at which we arrive is, then, that there go on in all organisms, certain changes of function and structure that are directly consequent on changes in the incident forces--inner changes by which the outer changes are balanced, and the equilibrium restored. Such re-equilibrations, which are often conspicuously exhibited in individuals, we have reason to believe continue in successive generations; until they are completed by the arrival at structures fitted to the modified conditions. But, at the same time, we see that the modified conditions to which organisms may be adapted by direct equilibration, are conditions of certain classes only. That a new external action may be met by a new internal action, it is needful that it shall either continuously or frequently be borne by the individuals of the species, without killing or seriously injuring them; and shall act in such way as to affect their functions. And we find that many of the environing agencies--evil or good--to which organisms have to be adjusted, are not of these kinds: being agencies which either do not immediately affect the functions at all, or else affect them in ways that prove fatal.
Hence there must be at work some other process which equilibrates the actions of organisms with the actions they are exposed to. Plants and animals that continue to exist, are necessarily plants and animals whose powers balance the powers acting on them; and as their environments change, the changes which plants and animals undergo must necessarily be changes towards re-establishment of the balance. Besides direct equilibration, there must therefore be an indirect equilibration. How this goes on we have now to inquire.
XII. INDIRECT EQUILIBRATION
§ 164. Besides those perturbations produced in any organism by special disturbing forces, there are ever going on many others--the reverberating effects of disturbing forces previously experienced by the individual, or by ancestors; and the multiplied deviations of function so caused imply multiplied deviations of structure. In § 155 there was re-illustrated the truth, set forth at length when treating of Adaptation (§ 69), that an organism in a state of moving equilibrium, cannot have extra function thrown on any organ, and extra growth produced in such organ, without correlative changes being entailed throughout all other functions, and eventually throughout all other organs. And when treating of Variation (§ 90), we saw that individuals which have been made, by their different circumstances, to deviate functionally and structurally from the average type in different directions, will bequeath to their joint offspring, compound perturbations of function and compound deviations of structure, endlessly varied in their kinds and amounts.
Now if the individuals of a species are thus necessarily made unlike in countless ways and degrees--if in one individual the amount of energy in a particular direction is greater than in any other individual, or if here a peculiar combination gives a resulting action which is not found elsewhere; then, among all the individuals, some will be less liable than others to have their equilibria overthrown by a particular incident force previously unexperienced. Unless the change in the environment is so violent as to be universally fatal to the species, it must affect more or less differently the slightly-different moving equilibria which the members of the species present. Inevitably some will be more stable than others when exposed to this new or altered factor. That is to say, those individuals whose functions are most out of equilibrium with the modified aggregate of external forces, will be those to die; and those will survive whose functions happen to be most nearly in equilibrium with the modified aggregate of external forces.
But this survival of the fittest[52] implies multiplication of the fittest. Out of the fittest thus multiplied there will, as before, be an overthrowing of the moving equilibrium wherever it presents the least opposing force to the new incident force. And by the continual destruction of the individuals least capable of maintaining their equilibria in presence of this new incident force, there must eventually be reached an altered type completely in equilibrium with the altered conditions.
§ 165. This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called "natural selection, or the preservation of favoured races in the struggle for life." That there goes on a process of this kind throughout the organic world, Mr. Darwin's great work on the Origin of Species has shown to the satisfaction of nearly all naturalists. Indeed, when once enunciated, the truth of his hypothesis is so obvious as scarcely to need proof. Though evidence may be required to show that natural selection accounts for everything ascribed to it, yet no evidence is required to show that natural selection has always been going on, is going on now, and must ever continue to go on. Recognizing this as an à priori certainty, let us contemplate it under its two distinct aspects.
That organisms which live, thereby prove themselves fit for living, in so far as they have been tried, while organisms which die, thereby prove themselves in some respects unfitted for living, are facts no less manifest than is the fact that this self-purification of a species must tend ever to insure adaptation between it and its environment. This adaptation may be either so maintained or so produced. Doubtless many who have looked at Nature with philosophic eyes, have observed that death of the worst and multiplication of the best, tends towards maintenance of a constitution in harmony with surrounding circumstances. That the average vigour of any race would be diminished did the diseased and feeble habitually survive and propagate; and that the destruction of such, through failure to fulfil some of the conditions to life, leaves behind those which are able to fulfil the conditions to life, and thus keeps up the average fitness to the conditions of life; are almost self-evident truths. But to recognize "Natural Selection" as a means of preserving an already-established balance between the powers of a species and the forces to which it is subject, is to recognize only its simplest and most general mode of action. It is the more special mode of action with which we are here concerned. This more special mode of action, Mr. Darwin has been the first to recognize as an all-important factor, though, besides his co-discoverer Mr. A. R. Wallace, some others have perceived that such a factor is at work. To him we owe due appreciation of the fact that natural selection is capable of producing fitness between organisms and their circumstances. He has worked up an enormous mass of evidence showing that this "preservation of favoured races in the struggle for life," is an ever-acting cause of divergence among organic forms. He has traced out the involved results of the process with marvellous subtlety. He has shown how hosts of otherwise inexplicable facts, are accounted for by it. In brief, he has proved that the cause he alleges is a true cause; that it is a cause which we see habitually in action; and that the results to be inferred from it are in harmony with the phenomena which the Organic Creation presents, both as a whole and in its details. Let us glance at a few of the more important interpretations which the hypothesis furnishes.
