Let us return to this primordial anatomical element which we may call individual-element. It virtually represents a vertebrate animal just as the ovum detached from the ovary of the female represents a man, who is only waiting for favourable circumstances in order to develope himself. This individual-element, according to our hypothesis, is at first simply reproduced; then, after some considerable time, its descendants, will, little by little, in their own sphere of activity, give birth to other elements in juxtaposition to themselves, in this manner perfecting it and identifying it more and more with the vertebrate type which it offers for our consideration. After some considerable time vertebrates of as simple an organism as mixinæ and lampreys will have thus appeared. Then, again, after another considerable lapse of time—millions of centuries, rather than thousands—these animals with elementary vertebra will have successively produced, by transformation, all the vertebrata which stock the globe at the present day.
We must here make an important remark. We have inferred by all which precedes this, that the vertebrata of the present day and the fossil vertebrata all descend from the same individual-element prototype, whose existence we have admitted. In one word, we think that all the vertebrata, both present and past, have the same genealogy, and are all relations. That may doubtless be the case; but nothing will make us admit that there once existed on our planet conditions fit for the birth of this individual-element prototype, and that these circumstances have never since been represented; so that the most simple vertebrata of our time may very well descend from a less ancient spontaneous genesis than the mammalia and man himself. Nothing hinders such a supposition. It does not cost us any more to admit that one day or other a simple organic element is formed, endowed with a life of its own, and, even more, with a latent life, which it can, by means of time and circumstances, diffuse around it; it does not cost us more to admit this than to admit that similar elements have arisen at different periods of time. This last supposition may even be regarded as so much more probable, that we must renounce entirely, in order to explain specific transformations, the influence of the geological revolutions of which Étienne Geoffroy took so much account. We have seen higher up that these were far from being proved; we can add, in support of our assertion, a fact which we think has not been sufficiently remarked. If these revolutions ever existed, we have a strong proof that they have only very slightly altered the conditions of life on the surface of the globe, at least since the ancient periods during which the first alluvium was deposited; if we dredge some yards deep in the ocean, the drag brings up terebratulæ and encrini; that is to say, animals identical with those which we find in the most ancient alluvia. Is it not remarkable that the lowest placed fossil in the stratigraphic ladder of the beds of the terrestrial surface, the most ancient fossil which we know, is precisely this same terebratula, which still lives in our seas? What must we hence conclude? That there once existed on the globe, at least to a certain extent, conditions of aquatic life sensibly identical with those which exist at the present day.
Whether all the species of vertebrata descend from one original spontaneous beginning, or from many successive ones, signifies very little, since, in the second case, the primordial individual-elements which have thus appeared at various times, would always show a great analogy to one another.
Now, after all that we have said, this is how we may, in our opinion, represent by a graphic figure the whole of the vertebrate kingdom,[315] in the present and in the past. Let us image a conical figure: the individual-element of which we have spoken will occupy its summit. From this point a number of straight lines, few at first, will start, branching off and always multiplying themselves with more or less regularity, but so as to form an immense cone.[316]
Each of these straight lines would represent a specific modification, accomplished after a certain number of generations under the combined influence of the ambient medium and of some considerable time: in other terms, each ramification would represent a species having once existed or now existing on our planet. The length of each line would measure the time which the species in question has existed. These lines would never converge, because we do not believe in the creation of permanent species by means of hybridity.
Now the mind must admit here all possible combinations; certain species have disappeared without producing any others after them:—others exist actually without our having any idea of one of the intermediary species which have been allied to primitive species;—others have subsisted slightly or not at all altered from the remotest antiquity up to our own days, thus becoming through contemporaneous time the transformed descendants of fossil species, of which they were also formerly the contemporaries; it is even not impossible but that certain species succeeding one another may have presented a retrograde evolution, so that we must not always conclude that because one animal is only inferior to another, it has therefore preceded it:—without going so far as all this, the evolution of certain species may have presented a long time of cessation whilst all others were progressing around them, so that they appear to have retrograded. This is what has made M. Michelet[317] say, “Nature has not progressed with a continuous flow, but with retrograde movements, and stoppages, which allow her to harmonise everything.” These times of repose in a specific evolution, as well as the hypothesis of successive geneses which are already admitted, explain how the stratified beds of the earth’s surface, in showing from low to high what we may call more perfect organic means, unveil at the same time to our eyes here and there a certain number of species, inferior in organisation to those in the most ancient rocks.
