Chapter VI (2)

The question here is, not whether descent of organisms from organisms, with modifications upon modifications, is a supposable theory, but whether it is so satisfactorily shown that it can be said to exclude the hypothesis of a special creation of each organism. There may be parts of structure in one animal which seem to have no functional use, although we should be cautious in making the assumption that they are of no use because we have not yet discovered that use. But let it be assumed that these apparently useless parts in one animal correspond to parts which in another animal are functionally useful. If there was established for these two separately created animals a like system of procreation and gestation, that system, affected at the same time by a law of growth imposed by the special type of the species, might in one species lead to the presence of parts of which we can not recognize the use, and might in other species lead to the presence of parts of which we can see the use. It does not help to a better explanation to say that there has been an accumulation of modifications upon modifications in the course of an unknown descent of one organism from another. Why did these modifications stop short of the production of a species or of several species in which no resemblance of parts more or less functionally useful could be found? The supposition is that the modifications have been going on through millions of years. Time enough, therefore, has elapsed for the destruction of all uniformity of structure; and the causes of modification are as immeasurable as the period through which they are supposed to have been operating. The imaginary ancestral stock, wherever it is placed in the line of remote descent, had, in its first distinctive existence, a peculiar structure, which it bequeaths to its offspring. In the countless generations of its descendants, modifications of that structure take place, until a new animal is evolved. What preserved any unity of type from the modifying influences? It was not choice on the part of the several descending species; not a conscious exertion to preserve something; it was nothing but the propinquity of descent, which by the law of heredity transmitted certain resemblances. But why was that law so potent that it could preserve a certain unity of type, and at the same time so powerless as not to prevent the modifications which the successive organisms have undergone in all other respects? Or, to reverse the terms of the question, why were the causes of modification sufficiently powerful to produce distinct species, and yet not powerful enough to eliminate the resemblances which we find obtaining throughout the whole group of animals to which these several species belong? It would seem that here we are not to lose sight of the fact that, in the animal kingdom, procreation never takes place between a male and a female of distinct species, and that we have no reason to believe that it ever did take place. Now, although the evolution hypothesis supposes that, starting from an ancestral stock, the modifications of structure have been produced in offspring descended from parents of that same stock, which have transmitted acquired peculiarities to their immediate progeny, and so on indefinitely, yet there must have been a time when the diverging species became distinct and peculiar organisms, and when it became impossible for any crossing of these organisms to take place. All the supposed modifications, therefore, have taken place within the limits of an actual descent of one kind of animal from another, each successive pair belonging to the species from which they were individually generated. In this descent of lives from lives, there came about changes which in progress of time led to two animals as wide asunder as the man and the ostrich, or as the man and the horse, and yet the causes which were powerful enough to produce these widely diverging species were not powerful enough to break up all unity of plan in some one or more respects. If naturalists of the evolution school would explain how there has come to be, for example, in the skeleton of the _vertebrata_, a bony structure called the spine, in which a certain resemblance and a certain function obtain throughout the whole class, and yet one species creeps upon its belly, another walks on four legs, and another on two, and one flies in the air and another never can do so, and how this could be without any design or special interposition of a creating power, but that the whole of this uniformity amid such diversity has arisen from acquired habits among the different descendants from an aboriginal stock that had no such habits in either mode of locomotion, and no organs for such modes of life, they would at least be able to commend their theory to a better appreciation of its claims than is now possible to those who want "grounds more relative" than a naked hypothesis.

3. The argument from embryology requires for its appreciation a careful statement of its abstract proposition, and a statement of it in a concrete form. As an abstract proposition, embryology, or the comparison of the development of different organisms under their embryonic stages, shows that in the earliest stage of any organism it has the greatest number of characters in common with all other organisms in their earliest stage; that at a later stage its structure is like the structures displayed at corresponding phases by a less extensive number of organisms; that at each subsequent stage the developing embryo becomes more and more distinguished from the groups of embryos that it previously resembled; and that this divergence goes on, until we reach the species of which the embryo is a member, in which the class of similar forms is finally narrowed to that species.