A soil possessing some ingredient in unusual quantity, may supply to a plant an excess of the matter required for certain of its tissues; and may cause all the parts formed of such tissues to be abnormally developed. Suppose that among these are the hairs clothing its surfaces, including those which grow on its seeds. Thus furnished with somewhat longer fibres, its seeds, when shed, are carried a little further by the wind before they fall to the ground. The plants growing from them, being rather more widely dispersed than those produced by other individuals of the same species, will be less liable to smother one another; and a greater number may therefore reach maturity and fructify. Supposing the next generation subject to the same peculiarity of nutrition, some of the seeds borne by its members will not simply inherit this increased development of hairs, but will carry it further; and these, still more advantaged in the same way as before, will, on the average, have still more numerous chances of continuing the race. Thus, by the survival, generation after generation, of those possessing these longer hairs, and the inheritance of successive increments of growth in the hairs, there may result a seed deviating greatly from the original. Other individuals of the same species, subject to the different physical conditions of other localities, may develop somewhat thicker or harder coatings to their seeds: so rendering their seeds less digestible by the birds which devour them. Such thicker-coated seeds, by escaping undigested more frequently than thinner-coated ones, will have additional chances of growing and leaving offspring; and this process, acting in a cumulative manner season after season, will produce a seed diverging in another direction from the ancestral type. Again, elsewhere, some modification in the physiologic actions of the plant may lead to an unusual secretion of an essential oil in the seeds; rendering them unpalatable to creatures which would otherwise feed on them: so giving an advantage to the variety in its rate of multiplication. This incidental peculiarity, proving a preservative, will, as before, be increased by natural selection until it constitutes another divergence. Now in such cases, we see that plants may become better adapted, or re-adapted, to the aggregate of surrounding agencies, not through any direct action of such agencies on them, but through their indirect action--through the destruction by them of the individuals least congruous with them, and the survival of those most congruous with them. All these slight variations of function and structure, arising among the members of a species, serve as so many experiments; the great majority of which fail, but a few of which succeed. Just as each plant bears a multitude of seeds, out of which some two or three happen to fulfil all the conditions required for reaching maturity and continuing the race; so each species is ever producing numerous slightly-modified forms, deviating in all directions from the average, out of which most fit the surrounding conditions no better than their parents, or not so well, but some few of which fit the conditions better; and, doing so, are enabled the better to preserve themselves, and to produce offspring similarly capable of preserving themselves. Among animals the like process results in the like development of various structures which cannot have been affected by the performance of functions--their functions being purely passive. The thick shell of a mollusk cannot have arisen from direct reactions of the organism against the external actions to which it is exposed; but it is quite explicable as an effect of the survival, generation after generation, of individuals whose thicker coverings protected them against enemies. Similarly with such dermal structure as that of the tortoise. Though we have evidence that the skin, where it is continually exposed to pressure and friction, may thicken, and so re-establish the equilibrium by opposing a greater inner force to a greater outer force; yet we have no evidence that a coat of armour like that of the tortoise can be so produced. Nor, indeed, are the conditions under which alone its production in such a manner could be accounted for, fulfilled; since the surface of the tortoise is not exposed to greater pressure and friction than the surfaces of other creatures. This massive carapace, and the strangely-adapted osseous frame-work which supports it, are inexplicable as results of evolution, unless through the process of natural selection. So, too, is it with the formation of odoriferous glands in some mammals, or the growth of such excrescences as those of the camel. Thus, in short, is it with all those organs of animals which do not play active parts.
Besides giving us explanations of structural characters that are otherwise unaccountable, Mr. Darwin shows how natural selection explains peculiar relations between individuals in certain species. Such facts as the dimorphism of the primrose and other flowers, he proves to be in harmony with his hypothesis, though stumbling-blocks to all other hypotheses. The various differences which accompany difference of sex, sometimes slight, sometimes very great, are similarly accounted for. As before suggested (§ 79), natural selection appears capable of producing and maintaining the right proportion of the sexes in each species; and it requires but to contemplate the bearings of the argument, to see that the formation of different sexes may itself have been determined in the same way.
To convey here an adequate idea of Mr. Darwin's doctrine, throughout the immense range of its applications, is of course impossible. The few illustrations just given, are intended simply to remind the reader what Mr. Darwin's hypothesis is, and what are the else insoluble problems which it solves for us.
§ 166. But now, though it seems to me that we are thus supplied with a key to phenomena which are multitudinous and varied beyond all conception; it also seems to me that there is a moiety of the phenomena which this key will not unlock. Mr. Darwin himself recognizes use and disuse of parts, as causes of modifications in organisms; and does this, indeed, to a greater extent than do some who accept his general conclusion. But I conceive that he does not recognize them to a sufficient extent. While he shows that the inheritance of changes of structure caused by changes of function, is utterly insufficient to explain a great mass--probably the greater mass--of morphological phenomena; I think he leaves unconsidered a mass of morphological phenomena which are explicable as results of functionally-produced modifications, and are not explicable as results of natural selection.
