On the other hand, the hypothesis of the distinct creation of different species deals with the phenomena of embryologic life in a very different way. It supposes the creation of a pair, male and female, and a law of procreation, designed for the multiplication of individuals of a fixed type. It supposes many such creations, each having in its own peculiar germ the characteristic type of organism that will distinguish the mature animal from all the others. It supposes finally a law of development common to all the species the individuals of which are multiplied by the sexual union of male and female; a law of growth under like conditions, which leads to a parallelism of development until the typical plan of form and structure designed for each distinct animal, and implanted in its germ, begins to take on a mode of development peculiar to that species, and at length the perfect individual of that species is the result. In this hypothesis, therefore, there is no necessity for resorting to any connection with an imaginary ancestral stock of a different type, or for resorting to a theoretical process by which successive generations may be supposed to have gradually arisen out of the ancestral stock by successive changes which have at length resulted in a totally new species. The new species is what is supposed to have been aboriginally created, and to have been placed under its own law for the multiplication of individuals of the same type. In point of simplicity, of comparative certainty, of freedom from accidental causes of variation of which we can predicate no specific result, this hypothesis seems to have a far greater degree of probable evidence in its favor than the theory which entirely lacks the requisite evidence of intermediate connections between the lives of one species with the lives of a remote and different species. For, while it may be truly said that no man ever saw a special creation take place, and while such an act of the infinite power is of a nature that places it beyond the observation of our senses, it is neither inconceivable nor improbable, nor inconsistent with the idea of the divine attributes which we derive from the study of nature. On the other hand, it is not only equally true that no man ever saw, or in the nature of things ever can see, an evolution of distinct species out of other distinct species, but the whole nature of the supposed process of transformation involves an element of chance which forbids all calculation of the results. How, for example, in this very matter of comparative embryological development on the hypothesis of descent of all the species of the vertebrate animals from a common ancestral stock of a different type, are we to account for the fact that the embryo of any one of the descended species has come to be developed in a mode peculiar to itself and differing from the mode in which the embryo of the ancestral stock was developed? The law of sexual union, under which the individuals of the supposed ancestral stock were multiplied, must have imposed on that species an invincible necessity of reproducing in its offspring the same type that constituted the peculiar organism of the parents, whether these parents were or were not the fittest survivors of their race after the severest struggle for existence which they may have had to undergo. If the pair, or the male of that pair, has in the course of that struggle acquired a new organ, or more completely developed an old one, before the act of procreation takes place, how is it that the ovum is developed into the fœtus, and the fœtus into the newly born infant, in an invariable mode peculiar to the species to which the parents belonged? Why did not the same causes of variation which are supposed to have changed the ancestral type into one of a new and entirely distinct character, also vary the mode of fœtal development? When and how did the new organs become fixed in the type which the parents have transmitted to the offspring? And if they became so fixed in the germ which was formed out of the cellular substance contributed by each of the parents, why do we find in every known species participating in this process of reproduction a uniform mode of embryologic development peculiar to the species, and exhibiting its own suppressions and substitutions of organs, irrespective of any newly acquired peculiarities in the individual structures of the parents?

The believer in special creations has to answer no such questions as these. His hypothesis assumes the creation of a pair of animals of a certain distinct species; a law of procreation and gestation common to a vast multitude of organisms; and a law of embryologic growth peculiar to each species. Whatever peculiarities of structure may have been possessed by the immediate parents of any individual of any one of these different species—peculiarities which did not separate the parents from their race, but only made them the fittest survivors of their race—those peculiarities would or would not descend to their immediate offspring, according to varying and very inappreciable circumstances. But that which constituted the special type of the race, and especially that which constituted its peculiar mode of development during the embryonic stage, would remain unaffected by these incidental and accidental peculiarities of the parents, because, from all that we can discover, that special type was impressed upon the embryo at the earliest stage of its existence, and constituted the living model that was to be developed into the perfect animal of that species, by a law which placed it beyond the influence of any adventitious and non-essential advantages which the male or female parent may have acquired over other individuals of the same race. So that, if the postulate of a special creation of species be assumed as the groundwork of the reasoning, we have to go through with no speculations about a common ancestral stock of all the species, and we have to account for no phenomena that are exposed to chances which might have produced very different results from those which are open to our observation, and results of which we can predicate nothing with any degree of certainty. On the hypothesis of the special creation of a species, and an aboriginal pair of each species, with all that this implies, we can with a high degree of certainty predicate most of the phenomena that we have to observe, and more especially so much of the phenomena of embryologic growth of the different species as are open to our investigation after the life of both mother and embryo has become extinct.

It only remains for me to give to this reasoning a concrete application. Take the case made use of by Mr. Spencer in the passage above cited—that of the "allantois," a vascular membrane, which is said to be in the mammalian embryo homologous with one which in the higher oviparous vertebrates, such as the birds and reptiles, replaces what was at first a breathing apparatus, and becomes for them, during the rest of embryonic life, a sort of lung, or an organ that aërates the blood until the permanent respiratory organs come into play. In the mammalian embryo, the first appliance for aërating the blood is described as a system of vessels distributed over the area vasculosa, and like that which is first observable for the same purpose in fishes. But, as the mammalian embryo continues to grow, a change takes place. There buds out from it the vascular membrane called the "allantois," which is substituted in the place of the first aërating apparatus. Then a further change takes place, as between the higher oviparous vertebrates and the mammalian vertebrates. In the former, the "allantois" continues to perform the breathing function through the rest of the embryonic life. In the mammalian vertebrates it undergoes two changes: In the implacental mammals, it aborts, having no function to discharge; in the placental mammals it becomes modified into another organ, namely, that which serves to convey nutrition from the mother to the offspring. After birth, it is of course ended.

