Along with these phenomena of distribution in space and in medium of life, we have the further element of distribution in time; the facts of which are admitted, however, to be too fragmentary to be conclusive either for or against the doctrine of evolution. Still it is claimed that there is one general truth respecting distribution in time, which is "profoundly significant, namely, that the relations between the extinct forms of life, found by geological exploration, and the present forms of life, especially in each great geographical region, show in the aggregate a close kinship, and a connection which is in perfect harmony with the belief in evolution, but quite irreconcilable with any other belief. As Mr. Darwin has expressed it, there is 'a wonderful relationship in the same continent between the living and the dead.'"[91]
The argument from distribution is thus summed up by Mr. Spencer:
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 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 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 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 connections between successive groups of species from early eras down to our own, cease to be inexplicable.[92]
Passing by what is here said of the aptitude of the human race to multiply, to spread, to separate, and to differentiate—an aptitude which has never resulted in the production of an essentially different animal, or in anything but incidental variations within the limits of the same species—I propose now to apply to this argument from distribution a test which seems to me to be a perfectly fair one, and one which it ought to be able to encounter. If the theory that the different species of animals now known to us have been evolved successively by descent from some primordial simplest form through modifications induced by change of habitation, of medium of life, and accumulation of new structures occurring through an immense period of time, be a sound hypothesis, the process which has evolved superior out of inferior organizations ought, in consistency with itself and with all its supposed conditions, to be capable of being reversed, so as to lead to the evolution of inferior out of superior organisms. For, although the doctrine of evolution has thus far been applied only to facts which are supposed to show an ascent in the scale of being, the argument ought to be equally good for a descent in the scale of being, provided we take care to include all the elements and causes of a change of structure, mode and medium of life, and the necessary element of time, in the operation of the process. The imaginary case that is about to be put shall include all the elements of the evolutionary hypothesis, and will serve to test at least the rationality of that theory.
Let it be supposed, then, that there was a period in the history of this earth when the whole human race, however it originated, was confined to an island, thousands of miles from any other land. This race of men adapted to a life in one medium, the air, may be supposed to have so far advanced in the ruder arts of hunting and fishing, and in the higher art of tillage, as to be able for many generations to support life by what the sea and the land would put within their reach, and by the product which their rude agriculture could extract from the soil, or which the soil would spontaneously yield. But as the centuries flow on, the population begins to press upon the resources of the territory, and the struggle for life becomes very great. At length a point is reached where the supply of food from the land becomes inadequate to sustain the population, and what can be made up from the sea will not supply the deficiency. The population will then slowly decrease, but, while this decrease goes on, there comes in a disturbing cause which will prevent any adjustment of the supply of food to the diminished number of the consumers. The sea begins by almost imperceptible but steadily progressing encroachments to diminish the area of dry land; a change of climate reduces the number of other animals available for human food, and reduces the productive capacity of the earth. Then ensues that struggle for existence which is supposed to entail changes of medium of life, and to induce transformations of structure. The conditions of existence have become wholly changed. The wretched descendants of a once comparatively thriving race are dwelling on a territory which has become a marsh. They have no means of migrating to another territory; they can only migrate to another medium. They begin by feeding exclusively on what the water will afford. They pass their lives in the pursuit of a prey which lives only in the water, and in this change of life they acquire or develop organs adapted to the new condition, organs which, in such miserable reproduction of their own species as can go on, they transmit to their offspring. Modifications upon modifications accumulate in this way through untold periods of time, until at last a new aquatic or a new amphibious creature is formed, and the difference between that creature and his remote ancestral human stock is as great as that between man and the seal, or between man and any fish that swims. Still, there will be peculiarities of structure retained, which might lead any inhabitant of another world, alighting on this globe and undertaking to trace the origin of this new creature, to the supposition that he was akin to a race of men whose fossil remains he might find buried in some stratum beneath the marsh which was the last habitat of this unfortunate race, when it had all the characteristics of its original type.
Is it conceivable that this transformation could take place? Could such a condition and situation result in anything but the utter extinction of the human race, or, in other words, in an absolute break? Could there be any modifications exhibited by the last survivors of that race other than those which are familiar to us among the varieties of the human species which have never separated themselves from their race, and between whom and their ancestral stock, wherever it was originally placed on this globe, we recognize no fundamental difference of structure, whatever may have been the changes of habitat or conditions of life? Yet the conditions and elements of this imaginary case, which is simply the process of evolution reversed, are just what the evolution theory assumes as the causes of that modification which proceeds from a lower to a higher organism; and whatever may be said of the tendency, through "the survival of the fittest," to evolve higher out of lower forms of animal life, if we allow time enough for the process, there is no reason, in the nature of things, why corresponding conditions should not lead to a degradation as well as to an elevation in the scale of beings. There is, however, one reason why no such potency should be ascribed to the conditions, either in respect to the one result or the other. That reason is that all such causes of modification, either in the ascending or the descending scale, are so limited in their effects that distinct beings can not be rationally predicated as their product, whereas the power of the Infinite Artificer to give existence to distinct beings is absolutely without limit. If naturalists would turn their attention to the limitations upon the power of all such causes as those which are supposed to work in the process of evolution, and would give us the explanations to which those limitations point, in those cases of local variation which are exhibited by animals that can clearly be traced to a parent form, they would not be compelled to resort to a sweeping theory that refuses all force to any hypothesis but its own.
But now let us go a step further in this imaginary case. Let us suppose that after this new creature, fish or amphibian, descended from the human race, has inhabited the water surrounding the ill-fated island for a million of years, another great change takes place. The water begins to recede from the land by gradations as slow as those by which in the former period it encroached. The land rises from the low level to which it had sunk, by volcanic action. Forests spring up upon the sides of mountains. The soil becomes firm; verdure overspreads the fields; the climate grows genial; the wilderness blossoms as the rose. Allow another million years for this restoration of the territory to an inhabitable condition. Slowly and in an unbroken series of generations the aquatic creatures, descended from the ancient human inhabitants of the island, emerge from the sea and betake themselves to the land. Modifications upon modifications accumulate, new organs are acquired; the survival of the fittest perpetuates them; the animals ascend in the scale of being, until the human type is again evolved out of the degraded descendants of the population which two millions of years previously dwelt as men upon the island, and carried on in some primitive fashion the simpler arts of human life. Is not this just as supposable as the evolution of the human race out of some lower form of organism? Are not all the elements—time, migration from one medium to another, change of conditions, and what is supposed to lead to the production of different organisms—just as powerful to produce the inferior out of the superior as to produce the superior out of the inferior, and so on interchangeably? The answer in each case is, that all such causes of modification in the animal kingdom are limited; that when once a distinct species is in existence, we have no evidence that it loses its distinct type or merges itself in another, although the earth may be full of evidence that types which formerly existed are no longer among the living organisms.