By induction, as well as by inference from the hypothesis of natural selection, we know that there exists a balance among the powers of organs which habitually act together--such proportions among them that no one has any considerable excess of efficiency. We see, for example, that throughout the vascular system there is maintained an equilibrium of the component parts: in some cases, under continued excess of exertion, the heart gives way, and we have enlargement; in other cases the large arteries give way, and we have aneurisms; in other cases the minute blood-vessels give way--now bursting, now becoming chronically congested. That is to say, in the average constitution, no superfluous strength is possessed by any of the appliances for circulating the blood. Take, again, a set of motor organs. Great strain here causes the fibres of a muscle to tear. There the muscle does not yield but the tendon snaps. Elsewhere neither muscle nor tendon is damaged, but the bone breaks. Joining with these instances the general fact that, under the same adverse conditions, different individuals show their slight differences of constitution by going wrong some in one way and some in another; and that even in the same individual, similar adverse conditions will now affect one viscus and now another; it becomes manifest that though there cannot be maintained an accurate balance among the powers of the organs composing an organism, yet their excesses and deficiencies of power are extremely slight. That they must be extremely slight, is, as before said, a deduction from the hypothesis of natural selection. Mr. Darwin himself argues "that natural selection is continually trying to economise in every part of the organization. If under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure." In other words, if any muscle has more fibres than are required, or if a bone is stronger than needful, no advantage results but rather a disadvantage--a disadvantage which will decrease the chances of survival. Hence it follows that among any organs which habitually act in concert, an increase of one can be of no service unless there is a concomitant increase of the rest. The co-operative parts must vary together; otherwise variation will be detrimental. A stronger muscle must have a stronger bone to resist its contractions; must have stronger correlated muscles and ligaments to secure the neighbouring articulations; must have larger blood-vessels to bring it supplies; must have a more massive nerve to transmit stimulus, and some extra development of a nervous centre to supply the extra stimulus. The question arises, then,--do variations of the appropriate kinds occur simultaneously in all these co-operative parts? Have we any reason to think that the parts spontaneously increase or decrease together? The assumption that they do seems to me untenable; and its untenability will, I think, become conspicuous if we take a case, and observe how extremely numerous and involved are the variations which must be supposed to occur together. In illustration of another point, we have already considered the modifications required to accompany increased weight of the head (§ 155). Instead of the bison, the moose deer, or the extinct Irish elk, will here best serve our purpose. In this last species the male has enormously-developed horns, used for purposes of offence and defence. These horns, weighing upwards of a hundred-weight, are carried at great mechanical disadvantage: supported as they are, along with the massive skull which bears them, at the extremity of the outstretched neck. Further, that these heavy horns may be of use in fighting, the supporting bones and muscles must be strong enough, not simply to carry them, but to put them in motion with the rapidity needed for giving blows. Let us, then, ask how, by natural selection, this complex apparatus of bones and muscles can have been developed, pari passu with the horns? If we suppose the horns to have been originally of like size with those borne by other kinds of deer; and if we suppose that in some individual they became larger by spontaneous variation; what would be the concomitant changes required to render their greater size useful? Other things equal, the blow given by a larger horn would be a blow given by a heavier mass moving at a smaller velocity: the momentum would be the same as before; and the area of contact with the body struck being somewhat increased, while the velocity was decreased, the injury done would be less. That horns may become better weapons, the whole apparatus concerned in moving them must be so strengthened as to impress more force on them, and to bear the more violent reactions of the blows given. The bones of the skull on which the horns are seated must be thickened; otherwise they will break. The vertebræ of the neck must be further developed; and unless the ligaments which hold together these vertebræ, and the muscles which move them, are also enlarged, nothing will be gained. Again the upper dorsal vertebræ and their spines must be strengthened, that they may withstand the stronger contractions of the neck-muscles; and like changes must be made on the scapular arch. Still more must there be required a simultaneous development of the bones and muscles of the fore-legs; since these extra growths in the horns, in the skull, in the neck, in the shoulders, add to the burden they have to bear; and without they are strengthened the creature will not only suffer from loss of speed but will fail in fight. Hence, to make larger horns of use, additional sizes must be acquired by numerous bones, muscles, and ligaments, as well as by the blood-vessels and nerves on which their actions depend. On calling to mind how the spraining of a single small muscle in the foot incapacitates for walking, or how permanent weakness in a knee-ligament will diminish the power of the leg, it will be seen that unless all these many changes are simultaneously made, they may as well be none of them made--or rather, they would better be none of them made; since the enlargements of some parts, by putting greater strains on connected parts, would render them relatively weaker if they remained unenlarged. Can we with any propriety assume that these many enlargements duly proportioned will be simultaneously effected by spontaneous variations? I think not. It would be a strong supposition that the vertebræ and muscles of the neck suddenly became bigger at the same time as the horns. It would be a still stronger supposition that the upper dorsal vertebræ not only at the same time became more massive, but appropriately altered their proportions, by the development of their immense neural spines. And it would be an assumption still more straining our powers of belief, that along with heavier horns there should spontaneously take place the required strengthenings in the bones, muscles, arteries, and nerves of the scapular and the fore-legs.