Now, the reasoning, or rather the assertion, that these substitutions are unaccountable as the results of design, appears to me to be singularly inconclusive. It is quite illogical, according to all philosophic meaning of design as applied to the works of the Creator, or to the works of nature, if that term is preferred, to argue that a particular object could have been better accomplished directly, than by a metamorphosis of an organ from one function to another, or by substitution. The metamorphosis, or substitution, which in such cases we find in nature, is of itself the very highest evidence that the indirect method was the best, if we admit the idea of a Creator, because it was the method chosen by a being of infinite perfections for reasons which we may not be able to discover, but which we must presume to have existed, if we concede that hypothesis of attributes which "design" in this case necessarily implies. But how are these metamorphoses and substitutions any more accountable upon the supposition that the mammalian type arose by generation out of the lower vertebrate types which in their embryonic life exhibited the same changes? The doctrine or theory of evolution does not account for them at all; for, while the doctrine supposes, as matters of pure theory, that there were certain states through which the mammalian embryo passed, which represented more or less distinctly those which it had in common with its assumed remote ancestors, the lower vertebrata, it does nothing more than to suggest the theoretical idea that the mammalian embryo came to develop these subsidiary organs in the mode in which they were developed in the embryo of the lower vertebrata, because it was descended from the lower vertebrata. The varying states through which the embryo passed from the lower vertebrata to the mammalian type, are all hypothetical, and there is, therefore, no basis of fact on which to rest the belief in a common mode of development, as resulting from a connection of lives with lives between the mammalian type and the types of birds, reptiles, or fishes.

On the other hand, the hypothesis of the special creation of a species implies the simple fact of a designed process of embryonic development for each species, with substitutions of organs and changes of function in certain organs peculiar to that species; a fact which may well consist in a certain parallelism in the different metamorphoses, and a preservation of the same unvarying changes in the development of each separate embryo. Why these changes should exist, we can not tell; but their existence is very strong proof that they were designed, or made to take place, for some reason, if we admit the hypothesis of a Creator. For that hypothesis, we must look to a wider class of facts, and to the whole phenomena of nature.

4. We now come to the argument from distribution. This is one of the weakest of the indirect supports of the doctrine of evolution; but, as it is much relied upon, it must be stated with all the force that it is supposed to have. The facts that are relied upon are these: When we survey the whole surface of the globe, so far as it is known to us, we find, in the first place, that the areas which have similar conditions (of soil and climate), and sometimes, where the areas are nearly adjacent, are occupied by quite different faunas. On the other hand, it is said that areas remote from each other in latitude, and contrasted in soil and climate, are occupied by closely allied faunas. The inference drawn is, that there is no manifest predetermined adaptation of the organisms to the areas, or habitats, in which they are found, because we do not find that like organisms are universally or generally found in like habitats, nor very unlike organisms in very unlike habitats. The conclusion is, that the facts of distribution in space do not conform to the hypothesis of design. In other words, the different animals found in different regions were not specially designed for those regions, but some of them have extended into regions of a different character; and when the regions are very unlike there are not found very unlike organisms, but there is a general similarity, or a less extensive variety. There is said, also, to be another important fact, namely, 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." Hence is drawn the conclusion that "each species of organism tends ever to expand its sphere of existence—to intrude on other areas, other modes of life, other media."[88] A good deal of aid is supposed to be derived for this argument respecting animal life by analogies drawn from the vegetable kingdom; but I can not help thinking that there is much caution to be observed in formulating such analogies into a law of universal application, or into one that relates to the existence of animal organisms. The origin, the multiplication, and the spread of animals involve a principle of life, organization and development which is very different in some important respects from that which obtains in the vegetable world. But, without laying any stress upon this distinction, and without intending to deprive the argument for animal evolution of any aid which it can derive from such supposed analogies, I pass to the specific argument respecting animal distribution. The argument is this: Races of organisms become distributed over different areas, and also through different media. They are thrust by the pressure of overpopulation from their old into new habitats, and as they diverge more widely in space they undergo more and more modifications of structure, by reason of the new conditions on which they enter. Thus, these powerfully incident forces, the new conditions on which the migrating races enter in new regions, vary the structure which they originally brought with them, and which descended to them from the common stock of which they were modified descendants. The widest divergences in space, under such circumstances, will indicate the longest periods of time during which these various descendants from a common stock have been subject to modifying conditions. There will, therefore, come to be, it is said, among organisms of the same group, smaller contrasts of structure in the smaller areas; and, where the varying incident forces vary greatly within given areas, the alterations will become more numerous than in equal areas which are less variously conditioned: that is to say, in the most uniform regions there will be the fewest species, and in the most multiform regions there will be the most numerous species. These hypotheses are said to be in accordance with the facts of distribution in space.[89]

But there are also facts of distribution through different media. The meaning of this is, that, whereas all forms of organisms have descended from some primordial simplest form, which inhabited some one medium, such as the water, its descendants, by migration into some other medium or other media, underwent adaptations to media quite unlike the original medium. In other words, the earth and the air have been colonized from the water. Numerous facts are adduced in support of this conclusion, which are thus summarized:

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 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 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 those 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.

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 that there has even been going on that redistribution 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 unfitness. 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.[90]

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.