It seems that Von Baer formulated this generalization of embryologic development into an "embryologic law," which, according to Mr. Spencer, becomes a support to the hypothesis of evolution in this way: Species that had a common ancestry will exhibit a parallelism in the embryonic development of their individual members. As the embryos of the ancestral stock were developed in their growth, so the embryos of the descended species would be developed at corresponding phases in a similar way. As one species diverged from its ancestral stock, there would come about modifications in the development of its embryos, and thus a later ancestral stock would be formed, which would in turn transmit to its descendants in the development of the embryo less and less resemblances, and so on, until finally the individual animal, at birth, would structurally resemble only the individual infants of its own race.

Here, then, is another remarkable instance of the force of an adopted theory. First, we have a comparison of the embryonic development of different animals from their seminal germs which displays certain phenomena of resemblances and departures. Next, we have the assumption of an ancestral stock, the common origin of all the organisms in the development of whose embryos among its descendants an embryologic law was to work, starting from the visible resemblance of all the germs, then exhibiting structural changes into later ancestral stocks, and so on, until the resemblances are reduced to those which obtain only among individuals of the same species. So that, without the hypothesis, the assumption of an ancestral stock of all the organisms, formed somehow in the course of descent from a germ that gave rise to an animal of some kind, we have nothing to which to apply the embryologic law. We are to infer the embryologic law from the parallelism of embryonic development which prevails in the whole series of animal generation, or from its divergences, or from both, and then we draw from _this law_ the inference that the whole series of animals came from some common stock. The difficulty with this whole theory is, as I have more than once suggested, that we have no means, aside from the theory itself, of connecting lives with lives, in the generation of one distinct species out of another. Without some proof of the fact that the human fœtus was a diverging growth out of some ancestral stock that was the same as that from which the fœtus of another animal was a different diverging growth, the embryologic law is no help to us whatever. If this kinship of the human fœtus with the fœtus of some other animal can not be found, by tracing the intermediate links which carry them respectively back to their common ancestor, between what animals in respect to their embryonic development can such kinship be found, excepting upon the theoretical assumption of a common origin of the whole vertebral class? If there was such a common ancestral stock, where is it to be placed, what was its character, when did the law of embryologic development begin to operate upon its descendants? Until some facts can be adduced which will have a satisfactory tendency to show the kinship of one animal with another by reason of ancestral descent from a common ancestral stock that was unlike either of them, the phenomena of embryologic development have no tendency to displace the hypothesis of special creations; for, on the latter hypothesis, the phenomena of resemblances and differences in the growth from the germ into the fœtus and from the fœtus into the newly born infant, evinced by any range of comparison of the different species, would be the same. If man was a special creation, and one of the higher quadrumana was also a distinct and separate creation, the establishment for each of a like process of procreation and gestation would produce all the resemblances of fœtal growth that obtain between them, and the ordained differences of their animal destinies would explain all the divergences. Let us see if this is not a rational conclusion.

It is exceedingly difficult for the common reader of such a work as that of Mr. Spencer, on which I am now commenting, to avoid the influence of the perpetual assertion that facts are explicable upon one hypothesis alone. At each step in the argument, the array of facts terminates with the assertion that, upon the hypothesis of design, the facts are inexplicable; and yet we are furnished with no reasoning that has a tendency to show that the facts necessarily exclude the hypothesis of design, or, in other words, that the facts are inconsistent with that hypothesis. It is essential to understand what is the true scope of the hypothesis of special creation; for, without a definite idea of what that term implies, we have no proper means of comparing the facts of animal resemblances or differences with the rationality of the hypothesis that they resulted from an intentional design. Recollecting, then, that we are now pursuing the resemblances and divergences that are found in a comparison of the embryologic development of different species of animals, let us endeavor to understand the meaning of what I have suggested at the close of the last preceding paragraph; namely, the establishment for a large class of animals of a like general system of procreation and gestation, and the ordination of different destinies for the different species of animals belonging to that class. I have said that the two branches of this hypothesis would account for the resemblances in the embryological growth of different animals, and would explain the divergences which obtain among their embryological developments. The first inquiry is, whether this hypothesis presents a true philosophic idea of special creation. The next inquiry is, whether it affords a satisfactory explanation of the phenomena of comparative embryologic development.