Besides the multiplicity of directly-coöperative organs, the multiplicity of organs which do not coöperate, save in the degree implied by their combination in the same organism, seems to me a further hindrance to the development of special structures by natural selection alone. Where the life is simple, or where circumstances render some one function supremely important, survival of the fittest may readily bring about the appropriate structural change, without aid from the transmission of functionally-caused modifications. But in proportion as the life grows complex--in proportion as a healthy existence cannot be secured by a large endowment of some one power, but demands many powers; in the same proportion do there arise obstacles to the increase of any particular power by "the preservation of favoured races in the struggle for life." As fast as the faculties are multiplied, so fast does it become possible for the several members of a species to have various kinds of superiorities over one another. While one saves its life by higher speed, another does the like by clearer vision, another by keener scent, another by quicker hearing, another by greater strength, another by unusual power of enduring cold or hunger, another by special sagacity, another by special timidity, another by special courage; and others by other bodily and mental attributes. Conditions being alike, each of these life-saving attributes is likely to be transmitted to posterity. But we may not assume that it will be increased in subsequent generations by natural selection. Increase of it can result only if individuals possessing average endowments of it are more frequently killed off than individuals highly endowed with it; and this can happen only when the attribute is one of greater importance, for the time being, than most of the other attributes. If those members of the species which have but ordinary shares of it, nevertheless survive by virtue of other superiorities which they severally possess; then it is not easy to see how this particular attribute can be developed by natural selection in subsequent generations. The probability seems rather to be that, by gamogenesis, this extra endowment will, on the average, be diminished in posterity--just serving in the long run to make up for the deficient endowments of those whose special powers lie in other directions; and so to keep up the normal structure of the species. As fast as the number of bodily and mental faculties increases, and as fast as maintenance of life comes to depend less on the amount of any one and more on the combined actions of all; so fast does the production of specialities of character by natural selection alone, become difficult. Particularly does this seem to be so with a species so multitudinous in its powers as mankind; and above all does it seem to be so with such of the human powers as have but minor shares in aiding the struggle for life--the æsthetic faculties, for example.
It by no means follows, however, that in cases of this kind, and cases of the preceding kind, natural selection plays no part. Wherever it is not the chief agent in working organic changes, it is still, very generally, a secondary agent. The survival of the fittest must nearly always further the production of modifications which produce fitness, whether they be incidental modifications, or modifications caused by direct adaptation. Evidently, those individuals whose constitutions have facilitated the production in them of any structural change consequent on any functional change demanded by some new external condition, will be the individuals most likely to live and to leave descendants. There must be a natural selection of functionally-acquired peculiarities, as well as of spontaneously-acquired peculiarities; and hence such structural changes in a species as result from changes of habit necessitated by changed circumstances, natural selection will render more rapid than they would otherwise be.
There are, however, some modifications in the sizes and forms of parts, which cannot have been aided by natural selection; but which must have resulted wholly from the inheritance of functionally-caused alterations. The dwindling of organs of which the undue sizes entail no appreciable evils, furnishes the best evidence of this. Take, for an example, that diminution of the jaws and teeth which characterizes the civilized races, as contrasted with the savage races.[53] How can the civilized races have been benefited in the struggle for life, by the slight decrease in these comparatively-small bones? No functional superiority possessed by a small jaw over a large jaw in civilized life, can be named as having caused the more frequent survival of small-jawed individuals. The only advantage accompanying smallness of jaw, is the advantage of economized nutrition; and this cannot be great enough to further the preservation of those distinguished by it. The decrease of weight in the jaw and co-operative parts, which has arisen in the course of thousands of years, does not amount to more than a few ounces. This decrease has to be divided among the many generations which have lived and died in the interval. Let us admit that the weight of these parts diminished to the extent of an ounce in a single generation (which is a large admission); it still cannot be contended that the having to carry an ounce less in weight, and to keep in repair an ounce less of tissue, could sensibly affect any man's fate. And if it never did this--nay, if it did not cause a frequent survival of small-jawed individuals where large-jawed individuals died; natural selection could neither cause nor aid diminution of the jaw and its appendages. Here, therefore, the decreased action which has accompanied the growth of civilized habits (the use of tools and the disuse of coarse food), must have been the sole cause at work. Through direct equilibration, diminished external stress on these parts has resulted in diminution of the internal forces by which this stress is met. From generation to generation, this lessening of the parts consequent on functional decline has been inherited. And since the survival of individuals must always have been determined by more important structural traits, this trait can have neither been facilitated nor retarded by natural selection.
§ 167. Returning from these extensive classes of facts for which Mr. Darwin's hypothesis does not account, to the still more extensive classes of facts for which it does account, and which are unaccountable on any other hypothesis; let us consider in what way this hypothesis is expressible in terms of the general doctrine of evolution. Already it has been pointed out that the evolving of modified types by "natural selection or the preservation of favoured races in the struggle for life," must be a process of equilibration; since it results in the production of organisms which are in equilibrium with their environments. At the outset of this chapter, something was done towards showing how this continual survival of the fittest may be understood as the progressive establishment of a balance between inner and outer forces. Here, however, we must consider the matter more closely.
