CHAPTER I

THE REALITY OF EVOLUTION

The widening of the horizon of biological knowledge within recent years has been remarkable and has cast a flood of light on the question of ‘Man’s Place in Nature.’ At the present day well-informed persons have abandoned the idea of a ‘miraculous’ or special creation of man, yet it is curious to note with what tenacity tradition adheres, and how speculative theories and poetical imaginations to a large extent still prejudice the mind to shut out pure reasoning and rational philosophical thinking. While it has become very general now-a-days to accept the idea of evolution as that method by which man came to inhabit this planet, one still asks do most of us thoroughly grasp the principles of the doctrine which we profess to accept? While many of us do, still it would appear that there are others who vaguely accept the doctrine because it is becoming more fashionable to do so every day. That is to say man, a highly gregarious animal, is carried away to follow the strong leaders of the flock. But to get a clear conception of the wonders of organic descent one must insist that it is not enough to listen only to lectures: we must be students of Nature, endowed with a wholesome amount of scepticism, and not content ourselves with accepting _en masse_ the evidence of others without verifying for ourselves as far as we can the revelations made in the study of the biological sciences.

I introduce these few sentences at the outset because I wish to point out what a very strong attitude of mind in favour of the

[Sidenote: _Evolution a reality_]

reality of human evolution is fostered by those of us who have had the opportunities of making a special study of biology, more

## particularly when this includes a detailed knowledge of human anatomy

and embryology. But it might be asked: Why then do our medical brethren who study anatomy in detail not act more fervently as propagandists of the theme of evolution? As a matter of fact, I have seldom met with a medical student who at the end of his anatomical course has not, at least in an unprejudiced and general way, accepted evolutionary principles; but, even if such fail to occupy a foremost place in his mind, it is not surprising considering the strongly utilitarian view he takes of the study in question. The medical student, and he who teaches the subject of anatomy from this utilitarian standpoint alone, obtain but a limited view of the great principles of human structure. This is brought home to us at once by taking one or two familiar examples. The medical student seldom stops to consider the significance of the presence of the mammary gland in the male. To him its presence is probably a matter of little import. But in the

[Sidenote: _Evidence of Evolution from vestigial Sex-organs_]

mind of the anatomist the question at once arises: Why is the organ there at all, if it be useless? And he finds by further examination in an early stage of the intra-uterine development of the individual that the gland is already present when the external sex-organs are indistinguishable, and when it would be impossible to say which sex the individual would ultimately assume. The logical conclusion arrived at, then, is that this gland is only suppressed, in one sex, so that the male has incorporated in its body structural features, more fully developed and functional in the female, a distinction merely of degree but not of kind. _Vice versa_, an examination of the female generative organs reveals to us the fact that the male homologues have not vanished, but are merely suppressed. This short chain of reasoning from objective biological evidence regarding the presence of structures which can be examined without even resorting to dissection has a most important bearing on the whole question of the evolution of sex from an ancestral hermaphrodite stock. And, indeed, we find on comparing our human embryo in certain very early stages with some lower forms of life which are hermaphrodite, that a marked similarity can be made out. The evolutionary history of the mammary glands is highly interesting, and deserves short notice as shedding light on the reality of evolution. Sir Arthur Keith in his delightful work _Human Embryology and Morphology_ (4th ed., 1921) says that “it is a remarkable fact that although the milk glands do not come into use until adult life, and although they must be regarded as among the later evolved structures of vertebrate animals, yet they are _the first of all the glands arising from the epidermis to appear during development of the embryo_. In the human embryo of the 6th week, or in the corresponding stage of a pig or of any other mammal, the primary mammary ridge or milk line――a mere surface thickening of the ectoderm――is seen extending along the body wall on either side from axilla to groin. Breslau[1] regards these primary ridges as representatives of the brooding organs of the ancestors of mammals, from which structures he supposed that the mammary glands were evolved. In a large number of human beings (15%) one or more supernumerary nipples are to be found between the axilla and the groin, indicating the wide distribution of ancestral glands. The mammary ridge appears in both sexes alike, but this may not mean that both sexes of ancestral mammals were concerned in brooding or gave milk. The male is the father of girls as well as of boys; it is therefore necessary to provide both father and mother with a complete sexual outfit if each sex is to provide equal shares to the making of their progeny. In females the breasts undergo a great development at puberty while in males they retain their infantile form.” Many other instances could easily be cited of the presence of structures which afford us incontestable evidence of the evolution of the human body from ancestors not necessarily of human form. Suffice it to draw attention to certain muscles which in _Herbivores_ and other Orders are well developed and functional but are only vestigial in man, and to other muscles present in certain groups of lower animals,