We must never lose sight of the one grand postulate of an infinite Creator. This postulate must be conceded to the believers in special creations, because any idea of creation implies a creating power. If we conceive of creation without a Creator, we must stop all argument. Now, the hypothesis of creation, as I have more than once said, implies a being of boundless faculties. There can be absolutely no limitation to the power of such a being, either in respect to the methods by which he will accomplish his objects, or to the number and variety of these objects, or to the purposes for which they are to exist. If we narrow our conception of creating power to anything less than an infinite faculty; if we suppose it to be restricted in any direction; if we argue about it as if there were things that it can not do, we shall be without the means of reasoning soundly upon anything that it is supposed to have done. It is quite otherwise when we are reasoning about the operation and effect of secondary causes. There is no secondary cause--no imaginable operation of a fixed quality of substance--no action of any of the properties of substance--that is not limited. The scope of its action may be very wide; within its sphere it may be enormously potent; but in its very nature it is bounded.[86] It is not so with the First Cause of all things; not so with the Infinite Power which, upon the hypothesis of a First Cause, has established all the physical laws of the universe and all the properties of matter. So that, when we reason about the methods of that infinite creating power, if we find a general system established, or a pattern repeated through a very large class of organisms, the proper inference is, not that the power was limited, but that it has been exercised to the whole extent of what was useful, and in that direction has been exercised no further; and if we find variations or additional structures incorporated with the repetition of a general pattern, the proper inference is that the unlimited creating power has put forth all the additional exertion and skill needful for the formation of new beings.

What, then, does the establishment of a like system of procreation and gestation imply, upon the supposition of the distinct creation of species? It implies a certain parallel embryonic development, from the germ to the fœtus and from the fœtus to the new-born infant, throughout a large group of different animals; and this parallelism would in certain stages of the embryonic growth display identity or close similarity of form and structure. But as in each species of animal the distinct creation would necessarily imply a distinct destiny, the parallelism of embryonic form and structure would cease at the point of development at which the characteristic structure of the species would begin to unfold itself. The general system of procreation and gestation common to a whole class of different animals, and the ordained diversity of species, would present the same phenomena of resemblances and differences in the embryonic development that are supposed to be explicable only by the hypothesis of a descent of all the species from a common ancestral stock through the process of evolution.

Notwithstanding the mystery and obscurity in which the process of animal procreation is involved--a mystery and obscurity which will perhaps never be fully solved--we can see enough to warrant some definite conclusions. One of these conclusions is that, in the formation of the germ which becomes developed into the fœtus, the male and female parent each contributes some cellular substance to the compound which constitutes that germ. We may safely infer this, because the individual animal becomes a union of characteristics belonging to both the parents, although the traits that are peculiar to one of the parents may be more or less marked in their different offspring, so that in one of the descendants the paternal and in another the maternal traits will predominate. But in every descendant from the same pair there is more or less of the peculiarities of each parent plainly discernible. The inference, therefore, may be safely drawn that the male and the female parent each contributes to the formation of the ante-fœtal germ some cellular substance, in which resides the typical characteristic of animal organism which each parent possesses. The compound germ that is thus formed is endowed with the mysterious principle of animal life which admits of growth and development; and whether after its formation the female parent bestows most or bestows least upon the product, that product consists of a union of cellular substances contributed by both the male and the female parent in the sexual act of procreation. This compound resultant germ, in the earliest stage of its formation, like the separate cells of which it is a union, exhibits no visible difference when we compare the ante-fœtal germ of one animal with that of a different animal. Perhaps we shall never be able to detect either chemical or mechanical differences in the cellular substances or in the earliest stage of the compound product which has resulted from their union. But in that compound product there resides a contributory cellular substance derived from each of the parents; and it is a just inference from this fact, and from what we learn when we trace the further development, that there is a peculiar and typical structure impressed upon and inwrapped in this compound germ, which is to grow into a fœtal development by a law of its own. There will at the same time be a particular law of development for each distinct species of animal, and a general law of development for a great variety of species among whom there obtains a common process of the sexual union and of the contribution of male and female cellular substance. When the fœtus becomes formed, there will still be marked resemblances in the different species, before the stage is reached at which the characteristic structure of each species is to begin to unfold itself. But at some time the fundamental difference of structure originally lodged in the cellular substances of which the compound ante-fœtal germ was composed, and impressed upon that germ as the type which was gradually to unfold itself into a distinct being, will begin to exert its force. The resemblances of structure will become less and less, as the fœtus of the different animals approaches to the time of birth. Organs, or appearances of organs, which at one stage of the comparison have seemed to indicate descent from a common ancestral stock, but which may have been only the result of a common process of fœtal development, will be found to be varied by force of the original diversity of structure and destiny that was made to reside in the seminal substance of each distinct species of animal; and, at length, this original and intentional peculiarity of structure and being would become perfected at or before the period when birth is to take place, leaving only those resemblances which must obtain in all organisms constructed in certain respects upon a uniform plan, and brought into being by a common process of procreation and gestation.