On previous occasions we have contemplated the assemblage of individuals composing a species, as an aggregate in a state of moving equilibrium. We have seen that its powers of multiplication give it an expansive energy which is antagonized by other energies; and that through the rhythmical variations in these two sets of energies there is maintained an oscillating limit to its habitat, and an oscillating limit to its numbers. On another occasion (§ 96) it was shown that the aggregate of individuals constituting a species, has a kind of general life which, "like the life of an individual, is maintained by the unequal and ever-varying actions of incident forces on its different parts." We saw that "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." Hence, to understand how a species is affected by causes which destroy some of its units and favour the multiplication of others, we must consider it as a whole whose parts are held together by complex forces that are ever re-balancing themselves--a whole whose moving equilibrium is continually disturbed and continually rectified. Thus much premised, let us next call to mind how moving equilibria in general are changed. In the first place, a new incident force falling on any part of an aggregate with balanced motions, produces a new motion in the direction of least resistance. In the second place, the new incident force is gradually used up in overcoming the opposing forces, and when it is all expended the opposing forces produce a recoil--a reverse deviation which counter-balances the original deviation. Consequently, to consider whether the moving equilibrium of a species is modified in the same way as moving equilibria in general, is to consider whether, when exposed to a new force, a species yields in the direction of least resistance; and whether, by its thus yielding, there is generated in the species a compensating change in the opposite direction. We shall find that it does both these things.
For what, expressed in mechanical terms, is the effect wrought on a species by some previously-unknown enemy, that kills such of its members as fail in defending themselves? The disappearance of those individuals which meet the destroying forces by the smallest preserving forces, is tantamount to the yielding of the species as a whole at the places where the resistances are the least. Or if by some general influence, such as alteration of climate, the members of a species are subject to increase of external actions which are ever tending to overthrow their equilibria, and which they are ever counter-balancing by certain physiological actions, which are the first to die? Those least able to generate the internal energies which antagonize these external energies. If the change be an increase of the winter's cold, then such members of the species as have unusual powers of getting food or of digesting food, or such as are by their constitutional aptitude for making fat, furnished with reserve stores of force, available in times of scarcity, or such as have the thickest coats and so lose least heat by radiation, survive; and their survival implies that in each of them the moving equilibrium of functions presents such an adjustment of internal forces, as prevents overthrow by the modified aggregate of external forces. Conversely, the members which die are, other things equal, those deficient in the power of meeting the new action by an equivalent counter-action. Thus, in all cases, a species considered as an aggregate in a state of moving equilibrium, has its state changed by the yielding of its fluctuating mass wherever this mass is weakest in relation to the special forces acting on it. The conclusion is, indeed, a truism. But now what must follow from the destruction of the least-resisting individuals and survival of the most-resisting individuals? On the moving equilibrium of the species as a whole, existing from generation to generation, the effect of this deviation from the mean state is to produce a compensating deviation. For if all such as are deficient of power in a certain direction are destroyed, what must be the effect on posterity? Had they lived and left offspring, the next generation would have had the same average powers as preceding generations: there would have been a like proportion of individuals less endowed with the needful power, and individuals more endowed with it. But the more-endowed individuals being alone left to continue the race, there must result a new generation characterized by a larger average endowment of this power. That is to say, on the moving equilibrium of a species, an action producing change in a given direction is followed, in the next generation, by a reaction producing an opposite change. Observe, too, that these effects correspond in their degrees of violence. If the alteration of some external factor is so great that it leaves alive only the few individuals possessing extreme endowments of the power required to antagonize it; then, in succeeding generations, there is a rapid multiplication of individuals similarly possessing extreme endowments of this power--the force impressed calls out an equivalent conflicting force. Moreover, the change is temporary where the cause is temporary, and permanent where the cause is permanent. All that are deficient in the needful attribute having been killed off, and the survivors having the needful attribute in a comparatively high degree, there will descend from them, not only some possessing equal amounts of this attribute with themselves, but also some possessing less amounts of it. If the destructive agency has not continued in action, such less-endowed individuals will multiply; and the species, after sundry oscillations, will return to its previous mean state. But if this agency be a persistent one, such less endowed individuals will be continually killed off, and eventually none but highly-endowed individuals will be produced--a new moving equilibrium, adapted to the new environing conditions, will result.
