The theologians have come to their last ditch in contesting that the mental differences between man and the lower animals are fundamental, being differences of kind, and therefore that no gradual process from the mental faculties of the one to those of the other has taken place. This struggle against the application of the theory of Evolution to man’s intellectual and spiritual nature will be long and stubborn. It is a matter of life and death to the theologian to show that he has in revelation, and in the world-wide belief of mankind in spiritual existences without, and in a spirit or soul within, evidence of the supernatural. The evolutionist has no such corresponding deep concern. When the argument against him is adduced from the Bible, he can only challenge the ground on which that book is cited as divine authority, or as an authority at all. Granting, for the sake of argument, that a revelation has been made, the writings purporting to contain it must comply with the twofold condition attaching to it, namely, that it makes known matters which the human mind could not, unaided, have found out; and that it embodies those matters in language as to the meaning of which there can be no doubt whatever. If there be any sacred books which comply with these conditions, they have yet to be discovered.
When the argument against the evolutionist is drawn from human testimony, he does not dispute the existence of the belief in a soul and in all the accompanying apparatus of the supernatural; but he calls in the anthropologist to explain how these arose in the barbaric mind.
Meanwhile, let us summarize the evidence which points to the psychical unity between man and the lower life-forms. As stated on p. [187], Mr. Herbert Spencer traces the gradual evolution of consciousness from “the blurred, indeterminate feeling which responds to a single nerve pulsation or shock.” There is no trace of a nervous system in the simplest organisms, but this counts for little, because there are also no traces of a mouth, or a stomach, or limbs. In these seemingly structureless creatures every part does everything. The amœba eats and drinks, digests and excretes, manifests “irritability,” that is, responds to the various stimuli of its surroundings, and multiplies, without possessing special organs for these various functions. Division of labour arises at a slightly higher stage, when rudimentary organs appear; the development of function and organ going on simultaneously.
Speaking broadly, the functions of living things are threefold: they feed; they reproduce; they respond to their “environment,” and it is this last-named function—communication with surroundings—which is the special work of the nervous system. It was an old Greek maxim that “a man may once say a thing as he would have said it: he cannot say it twice.” This is the warrant for transferring a few sentences on the origin of the nerves from my Story of Creation. They are but a meagre abstract of Mr. Spencer’s long, but luminous exposition of the subject.
“As every part of an organism is made up of cells, and as the functions govern the form of the cells, the origin of nerves must be due to a modification in cell shape and arrangement, whereby certain tracts or fibres of communication between the body and its surroundings are established.
“But what excited that modification? The all-surrounding medium, without which no life had been, which determined its limits, and touches it at every point with its throbs and vibrations. In the beginnings of a primitive layer or skin manifested by creatures a stage above the lowest, unlikenesses would arise, and certain parts, by reason of their finer structure, would be the more readily stimulated by, and the more quickly responsive to, the ceaseless action of the surroundings, the result being that an extra sensitiveness along the lines of least resistance would be set up in those more delicate parts. These, developing, like all things else, by use, would become more and more the selected paths of the impulses, leading, as the molecular waves thrilled them, to structural changes or modification into nerve-cells, and nerve-fibres, of increasing complexity as we ascend the scale of life. The entire nervous system, with its connections; the brain and all the subtle mechanism with which it controls the body; the organs of the senses alike begin as sacs formed by infoldings of the primitive outer skin.”
Biologists are agreed that a certain stage in the organization of the nervous system—the germs of which, we saw, are visible in the quivering of an amœba, and probably in plants as well as animals—must be reached before consciousness is manifest. Obscurity still hangs round the stage at which mere irritability passes into sensibility, but so long as the continuity of development is clear, the gradations are of lesser importance. And, for the present purpose, there is no need to descend far in the life-scale; if the psychical connection between man and the mammals immediately beneath him is proven, the connection of the mammals with the lowest invertebrate may be assumed as also established. Speaking only of vertebrates, the brain being, whether in fish or man, the organ of mental phenomena, how far does its structure support or destroy the theory of mental continuity? In Man’s Place in Nature, and its invaluable supplement, the second part of the monograph on Hume, this subject is expounded by Huxley with his usual clearness. In the older book he traces the gradual modification of brain in the series of backboned animals. He points out that the brain of a fish is very small compared with the spinal cord into which it is continued, that in reptiles the mass of brain, relatively to the spinal cord, is larger, and still larger in birds, until among the lowest mammals, as the opossums and kangaroos, the brain is so increased in proportion as to be extremely different from that of fish, bird, or reptile. Between these marsupials and the highest or placental mammals, there occurs “the greatest leap anywhere made by Nature in her brain work.” Then follows this important statement in favour of continuity.