Let us now see whether this reasoning involves any such unphilosophical or unscientific belief as is supposed. Passing by the often-repeated assertion that the facts of comparative embryologic development are reconcilable only with the belief in evolution, let us advert to some of those facts. "The substitutions," says Mr. Spencer, "of organs and the suppression of organs, are among those secondary embryological phenomena which harmonize with the belief in evolution, but can not be reconciled with any other belief. There are cases where, during its earlier stages of development, an embryo possesses organs that afterward dwindle away, as there arise other organs to discharge the same functions. And there are cases where organs make their appearance, grow to certain points, have no functions to discharge, and disappear by absorption." The concrete illustration of this substitution and suppression of organs is thus given by Mr. Spencer:

"We have a remarkable instance of this substitution in the successive temporary appliances for aërating the blood which the mammalian embryo exhibits. During the first phase of its development, the mammalian embryo circulates its blood through a system of vessels distributed over what is called the _area vasculosa_, a system of vessels homologous with one which, among fishes, serves for aërating the blood until the permanent respiratory organs come into play. After a time, there buds out from the mammalian embryo a vascular membrane called the allantois, homologous with one which, in birds and reptiles, replaces the first as a breathing apparatus. But while, in the higher oviparous vertebrates, the allantois serves the purpose of a lung during the rest of embryonic life, it does not do so in the mammalian embryo. In implacental mammals it aborts, having no function to discharge; and in the higher mammals it becomes "placentiferous, and serves as the means of intercommunication between the parent and the offspring"--becomes an organ of nutrition more than of respiration. Now, since the first system of external blood-vessels, not being in contact with a directly oxygenated medium, can not be very serviceable to the mammalian embryo as a lung; and since the second system of external blood-vessels is, to the implacental embryo, of no greater avail than the first; and since the communication between the embryo and the placenta among placental mammals might as well or better have been made directly, instead of by metamorphosis of the allantois--these substitutions appear unaccountable as results of design. But they are quite congruous with the supposition that the mammalian type arose out of lower vertebrate types. For, in such case, the mammalian embryo, passing through states representing, more or less distinctly, those which its remote ancestors had, in common with the lower _vertebrata_, develops these subsidiary organs in like ways with the lower vertebrata."[87]