It may be objected that this mode of expressing the facts does not include the cases in which a species becomes modified in relation to surrounding agencies of a passive kind--cases like that of a plant which acquires hooked seed-vessels, by which it lays hold of the skins of passing animals, and makes them the distributors of its seeds--cases in which the outer agency has no direct tendency at first to affect the species, but in which the species so alters itself as to take advantage of the outer agency. To cases of this kind, however, the same mode of interpretation applies on simply changing the terms. While, in the aggregate of influences amid which a species exists, there are some which tend to overthrow the moving equilibria of its members, there are others which facilitate the maintenance of their moving equilibria, and some which are capable of giving their moving equilibria increased stability: instance the spread into their habitat of some new kind of prey, which is abundant at seasons when other prey is scarce. Now what is the process by which the moving equilibrium in any species becomes adapted to some additional external factor furthering its maintenance? Instead of an increased resistance to be met and counterbalanced, there is here a diminished resistance; and the diminished resistance is equilibrated in the same way as the increased resistance. As, in the one case, there is a more frequent survival of individuals whose peculiarities enable them to resist the new adverse factor; so, in the other case, there is a more frequent survival of individuals whose peculiarities enable them to take advantage of the new favourable factor. In each member of the species, the balance of functions and correlated arrangement of structures, differ slightly from those existing in other members. To say that among all its members, one is better fitted than the rest to benefit by some before-unused agency in the environment, is to say that its moving equilibrium is, in so far, more stably adjusted to the sum of surrounding influences. And if, consequently, this individual maintains its moving equilibrium when others fail, and has offspring which do the like--that is, if individuals thus characterized multiply and supplant the rest; there is, as before, a process which effects equilibration between the organism and its environment, not immediately but mediately, through the continuous intercourse between the species as a whole and the environment.
§ 168. Thus we see that indirect equilibration does whatever direct equilibration cannot do. All these processes by which organisms are re-fitted to their ever-changing environments, must be equilibrations of one kind or other. As authority for this conclusion, we have not simply the universal truth that change of every order is towards equilibrium; but we have also the truth that life itself is a moving equilibrium between inner and outer actions--a continuous adjustment of internal relations to external relations; or the maintenance of a balance between the forces to which an organism is subject and the forces which it evolves. Hence all changes which enable a species to live under altered conditions, are changes towards equilibrium with the altered conditions; and therefore those which do not come within the class of direct equilibrations, must come within the class of indirect equilibrations.
And now we reach an interpretation of Natural Selection regarded as a part of Evolution at large. As understood in First Principles, Evolution is a continuous redistribution of matter and motion; and a process of evolution which is not expressible in terms of matter and motion has not been reduced to its ultimate form. The conception of Natural Selection is manifestly one not known to physical science: its terms are not of a kind physical science can take cognisance of. But here we have found in what manner it may be brought within the realm of physical science. Rejecting metaphor we see that the process called Natural Selection is literally a survival of the fittest; and the outcome of the above argument is that survival of the fittest is a maintenance of the moving equilibrium of the functions in presence of outer actions: implying the possession of an equilibrium which is relatively stable in contrast with the unstable equilibria of those which do not survive.
XIII. THE CO-OPERATION OF THE FACTORS
§ 169. Thus the phenomena of Organic Evolution may be interpreted in the same way as the phenomena of all other Evolution. Fully to see this, it will be needful for us to contemplate in their ensemble, the several processes separately described in the four preceding chapters.
If the forces acting on any aggregate remain the same, the changes produced by them will presently reach a limit, at which the outer forces are balanced by the inner forces; and thereafter no further metamorphosis will take place. Hence, that there may be continuous changes of structure in organisms, there must be continuous changes in the incident forces. This condition to the evolution of animal and vegetal forms, we find to be fully satisfied. The astronomic, geologic, and meteorologic changes that have been slowly but incessantly going on, and have been increasing in the complexity of their combinations, have been perpetually altering the circumstances of organisms; and organisms, becoming more numerous in their kinds and higher in their kinds, have been perpetually altering one another's circumstances. Thus, for those progressive modifications upon modifications which organic evolution implies, we find a sufficient cause. The increasing inner changes for which we thus find a cause in the perpetual outer changes, conform, so far as we can trace them, to the universal law of the instability of the homogeneous. In organisms, as in all other things, the exposure of different parts to different kinds and amounts of incident forces, has necessitated their differentiation; and, for the like reason, aggregates of individuals have been lapsing into varieties, and species, and genera, and orders. Further, in each type of organism, as in the aggregate of types, the multiplication of effects has continually aided this transition from a more homogeneous to a more heterogeneous state. And yet again, that increasing segregation and concomitant increasing definiteness, associated with the growing heterogeneity of organisms, has been aided by the continual destruction of those which expose themselves to aggregates of external actions markedly incongruous with the aggregates of their internal actions, and the survival of those subject only to comparatively small incongruities. Finally, we have found that each change of structure, superposed on preceding changes, has been a re-equilibration necessitated by the disturbance of a preceding equilibrium. The maintenance of life being the maintenance of a balanced combination of functions, it follows that individuals and species that have continued to live, are individuals and species in which the balance of functions has not been overthrown. Hence survival through successive changes of conditions, implies successive adjustments of the balance to the new conditions.
The actions that are here specified in succession, are in reality simultaneous; and they must be so conceived before organic evolution can be rightly understood. Some aid towards so conceiving them will be given by the annexed table, representing the co-operation of the factors.
§ 170. Respecting this co-operation, it remains only to point out the respective shares of the factors in producing the total result; and the way in which the proportions of their respective shares vary as evolution progresses.
