396 THE EVOLUTION OF LIFE.
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 it- self at the time when the shallow pools inhabited by the larva?, are being dried up by the summer's sun.* 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 w^hich 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 Amjyhlhia, are not amphibious in their habits — creatures the larvae of which do not pass their early lives in the water, and yet go through these same metamorphoses! Must we then think 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. Realizing the way in which such changes of media are in some cases gradvially imposed by physical conditions, and in other cases volmitarily commenced and slowly increased in the search after food; we shall begin to understand how, in the course of evolution, there have arisen * While these pages are passing through the press, Dr Hooker has obliged me by pointing out, that •' plants afford many excellent examples " of analogous transitions. He say§ that among true " water plants," there are found, in the smie species, varieties which have some leaves submerged and some floating; other varieties in which they are all floating; and other varieties in which they are all submerged. Further, that many plants characterized by floating leaves, and which have all their leaves floating when they grow in deeper water, are found with partly aerial leaves when they grow in shallower water; and that elsewhere they occur in almost dry soil with all their leaves aerial.
THE ARGUMENTS FROM DISTRIBUTION. 397 those strange obscurations of one type by tlie externals of another tj^pe. When we see hmd-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 en- abled to comprehend how, under pressure of population, aquatic habits may be acquired by creatures organized for aerial life; and how there may eventually arise an ornithic type, in which the traits of the bird are very much disguised. 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. Grant %at 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 w^hile it accounts for the occurrence in one medium of or- ganic 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 hin 1 limbs has a mean- ing; or else it was specially framed for its present habi- tat, in which case the structure of its hind limbs is incom- prehensible.
§ 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 habitually assumed, give us traces of the earliest organic forms; the evidence hence derived, for or against, would have had more 398 THE EVOLUTION OF LIFE.
weight than any other evidence. As it is, all we can do is to see whether such fragmentary evidence as remains, is con- gruous with the hypothesis.
Palaeontology 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 forma- tions, 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 expect to find successions of forms only slightly different from one another; while we may rationally look for consider- able 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 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 occur- rence in organic forms, of changes great enough to produce what are called different types. The result of inductive in- quiry 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 contem- plating the relations among past forms of life taken by them- selves, 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 irreconcil- able 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 ex- tinct; though, usually, species greatly resembling extant ones* Of the remains found in strata of still earlier date, the ex- tinct species form a larger per centage; and the differences be- tween 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 400 THE EVOLUTION OF LIFE.
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 com- pletely negatived.
For besides a close kinship between the aggregate of sur- viving 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 con- tinent 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 mar- supials were then specially created to fit the modified en- vironment; 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 simi- larity 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 re- divergence of organic forms, which we saw to be shadowed forth by the truths of classification and the truths of embry- ology, 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 or- ganisms inhabiting one medium, and strangely-modified organisms inhabiting another mediimi, are at the same time rendered comprehensible. And the appearances and disap- pearances 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.
26 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 accom- panied 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 indefi- nitely 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 pro- pensities, 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 anticipa- tions 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 organ- isation," 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 pro- vidence of God, the results, first, of an impulse which has been imparted to the forms of life, advancing them, in defi- nite 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 con- trasts 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, supernatur- ally 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 ex- planations 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, in- herent in organisms, and making them unfold into higher forms, is an assumption no more tenable than the assump- tion of special creations: of which, indeed, it is but a modi- fication; 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 pro- cesses 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 pos- terity." 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 ac- tions 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 sys- tems, 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 com- posant 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 com- position de I'organisation,'' * * * " alors on eut pu aperce- voir 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 406 THE EVOLUTION OF LIFE.
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 in- fluence 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 de- velopments 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 devia- tions 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 pur- poses required.'' And the creatures possessing these va- rious 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. La-