“As if to demonstrate, by a striking example, the impossibility of erecting any cerebral barrier between man and the apes, Nature has provided us, in the latter animals, with an almost complete series of gradations from brains little higher than that of a Rodent to brains little lower than that of Man.” After giving technical descriptions in proof of this, and laying special stress on the presence of the structure known as the “hippocampus minor” in the brain of man as well as of the ape—in the denial of which Owen cut such a sorry figure, Huxley adds:
“So far as cerebral structure goes, therefore, it is clear that Man differs less from the Chimpanzee or the Orang than these do even from the Monkeys, and that the difference between the brains of the Chimpanzee and of Man is almost insignificant when compared with that between the Chimpanzee brain and that of a Lemur.... Thus, whatever system of organs be studied, the comparison of their modifications in the ape series leads to one and the same result,—that the structural differences which separate Man from the Gorilla and the Chimpanzee are not so great as those which separate the Gorilla from the lower apes. But in enunciating this important truth I must guard myself against a form of misunderstanding which is very prevalent ... that the structural differences between man and even the highest apes are small and insignificant. Let me then distinctly assert, on the contrary, that they are great and significant; that every bone of a Gorilla bears marks by which it might be distinguished from the corresponding bone of a Man; and that, in the present creation, at any rate, no intermediate link bridges over the gap between Homo and Troglodytes. It would be no less wrong than absurd to deny the existence of this chasm; but it is at least equally wrong and absurd to exaggerate its magnitude, and, resting on the admitted fact of its existence, to refuse to inquire whether it is wide or narrow. Remember, if you will, that there is no existing link between Man and the Gorilla, but do not forget that there is a no less sharp line of demarcation, a no less complete absence of any traditional form, between the Gorilla and the Orang, or the Orang and the Gibbon.”
The brains of man and ape being fundamentally the same in structure, it follows that the functions which they perform are fundamentally the same. The large array of facts mustered by a series of careful observers prove how futile is the argument which, in his pride of birth, man advances against psychical continuity. Vain is the search after boundary lines between reflex action and instinct, and between instinct and reason. Barriers there are between man and brute, for articulate speech and the consequent power to transmit experiences has set up these, and they remain impassable. “The potentialities of language, as the vocal symbol of thought, lay in the faculty of modulating and articulating the voice. The potentialities of writing, as the visual symbol of thought, lay in the hand that could draw, and in the mimetic tendency which we know was gratified by drawing as far back as the days of Quaternary man” (Huxley’s Essays on Controverted Questions, p. 47). But these specially human characteristics are no sufficing warrant for denying that the sensations, emotions, thoughts, and volitions of man vary in kind from those of the lower creation. “The essential resemblances in all points of structure and function, so far as they can be studied, between the nervous system of man and that of the dog, leave no reasonable doubt that the processes which go on in the one are just like those which take place in the other. In the dog, there can be no doubt that the nervous matter which lies between the retina and the muscles undergoes a series of changes, precisely analogous to those which, in the man, give rise to sensation, a train of thought, and volition.” This passage occurs in Huxley’s Reply to Mr. Darwin’s Critics, which appeared in the Contemporary Review, 1871, and it may be supplemented by a quotation from the chapter on The Mental Phenomena of Animals in his Hume. “It seems hard to assign any good reason for denying to the higher animals any mental state or process in which the employment of the vocal or visual symbols of which language is composed is not involved; and comparative psychology confirms the position in relation to the rest of the animal world assigned to man by comparative anatomy. As comparative anatomy is easily able to show that, physically, man is but the last term of a long series of forms, which lead, by slow gradations, from the highest mammal to the almost formless speck of living protoplasm, which lies on the shadowy boundary between animal and vegetable life; so, comparative psychology, though but a young science, and far short of her elder sister’s growth, points to the same conclusion.”