Astronomic } changes } } alter the } Geologic } incidence } changes } of inorganic } } forces. } Meteorologic } } changes } } } } } on each species: affecting } | } | } | } | } | Enemies } } | Competitors } } | } varying in } } | Co-operators } number } } | Prey } } alter the } | } incidence } | Enemies } } of organic } | Competitors } } forces. } | } varying in } | Co-operators } kind } | Prey } | | ----------------------------------------------------------------- | | { which, partially in the first | { generation, and completely in | { the course of generations, are | { directly equilibrated with the | { changed agencies. | { immediately { | { through their { which have their direct | { functions; { equilibration with the changed | { { agencies, aided by indirect | { { equilibration, through the more | { { frequent survival of those in | { { which the direct equilibration | { { is most rapid. | { | { its individuals, { { positively--by aiding the | { { { multiplication of those whose | { { { moving equilibria happen to be | { { { most congruous with the | { { mediately { changed agencies: thus, in the | { { through the { course of generations, indirectly | { { aggregate of { equilibrating certain individuals | { { individuals; { with them. --{ { { { negatively--by killing those { { whose moving equilibria are { { most incongruous with the { { changed agencies: thus, in { { the course of generations, { { indirectly equilibrating each { { of its surviving individuals { { with them. { { { by acting on it in some parts of the habitat { { more than in others; and thus differentiating { its aggregate { the species into local varieties. { of individuals, { { { and thus causing { { differentiations of { { the species into { by acting differently { varieties, irrespective { on slightly-unlike { of locality. { individuals in the { { same locality; { and thus causing { modification of the { species as a whole, { by abstracting a { certain class of { its units.
At first, changes in the amounts and combinations of inorganic forces, astronomic, geologic, and meteorologic, were the only causes of the successive modifications; and these changes have continued to be causes. But as, through the diffusion of organisms and consequent differential actions of inorganic forces, there arose unlikenesses among them, producing varieties, species, genera, orders, classes, the actions of organisms on one another became new sources of organic modifications. And as fast as types have multiplied and become more complex, so fast have the mutual actions of organisms come to be more influential factors in their respective evolutions: eventually becoming the chief factors.
Passing from the external causes of change to the internal processes of change entailed by them, we see that these, too, have varied in their proportions: that which was originally the most important and almost the sole process, becoming gradually less important, if not at last the least important. Always there must have been, and always there must continue to be, a survival of the fittest; natural selection must have been in operation at the outset, and can never cease to operate. While yet organisms had small abilities to coordinate their actions, and adjust them to environing actions, natural selection worked almost alone in moulding and remoulding organisms into fitness for their changing environments; and natural selection has remained almost the sole agency by which plants and inferior orders of animals have been modified and developed. The equilibration of organisms that are almost passive, is necessarily effected indirectly, by the action of incident forces on the species as a whole. But along with the evolution of organisms having some activity, there grows up a kind of equilibration which is in part direct. In proportion as the activity increases direct equilibration plays a more important part. Until, when the nervo-muscular apparatus becomes greatly developed, and the power of varying the actions to fit the varying requirements becomes considerable, the share taken by direct equilibration rises into co-ordinate importance or greater importance. As fast as essential faculties multiply, and as fast as the number of organs which co-operate in any given function increases, indirect equilibration through natural selection becomes less and less capable of producing specific adaptations; and remains capable only of maintaining the general fitness of constitution to conditions. The production of adaptations by direct equilibration then takes the first place: indirect equilibration serving to facilitate it. Until at length, among the civilized human races, the equilibration becomes mainly direct: the action of natural selection being limited to the destruction of those who are constitutionally too feeble to live, even with external aid. As the preservation of incapables is secured by our social arrangements; and as very few save incarcerated criminals are prevented by their inferiorities from leaving the average number of offspring; it results that survival of the fittest can scarcely at all act in such way as to produce specialities of nature, either bodily or mental. Here the specialities of nature, chiefly mental, which we see produced, and which are so rapidly produced that a few centuries show a considerable change, must be ascribed almost wholly to direct equilibration.[54]
XIV. THE CONVERGENCE OF THE EVIDENCES
§ 171. Of the three classes of evidences that have been assigned in proof of Evolution, the à priori, which we took first, were partly negative, partly positive.
On considering the "General Aspects of the Special-creation hypothesis," we discovered it to be worthless. Discredited by its origin, and wholly without any basis of observed fact, we found that it was not even a thinkable hypothesis; and, while thus intellectually illusive, it turned out to have moral implications irreconcilable with the professed beliefs of those who hold it.
Contrariwise, the "General Aspects of the Evolution-hypothesis" begot the stronger faith in it the more nearly they were considered. By its lineage and its kindred, it was found to be as closely allied with the proved truths of modern science, as is the antagonist hypothesis with the proved errors of ancient ignorance. We saw that instead of being a mere pseud-idea, it admits of elaboration into a definite conception: so showing its legitimacy as an hypothesis. Instead of positing a purely fictitious process, the process which it alleges proves to be one actually going on around us. To which add that, morally considered, this hypothesis presents no radical incongruities.