As to explaining how a part of the ancient species has been able to modify itself whilst another has remained stationary, we must admit that all these influences of medium have always been exclusively local, so that all the coexisting vertebrata have never been able to submit at once to its influence. We must understand by medium, the whole of the circumstances, past or present, which are able to influence organism mediately or immediately in any manner whatsoever. The ancestors of an animal, as well as the sun which warms it, and the parasites which devour it, make up a part of this medium.
But if it is easy to explain variety by the medium, it is a difficulty against which the mind struggles. How can we explain ascending and progressive variety? must we believe in some finality, an end settled beforehand? We do not think so. Finality is a sort of divine prevision, and the world as regards this hypothesis is still in tutelage; we would rather believe in a creating intelligence. A simple example will make our meaning understood. In the vegetable world this strikes us forcibly:—the most simply formed plants are precisely those which approach most nearly to animals by reason of their physiological manifestations.[318] The plants which they call superior, by placing them in an organographic point of view, are in reality inferior, so that these plants are simple in reference to the dicotyledons which have necessarily succeeded them, and there has been in reality a retrograde march of life, instead of the ascending march of the animal kingdom. Must we seek for the reason of this difference in the presence of a nervous system? We think so. We then would admit that organism would tend to modify itself by an inconscient act of the will, analogous to those which rule most physiological actions; this would be something like the possible increase or growth of the head by reason of the influence of civilisation of which we have before spoken.[319] And whilst all the specific varieties would result among plants from the influence of the physical medium, we must add to the notion of this medium, as regards animals, the nervous activity of the ancestors.
By the side of this creating influence we must recognise in the medium a parallel destructive influence. Now, we can appreciate this every day. The present tells us about the past; we cannot doubt but that species formerly disappeared exactly as we see them still disappear under our eyes by the manifestation of some new condition of the medium; these may be sudden; volcanic phenomena, floods, extreme variations of temperature, diseases, famines, enemies—all these hypotheses are possible, and all equally reasonable: the dodo has disappeared some years ago, having been destroyed by the hand of man; they say that the apterix will soon disappear in the same way, devoured by cats. But actions only moderately destructive were doubtless otherwise very important, and we find here all the phenomena which have been so well described, and so well explained by Darwin[320] under the name of vital competition. By this we see, even since the most ancient historic periods, that certain savage animals, like the lion,[321] crocodile,[322] and hippopotamus,[323] retire before mankind; that the black rat is disappearing in Europe to give place to the field mouse, and that a race of savages disappears when their country begins to be inhabited by a more civilised race, even when the victors in this organic, as well as political, struggle, are not able to reproach themselves with any cruelty.
Now, let us apply to man the theory of the origin of species which we wish to be dominant, for there is no reason to think that man forms any exception to the common rule. Before all things, we must remember that human races cannot lend themselves to any classification in natural series. It is also as impossible for the naturalist to point out a race at the present day from which all the others are derived, either parallelly or successively, as for the historian to discover in the past any trace of a homogeneous humanity. If even such an uniformity had ever existed, how would the remembrance of it have been kept, for it is evident that this primitive form, constituting at the beginning all the human genus, would be the same inferior form, such as the Negro or the Bosjesman, for instance, nature rising in general from inferiority to perfection. This was for a long time Prichard’s idea, and certain monogenists think the same at the present day. This hypothesis, entering at its basis into the doctrine of evolution, has nothing in itself which is startling; we can only say one thing against it, and that is in its admitting as proved that filiation which would connect one with the other all the groups composing in our times the genus homo. For our part, we wish simply to extend the same manner of viewing the matter, to generalise it, and to place it in relation with this immense unknown which is behind us, and of which monogenists do not take enough notice. We maintain that there has existed in the night of time a certain species, less perfect than the most imperfect man, remounting by a certain number of intermediary species, of whose nature it is impossible for us at present to form any idea,—to this primordial vertebrate animal which we admit. This species, a rough outline of what man now is, gave birth, after a considerable time, to many other species, whose parallel and unequal evolution, following what we have said concerning animals, has at the present day as contemporaneous (but not the last) illustration, the different species of men designated by the name of races. So that all humanity would be in relationship, if the expression be allowed us, not in the serial sense, as monogenists take it, but in the collateral sense, and at a degree which we cannot determine; the prognathous races probably less deviated from the former type, the others more separated from this type, and more perfect.