[Sidenote: _Evidence of Evolution from atavistic myology_]

which, though long-since disappeared from the later human ancestry, nevertheless occasionally, by the strong strain of heredity, make their appearance again in human beings of the present day. The study of Human Embryology is most convincing, and carries with it incontrovertible proofs of evolution. We find that it is only at the later stages of development within the uterus that the human being is recognizable as such, when it is known as a foetus. In common with other higher animals, man _in utero_ repeats the stages of his ancestral-tree from the very lowest to the highest forms of animal life, due allowance

[Sidenote: _Evidences of Evolution from Human Ontogeny_]

being made for a blurred and transient picture owing to adaptative modifications which have arisen during countless ages and are purely secondary in character. Nor can it be argued that the process by which development proceeds is simply a mechanical one, built on a uniform plan or design of Nature. For, if it were, to take a simple example out of hundreds, one might ask why, just at the termination of foetal life, the digits of the limb are so specialized in different mammals? All arise alike; but compare the hoof of the horse, the flipper of the seal, the functionless and atrophied thumb of most quadrupeds, and so on, with the hand of man. There is no moulding within the uterus to produce these patterns mechanically. We are led to consider that, while we inherit through our non-human ancestors many features (more or less portrayed in our living non-human cousins), we also have had impressed upon us, demonstrable only at the termination of our embryological career, the features of our immediate predecessors, namely our own parents, and these features hall-mark us as the individual proper to which we belong, that is to say into which we have evolved. Even these few instances which I have cited regarding the study of human structure will, I think, suffice to remind us how intimately bound up become the thoughts of the anatomist with the evolution of his own body. Material for investigation is before him daily, and he cannot――even though he wished it――get away from the fact which may be expressed in Darwin’s words: “Man still bears in his bodily frame the indelible stamp of his lowly origin.” But here no thoughtful anatomist can stop.

[Sidenote: _Mental Evolution_]

With the material for the study of the development of the Brain in front of him, from the extremely simple membranous tubular condition of that organ to the adult form, when the scheme of its complexity seems an almost hopeless task to unravel; with the application of his knowledge of function supplementary to his knowledge of structure, he is carried onward ever more and more to consider as far as he can push his biological data, the physical basis of mental manifestations which go to form the phenomena grouped under habits, out of which the conduct or ethical aspect of the individual, relative to his fellow-creature, springs. As the processes of mental development are very imperfectly

[Sidenote: _Outline of the Evolution of the Human Brain_]

realized, I may here indicate very briefly the outlines along which the Brain develops, pointing out at the same time its correlation during phases of its development with the permanent, that is, the adult condition of the brains in several other animals. The expression ‘thin-skinned,’ often applied to persons who might be judged as mentally over sensitive, is not inappropriate when we bear in mind that