In what way, then, are these substitutions unaccountable as results of design, and why are they any more congruous with the supposition that the mammalian type arose out of the lower vertebrate type? In the first place, it is necessary to have a distinct conception of what is meant by design. In the present case, it means that for a certain large group of animals there was established a system of reproduction by the sexual union of male and female, each contributing a cellular substance peculiar to itself, in the formation of a compound cellular substance in which the separate substances are united, and which is to be developed into the fœtus by a law of growth; and as a further design there is wrapped up in the compound germ of each distinct species of animal a typical plan of ultimate form and structure. This typical plan can not be detected in the germ itself, as it is too subtile and obscure even for the microscope; but we have every reason to believe that it is there in all its distinctness of original purpose, because at a later stage of the embryonic development we find a distinct species of animal is the result. This is a conclusion that must be adopted by the evolutionist, as well as by the believer in special creations, because it has nothing to do with the question of how distinct species came to exist. Whether they were designedly and separately created, or were evolved out of one another, the reproductive process by which the individuals of the same species are brought into being alike involves the conclusion that, in the ante-fœtal germ of that species, there is somehow involved, in a form so minute that it can not be seen, the type of animal which is to belong to that species, and to no other. Here, then, we have the grand and compound design which is to obtain throughout a whole group of different animals; namely, that they shall multiply in the production of individuals of their own types, by a sexual union, in which the male and the female each contributes a cellular substance of its own to the formation of a compound germ, and in that germ there is made to reside the typical form and structure of a distinct organism, so minute that we can not see it, but which we must conclude from the result has been put there to be developed by a law of growth ordained for the accomplishment of a certain distinct order of beings. But the very obscurity of this type, in the earliest stage of embryonic development, leads to the conclusion that while it will never be lost, so long as its life is preserved, it will unfold itself in ways that will be equally beyond our ken, until the point is reached where it is no longer obscured, but where it is revealed in all its distinctness of outline and its peculiarity of structure. What is certain and invariable is, that the type peculiar to the species is at some time in the growth of the individual animal perfectly developed. But in the modes of its development through different embryonic stages, there will be variations and substitutions of organs in the different species, but in each distinct species these variations and substitutions will be uniformly the same, because the law of development imposed by the distinct type, while it may operate differently among different species, will always operate in the same way in the same species. Thus in one animal the development from the original type which was implanted in its seminal ante-fœtal germ may at one stage exhibit an organ for which at a later stage another organ will be substituted; and in another animal a seemingly corresponding organ may serve a different purpose, or may altogether abort. These embryologic phenomena, varying in different species, but occurring uniformly in the same species, are necessarily among the most obscure of all the phenomena of animal life, on account of the fact that they take place where we can not watch the changes or modifications as they are taking place during actual fœtal life. But they are no more explicable upon the hypothesis of the descent of distinct animals from a common stock, than they are upon the hypothesis of distinct creations of species. Upon the former hypothesis, the assumed propinquity of descent implies the preservation of the same mode of embryonic development until it becomes varied by the operation of causes that bring about a new habit of development, and then a fixation in this new habit after a new species or a new ancestral stock is formed; so that in each distinct species there comes at length to be a uniform process of substituting and suppressing organs, or changing the functions of organs. But how are we to account for the operation of causes that have preserved a parallelism of development, along with the operation of causes that have produced the different modes of development, when all the species are supposed to be derived from a common ancestral stock, which first began to procreate and to develop its descendants in one and the same way? What are the facts which will enable us to say that the mammalian type arose out of the lower vertebrate types, when we compare the different modes of their embryologic development? How are we to estimate the chances for a preservation of so much resemblance as exists between the two in their embryologic lives, and the chances for the variations that are observable? What we can safely conclude is that there is a law which holds each species in a constant repetition of its own fœtal growth, according to its unvarying development in the same series of changes, substitutions, or suppressions. But we can not safely conclude that this species became formed in the supposed process of descent from a remote ancestral stock, which may or may not have originally exhibited the same series of changes, substitutions, or suppressions. If the ancestors of the mammalian vertebrates were the kind of animal supposed, we have to find, in order to justify the supposed descent, those states which represent the correspondence between the mode in which the ancestral stock developed its own embryos, when compared with the mode in which the type of the lower vertebrata developed its embryos, so as to make it reasonably certain that these subsidiary organs derived their several substitutions or suppressions from the process of descent, and not from any special mode of development ordained for each distinct species. We may imagine these states through which the mammalian embryo has passed, but as yet we have only a theory which suggests their existence without facts to support it. The truth would seem to be that this whole subject of comparative embryology, upon the hypothesis of the kinship of all organized beings, or the descent of many distinct species from a common stock, is involved in very great difficulties; not the least of which is the difficulty of explaining how the diverging descendants from that stock came to be endowed with habits of embryologic life and growth that resulted in the production of very different modes of development, and at the same time preserved for each new species its own peculiar mode of development. To say, for example, that the mammalian embryo passed through states representing, more or less distinctly, those which its remote ancestors had in common with the lower vertebrata, and that it developed certain subsidiary organs in like ways with the lower vertebrata, is merely to state a theory, which, without some evidence that the mammalian embryo was a formation resulting from a connection of lives with lives back to a common ancestor whose embryo was developed as those of the lower vertebrata are, amounts to nothing. Often as this want of evidence has been adverted to, it must be here again pointed out: for the whole argument from embryology, like that derived from a comparison of the forms of mature animals, lacks the support of facts that are essential to show the connection of life with life which descent from a common ancestral stock necessarily implies.

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.