Passing from the supposed common progenitor to the resemblances between the brain of the higher apes and the brain of man, we come to the question whether these resemblances show that man was descended from any of the Simian stock through intermediate animals by gradual transformation. Here the case is in one respect different; for the animals that are to be compared are known, and their respective brains have been subjected to close anatomical scrutiny. This part of the process of evolution begins from one true species, the ape, and ends in another true species, the man. We are unable to trace the man and the ape to a common progenitor race; but we find the ape possessed of a brain which strongly resembles man's. I have searched diligently in the writings of naturalists for a sound reason which ought rationally to exclude the hypothesis that the brain of the ape was formed upon the same ideal plan as the brain of man, each animal being a distinct species and separately created. Anatomical comparison of the two brains shows that, whether they were separately planned upon the same general model, or the one was derived from the other by a process of gradual transformation through successive intermediate animals, the resemblances are consistent with either hypothesis. We are remitted, therefore, to an inquiry for the evidence which will establish the existence of a race or races of animals through whom there descended to man the peculiar structure of brain found in one of the classes of apes—namely, the Catarrhine or Old World monkeys. If such intermediate races could be found, their existence at any period anterior to the period of man's appearance on earth would have some tendency to show that man was descended from one of the families of apes, and this tendency would become stronger in proportion to the number of successive links in the family chain that could be made out. But not one of these links is known to have existed. There is an assumption that man, "from a genealogical point of view, belongs to the Catarrhine or Old World stock" of monkeys; and this assumption is claimed to be supported by the fact that the character of his brain is fundamentally the same as theirs.
A brain is an organ which, upon the hypothesis of an independent creation of distinct species of animals, would be expected to be found in very numerous species, although they might differ widely from each other. In all the vertebrate animals this organ is the one from which, by its connection with the spinal chord, the central portion of the nervous system, that system descends through the arches of the vertebræ, and thence radiates to the various other organs of the body. The brain is the central seat of sensation, to which are transmitted, along certain nerves, the impressions produced upon or arising in the other organs; and it is the source from which voluntary activity is transmitted along other nerves to organs and muscles that are subjected to a power of movement from within. The office which such an organ performs in a complex piece of animal mechanism is therefore the same in all the vertebrate animals in which it is found; and it would necessarily be found to be constructed upon the same uniform plan, and with just the degree of uniformity and adaptation which would fit it to perform its office in the particular species of animal to which it might be given. In point of fact, we find this office of the brain performed in all the vertebrate animals upon the same uniform plan, with the necessary adaptations to the various structures of the different animals. Resemblances, therefore, in the convolutions of different parts of this organ, as found in different vertebrate animals, however close they may be, prove nothing more than the adoption of a general plan for the production of objects common to the whole class of the vertebrate animals; and unless we can find other and independent proof that one species was descended from another by connection of lives with lives through successive generations, the hypothesis of special creations of the different species is not excluded by the facts.
Let us now further examine the supposed kinship of man with the monkey, as evidenced by the similarity of the structure of the brains of the two animals, in reference to the supposed process of evolution as the means of accounting for the origin of two species so essentially distinct. How has it happened that different species have become completed and final types, transmitting, after they have become completed, one and the same type, by the ordinary process of generation, and not admitting of the sexual union with any other distinct species? On the theory of the evolution of animal out of animal, we must suppose that at some time the secondary causes of natural and sexual selection have done their work. It ends in the production of a species which thereafter remains one and the same animal, and Nature has established a barrier to any sexual union with any other species. If we give the rein to our imaginations, and, taking the process of evolution as it is described to us, suppose that in the long course of countless ages the struggle for existence among very numerous individuals has led to gradual transformations of structure which the sexual selection has transmitted to offspring, and so a new animal has at length been formed through the successive "survivals of the fittest," we reach an animal of a new species, and that species, under no circumstances, produces any type but its own, so far as we have any means of knowledge. All the knowledge respecting the ape that has been accumulated shows only that this species of animal, since it became a completed type, has procreated its own type and no other. Whatever struggle for existence the individuals of this type have had to undergo, whatever modifications of structure or habits of life the survival of the fittest individuals of this type may have produced from the earliest imaginable period until the present time, the fact remains that this species of animal is a completed and final product. At the same time we have another completed and final type of animal known as man, which, so long as he has been known at all, is a distinct and peculiar species. Between the brain of this animal and the brain of the other we find certain strong resemblances. In each of them this organ is a structure performing the same office in the animal mechanism, with adaptations peculiar to the varying structure of each of them. In order to justify the conclusion that the one animal is a modified descendant from the other, so as to exclude the hypothesis that the resemblances of any one or of all of their respective organs was a result of the adoption of a general plan in special creations of distinct species, we ought to find some instance or instances in which the completed animal called the ape has been developed into an animal approaching more nearly to man than the man, as he is first known to us, approached to the first ape that is known to us. Without such intermediate connections, the analogy of the descent of individuals from other individuals of the same species will not hold. There is nothing left but resemblances of structure in one or more organs, which are just as consistent with the hypothesis of special creations as with that of evolution. Strong resemblances of structure and in the offices of different organs may be found between man and the horse, but upon no theory of evolution has it been suggested that man is descended from the horse, or from any other animal to which he bears more or less resemblance, excepting the monkey; and it is quite possible that naturalists have been led unconsciously to make this exception by external resemblances of the monkey and the man, by the imitative power of the inferior animal when it comes in contact with man, and by some of its habits when found in its wild and native haunts.