[Sidenote: _Brain and Skin_]

the Brain and Spinal cord, in fact the whole nervous system, originates from the skin-layer of the embryo; and, indeed, in the lower forms of Invertebrate animals the beginning of a nervous system is diffused over the skin-layer, in which are found indications of sensation. In such forms, for instance as the jelly-fishes, the brain-skin layer does not differentiate or split off into its two component parts; but in higher forms we find development proceeding in this wise; an elongated groove appears on the surface of a circumscribed area of an oval-shaped vesicle. The area is known as the embryonic shield, because it is on it that the embryo is afterwards laid down. But when the groove first appears there is, so to speak, but little else of an embryo, except that part which is now differentiating itself into the form of this groove. In other words, a very early indication of the appearance of the embryo is represented by its groove-shaped nervous system. But to continue. The surface-groove is soon converted into a simple straight tube, which, seeking a deeper situation, becomes surrounded by other tissue and cut off from the general surface-layer. Its wall then is extremely thin, comparable to a very fine membranous film, and the cellular elements of which it is composed are comparatively simple in shape, such as are found in many other parts of the permanent body.

[Sidenote: _Vesicular stage of the Human Brain_]

Very rapidly, however, the front portion of the tube dilates into three bulbs which are separated only by surface constrictions, so that their spacious cavities are continuous. These bulbs or vesicles are, in fact, the whole of the primitive Human Brain, out of which all other subdivisions of the organ are derived. Microscopical examination reveals to us here, and also in the lower portion of the tube (the latter forming the spinal cord), very thin membranous walls. However, with high magnifications of the microscope, the cellular elements are seen to be evolving speedily from simple to more complicated shapes. They give out branching processes which minutely interlace with those of neighbouring cells. These cells become very complicated in the ultimate analysis of their minute protoplasmic structure before the wall of the brain undergoes much thickening. They serve the purposes of allowing stimuli to pass from one cell to another, which, shooting along the innumerable branchings, can set up changes in the cellular elements, sometimes over a considerable area of the Brain. However, as long as the wall remains thin the cell-machinery remains, comparatively speaking, very limited in its action. In the lower forms of fishes, whose brains developmentally correspond more or less with the conditions of the early Human Brain, the higher mental manifestations, such as _memory_, _thought_, and so on, are feebly, if at all, capable of being called forth. If we now examine the fore-brain of a Human Foetus somewhat advanced, say at the stage when the organ is structurally comparable to the brain of an adult rabbit, we find that the walls have greatly thickened, giving the organ the appearance of being solid with a small hollow core. A very thin section of this wall shows vast numbers of complicated branching cells――what myriads, therefore, can the entire thickness of the wall accommodate! A step further and we behold in the Brain of the new-born babe a highly elaborate organ with immensely thickened walls stocked with cells which form the psychic machinery, and too intricate in their structure to call for special description here. And while now, from the structural point of view, we may regard the Human Brain as almost completed in its marvellous complexity, we are nevertheless struck with the great hiatus existing between the mental powers in parent and babe. It is true that many faculties of the Brain (which we would have as abstract in nature) manifest themselves at an extraordinarily early period, and that they seem to be the results of past experiences of the Human

[Sidenote: _The Brain of the babe and the Evolution of Mental Faculties_]

Race, which, having accrued, have been passed on by heredity to the offspring; yet others, and even the same faculties under different conditions, are put into action by experiences founded mainly on the child’s own observations and experiments. Regarding experiences inherited, Herbert Spencer points out that “an infant in arms, when old enough to gaze at objects around with some vague recognition, smiles in response to the laughing face and soft caressing voice of its mother. Let there come someone who, with an angry face, speaks to it in harsh tones. The smile disappears, the features contract into an expression of pain, and, beginning to cry, it turns away its head, and makes such movements of escape as are possible. What is the meaning of these facts? Why does not the frown make it smile, and the mother’s laugh make it weep? There is but one answer. Already in its developing brain there is coming into play the structure through which one cluster of visual and auditory impressions excites pleasurable feelings, and the structure through which another cluster of visual and auditory impressions excites painful feelings. The infant knows no more about the relation existing between a ferocious expression of face, and the evils which may follow perception of it, than the young bird just out of its nest knows of the possible pain and death which may be inflicted by a man coming towards it; and as certainly, in the one case as in the other, the alarm felt is due to a partially established nervous