Thus, even were we without further means of judging, there could be no rational hesitation which of the two views should be entertained.
§ 172. Further means of judging, however, we found to be afforded by bringing the two hypotheses face to face with the general truths established by naturalists. These inductive evidences were dealt with in four chapters.
"The Arguments from Classification" were these. Organisms fall into groups within groups; and this is the arrangement which we see results from evolution, where it is known to take place. Of these groups within groups, the great or primary ones are the most unlike, the sub-groups are less unlike, the sub-sub-groups still less unlike, and so on; and this, too, is a characteristic of groups demonstrably produced by evolution. Moreover, indefiniteness of equivalence among the groups is common to those which we know have been evolved, and those here supposed to have been evolved. And then there is the further significant fact, that divergent groups are allied through their lowest rather than their highest members.
Of "the Arguments from Embryology," the first is that when developing embryos are traced from their common starting point, and their divergences and re-divergences symbolized by a genealogical tree, there is manifest a general parallelism between the arrangement of its primary, secondary, and tertiary branches, and the arrangement of the divisions and sub-divisions of our classifications. Nor do the minor deviations from this general parallelism, which look like difficulties, fail, on closer observation, to furnish additional evidence; since those traits of a common ancestry which embryology reveals, are, if modifications have resulted from changed conditions, liable to be disguised in different ways and degrees in different lines of descendants.
We next considered "the Arguments from Morphology." Apart from those kinships among organisms disclosed by their developmental changes, the kinships which their adult forms show are profoundly significant. The unities of type found under such different externals, are inexplicable except as results of community of descent with non-community of modification. Again, each organism analyzed apart, shows, in the likenesses obscured by unlikenesses of its component parts, a peculiarity which can be ascribed only to the formation of a more heterogeneous organism out of a more homogeneous one. And once more, the existence of rudimentary organs, homologous with organs that are developed in allied animals or plants, while it admits of no other rational interpretation, is satisfactorily interpreted by the hypothesis of evolution.
Last of the inductive evidences, came "the Arguments from Distribution." While the facts of distribution in Space are unaccountable as results of designed adaptation of organisms to their habitats, they are accountable as results of the competition of species, and the spread of the more fit into the habitats of the less fit, followed by the changes which new conditions induce. Though the facts of distribution in Time are so fragmentary that no positive conclusion can be drawn, yet all of them are reconcilable with the hypothesis of evolution, and some of them yield it strong support: especially the near relationship existing between the living and extinct types in each great geographical area.
Thus of these four groups, each furnished several arguments which point to the same conclusion; and the conclusion pointed to by the arguments of any one group, is that pointed to by the arguments of every other group. This coincidence of coincidences would give to the induction a very high degree of probability, even were it not enforced by deduction. But the conclusion deductively reached, is in harmony with the inductive conclusion.
§ 173. Passing from the evidence that evolution has taken place, to the question--How has it taken place? we find in known agencies and known processes, adequate causes of its phenomena.
In astronomic, geologic, and meteorologic changes, ever in progress, ever combining in new and more involved ways, we have a set of inorganic factors to which all organisms are exposed; and in the varying and complicating actions of organisms on one another, we have a set of organic factors that alter with increasing rapidity. Thus, speaking generally, all members of the Earth's Flora and Fauna experience perpetual re-arrangements of external forces.
Each organic aggregate, whether considered individually or as a continuously-existing species, is modified afresh by each fresh distribution of external forces. To its pre-existing differentiations new differentiations are added; and thus that lapse to a more heterogeneous state, which would have a fixed limit were the circumstances fixed, has its limit perpetually removed by the perpetual change of the circumstances.
These modifications upon modifications which result in evolution structurally considered, are the accompaniments of those functional alterations continually required to re-equilibrate inner with outer actions. That moving equilibrium of inner actions corresponding with outer actions, which constitutes the life of an organism, must either be overthrown by a change in the outer actions, or must undergo perturbations that cannot end until there is a re-adjusted balance of functions and correlative adaptation of structures.
But where the external changes are either such as are fatal when experienced by the individuals, or such as act on the individuals in ways that do not affect the equilibrium of their functions; then the re-adjustment results through the effects produced on the species as a whole--there is indirect equilibration. By the preservation in successive generations of those whose moving equilibria are least at variance with the requirements, there is produced a changed equilibrium completely in harmony with the requirements.
§ 174. Even were this the whole of the evidence assignable for the belief that organisms have been gradually evolved, it would have a warrant higher than that of many beliefs which are regarded as established. But the evidence is far from exhausted.
At the outset it was remarked that the phenomena presented by the organic world as a whole, cannot be properly dealt with apart from the phenomena presented by each organism, in the course of its growth, development, and decay. The interpretation of either implies interpretation of the other; since the two are in reality parts of one process. Hence, the validity of any hypothesis respecting the one class of phenomena, may be tested by its congruity with phenomena of the other class. We are now about to pass to the more special phenomena of development, as displayed in the structures and functions of individual organisms. If the hypothesis that plants and animals have been progressively evolved be true, it must furnish us with keys to these phenomena. We shall find that it does this; and by doing it gives numberless additional vouchers for its truth.