[CHAPTER VI.]
The doctrine of evolution, according to Herbert Spencer, further considered.
In the last two preceding chapters I have examined what Mr. Spencer regards as the direct supports of the doctrine of evolution. I have now to consider the different orders of facts which, as he claims, yield to it indirect support. These are the facts derived from classification, from embryology, from morphology, and from distribution. An explanation is here needful of the sense in which he uses these respective terms, before the reader, who is not accustomed to them, is called upon to understand and appreciate the argument:
1. By classification is meant an arrangement of organic beings in some systematic manner, according to attributes which they have in common, and which may form the principle of a division into different classes or families. Pointing out that in the early history of botanical and zoölogical science the tendency was to make classifications according to a single characteristic, Mr. Spencer reminds us that later naturalists, by attending to a greater number of characteristics, and finally to the greatest number that can be found to be common to various classes of vegetable and animal organisms, have constructed systems of classification which, in place of a linear or a serial order, have exhibited the alliances of different groups, then the sub-groups, and the sub-sub-groups, so that the divergences and redivergences become developed, while the resemblances which obtain are preserved throughout the whole class. But it is at once apparent that, although classification, on whatever principle it is conducted, may be valuable as a means of fixing in the mind the resemblances or differences of structure that obtain in the different orders of organized beings, as, for example, among the vertebrate or the invertebrate animals, the flowering or the flowerless plants, the seeds naked or the seeds inclosed in seed-vessels, yet that any other system of classification, based upon other resemblances or differences which actually present means of grouping or separating the different families of organized beings, is just as valuable an aid in the investigation of facts. How far any classification affords an argument, or the means of constructing an argument, which will yield a support to the doctrine of evolution superior to that which it yields to the doctrine of special creations, is of course a question.
2. Embryology: This is the term employed to express that branch of inquiry which is concerned in a comparison of the increase of different organisms through the stages of their embryonic life, and in noting at different stages of this growth the characters which they have in common with each other; the resemblances of structure which at corresponding phases of a later embryonic stage are displayed by a less extensive multitude of organisms; and so on step by step, until we find the class of resembling embryos becoming narrower and narrower, and then we finally end in the species of which a particular embryo is a member. This process of tracing and eliminating embryonic resemblances is said to have "a profound significance"; because, beginning with a great multitude of resemblances between the embryonic development of different organisms, it reveals the divergences which they take on, and through every successive step we find new divergences, by means of which "we may construct an embryological tree, expressing the developmental relations of the organisms, resembling the tree which symbolizes their classificatory relations." We thus arrive at "that subordination of classes, orders, genera and species, to which naturalists have been gradually led," and which is said to be "that subordination which results from the divergence and redivergence of embryos, as they all unfold."[79] On this mode of comparing the embryonic development of different organized beings Mr. Spencer builds a scientific parallelism, which indicates, as he claims, a "primordial kinship of all organisms," and a "progressive differentiation of them," which justifies a belief in an original stock from which they have all been derived. In what way this method of investigation destroys or tends to destroy the hypothesis of special creations, or how it affords an important support to the doctrine of evolution, will be considered hereafter.[80]