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 obviously 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 particular type was evolved.
Though comparisons of ancient and modern organic forms, prove that many types have persisted through enormous j^eriods of time, without undergoing great changes; it was shown that such comparisons do not disprove the occurrence in 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 >ield signs of having been developed from ancient lower types; and while there are many modern types which may THE ARGUMKNTS FROM DISTRIBUTION. 399 liave been thus developed, thougli 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, in conformity to which there arise additional modifications answering to additional needs, will there be that increased heterogeneity which characterizes higher forms. Hence, though the facts of palaeontology cannot be held to prove evolution, yet they are in harmony with it; and some few of them yield it support.
§ 141. One general truth respecting distribution in Time, is, however, 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 perfect harmony with the belief in evolution, but quite irreconcilable with any other belief.
Note, first, how full of meaning is the close kinship that exists 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 per centage; 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 epoch 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 crea. tions. 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 that 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 completely in harmony with these new Australian conditions, that, when taken to Australia, they rapidly extrude the marsupials. ^Yhile, therefore, the similarity between the existing Australian Fauna and the Famia 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.
THE ARGUMENTS FROM DISTRIBUTION. 401 And so is it with parallel relations in New Zealand, in South Amcriea, 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 redivergence 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 difierentiate, 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 that kind of relation among the species, and genera, and orders, peopling the Earth's surface, which we find exists.
Those remarkable identities of type discovered between organisms inhabiting one medium, and strangely-modified organisms inhabiting another mediimi, 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.
CHAPTEIl VIII.
now IS ORGANIC EVOLUTION CAUSED?
§ 143. Alheady it lias been necessary to speak of tlie causes of organic evolution in general terms; and now we are prepared for considering them specifically. The task before us is to deduce the leading facts of organic evolution, from those same first principles which evolution at large conforms to.
Before attempting this, however, it will be instructive to glance at the causes of organic evolution that 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 Be 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 Darwin gave reasons (sundry of them valid ones) for believing that organized beings of every kind, have donow IS OllGANIC EVOLUTION CAUSED? 403 scendod from one, or a few, primordial germs; and along with some observable causes of modification, wliich 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. lie 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 general de la nature, et sa marche uniforme dans ses operations," Lamarck attributes " la progression evidente qui existe dans la composition de I'organisation des animaux; " and " la gradation reguliere qu'ils devroient oJffrir dans la composition de leur organisation," he thinks is rendered irregular by secondary causes. Essentially the same in kind, though somewhat different in form, was 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 general contrasts between lower and higher forms of life, are regarded by him as due to an innate aptitude to give birth to forms 404 THE EVOI-UTION OF LIFE.
of more perfect structures. The last to re-enunciate this doctrine has been Prof. Owen; who asserts " the a:iioni of the continuous operation of creative power, or of the ordained becoming of living things." Though these highly- general expressions do not suggest any very definite idea, yet they imply the belief that organic progress is a result of some in-dwelling tendency to develop, supernaturally impressed on living matter at the outset — some everacting 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 explains 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 forms, 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. Along with this intrinsic tendency to progress, supposed to be primordially impressed on them, Dr Darwin held that animals have a capacity for being modified by processes which their own desires initiate. lie 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 HOW IS ORGANIC EVOLUTION CAUSED? 405 lives, all animals undergo perpetual IransfurmationH; wliich are in part produced by their own exertions, in consequence of tlieir 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 a great deal is to 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 interieuTj'' 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 systeme nerveux, ne vivent qu'a I'aide des excitations qu'ils recoivent de Texterieur.'' Afterwards he says — "je reconnus que la nature, obligee d'abord d'emprunter des milieux environnans la puissance excitatrice des mouvemens vitaux et des actions des animaux imparfaits, sut, en composant de plus en plus I'organisation animale, transporter cette puissance dans I'interieur meme de ces etres, et qu'd la fin, elle parvint a mettre cette meme puissance a la disposition de I'individu." And still more definitely he contends that if one considers " la 'progression qui se montre dans la composition de I'organisation,'' * * * " alors on eut pu apercevoir comment les hesoins, d'abord reduits a nullite, et dont le nombre ensuite s'est accru graduellement, ont amene le penchant aux actions propres a y satisfaire; comment les actions devenues habituelles et energiques, ont occasionn^ le developpement des organes qui les executent."
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 tlie 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 sufiice to point out that the difiiculty 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 transferrence 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 arose 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 HOW IS OKGANTC EVOLUTION CAUSED? 407 type wliich exists among animals, he regards their deviations from one another, as caused by differences in their modes of life: such deviations being directly adaptive.
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 pm'pose, with great ingenuity, his extensive knowledge of the animal kingdom. From a passage in the Avertlssement, 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 I'interieur des animaux, mouvement qui c'est progressivement accelere avec la composition plus grande de Torganisation; et que Vinfluence des circonstances nouvelles, k mesure que les animaux s'y exposerent en se repandant dans tons les lieux habitables, furent les deux causes g^nerales qui ont amene les differens animaux a I'^tat ou nous les voyons actuellement." But elsewhere, the view he expresses appears decidedly different from this. He asserts that "dans sa marche, la nature a commence, et recoromence encore tons les jours, par former les corps organises les plus simples; " and that " les premieres ebauches de I'animal et du vegetal etant forme es dans les lieux et les circonstances convenables, les facultes d'une vie commencante et d'un mouvement organique etabli, ont necessairement developpe pen a 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 Vor- 408 THE EVOLrTIOiN OF LIFE.
ganisation suhit, ^d et Id, dans la serie generate des animaux, des anomalies operees /par Vinfluence des cir Constances d'hdbitation, et par celle des habitudes contractees.'' 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 a definite shape, 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 mde 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 as to the way in which natural agencies acted in the production of plants and animals, must not make us forget the merit of his intuition that animals and plants wei^e 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 HOW TS ORGANTC EVOTJTTTON CAUSED 409 the hypollicsis of evolution, but he does something towards elucidating tlie process of evolution. Ilis reasonings show us an unconscious mingling of the belief in a supernaturallyimpressed 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 gro^\Ti at the expense of the first. Lamarck, in elaborating this general conception, has given greater precision to both its truth and 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 evolution, 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 functional adaptation to conditions, produces either evolution in general, or the irregularities of evolution, is to raise the further question — why is there a functional adaptation to conditions? — why do use and disuse generate appropriate changes of structure? ^N^either 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.
CHAPTER IX.
CHAPTER IX.
§ 148. When illustrating the rhythm of motion {First PrincijjleSy § 94) 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 co-incidence which, though causing 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, it was pointed out 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 at which the Earth's orbit has least eccentricity, the temperate and intemperate climates which repeat their cj^cle 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 a response in the changed functions of organisms. The whole vegetal and animal kingdoms, are subject to a quadruply- compounded rhythm in the incidence of the forces on which life primarily depends — a rhythm so involved in its slow working round, that at no time during one of these vast epochs, can the incidence of these 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; and in some cases from salt water to fresh 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 that are able to move about with some rapidity; so must these alternations of temperate and intemperate climates produce extensions and restrictions of habitats. These extensions and restrictions, though slow, will be universal — will aifect 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 arc fixed, yet the species will move: the seeds of plants living at the limit, will produce individuals that 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 the 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 sj)ecies, will, during far longer intervals, undergo increasing rises and falls and then decreasing rises and falls. There will be an alternation of spring tides and neap tides, answering in its period 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.
§ 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 that are being formed, 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 altered; 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 which their detritus is left. And where volcanoes are formed, the ashes they occasionally send out, modify the character of the soil throughout large surrounding tracts.