[Sidenote: _Inherited Experiences of mental manifestations_]

structure. Why does this partially established nervous structure betray its presence thus early in the human being? Simply because in the past experiences of the human race smiles and gentle tones in those around have been the habitual accompaniments of pleasurable feelings; while pains of many kinds, immediate and more or less remote, have been continually associated with the impressions received from knit brows, and set teeth, and grating voice. Much deeper down than the history of the human race must we go to find the beginnings of these connections. The appearances and sounds which excite in the infant a vague dread indicate danger; and do so because they are the physiological accompaniments of destructive action, some of them common to man and inferior mammals, and consequently understood by inferior mammals as every puppy shows us. What we call the natural language of anger is due to a partial contraction of those muscles which actual combat would call into play; and all marks of irritation down to that passing shade over the brow which accompanies slight annoyance are incipient stages of these same contractions. Conversely with the natural language of pleasure, and of that state of mind which we call

[Sidenote: _Physiological interpretation of Anger and Pleasure_]

amicable feeling this too, has a physiological interpretation.” Let us now examine the same faculties, viz. _sorrow_ and _joy_ under different conditions, and see how the Brain machinery is called forth into

## action. The child trips over the door-mat and falls in its eagerness

to reach the sweetmeat held up in the parent’s hand at the other end of the room. The fall occasions pain, but only in a slight degree, not sufficient to warrant the burst of screams and sobs which follow. The experiment is repeated, and the child comes down again, this time more easily still, but the cries become worse and more prolonged. And, if the experiment is again repeated and the child falls, its sorrow instead of abating seems to increase. Why is this? It seems contrary to the more familiar cases of children who, after several upsets of an easy kind, i.e. involving little or no pain, become used to the mishap and get up smiling. But the particular child of whom we speak has made an important observation as it treads its ways hastily across the floor, and as it falls it continues in piteous sobs, to

[Sidenote: _The unfolding of the mental Faculties of Sorrow and Joy_]

observe――what? _the sweetmeat_. And it is in the great disappointment involved in the loss of time in securing the coveted tit-bit, coupled with sensation of pain, here only slightly felt but no doubt involving an unpleasant inherited sensation, that such an outburst of the mental manifestation――_Sorrow_――is now unfolded. In a short time the child tries the experiment of raising his feet higher in passing over the door-mat, and now, finding that in so doing he no longer comes tumbling down and consequently can scamper across the room without interruption to obtain the sweet, the mental manifestation of――_Joy_ is more and more unfolded and the outbursts of laughter, as the experiment is repeated, become more marked. And further, regarding this part of the subject, it may be said that while there is reason to believe that the basis of Memory is to a large extent the outcome of inherited experiences, still it undergoes rapid expansion as the child proceeds to build up its own vocabulary by associating sounds with ideas, and by showing a most earnest desire to reproduce those sounds as seen in the impatient and imperfect way in which they are blurted out, the parent often being at a loss to know what they mean.

[1] _The Mammary Apparatus of the Mammalia_, with Introduction by Prof. J. P. Hill, London, 1920.

I need dwell no further on the support of the truths of Evolution: it is clear that physically and mentally we undergo a gradual process of development from the simple to the complex organism. The evidences to be derived from the living forms of animal life around us need not here detain us. Let us just bear in mind that none of those now living

[Sidenote: _Phylogeny or Stem-Evolution_]

could closely represent in form our ancestors, as it is sometimes stated. Their kinship could only be that of a cousin: the ape a closer cousin than the cat; the cat a closer cousin than the jelly-fish. These creatures are in themselves modified from the common ancestral stocks (vast numbers of which have long since become extinct), from

[Sidenote: _Cousinship with all living beings_]

which their cousinship has diverged. A study of ancestral stocks would take us too far a-field in this treatise, so we must be content to accept the statement that pre-natal evolution or the evolution of one’s own being, and stem-evolution or evolution of the race are closely intertwined. But since I have asked you to give your support to Organic evolution, largely on the evidences derived from a study of pre-natal development, one question will probably suggest itself, namely, what is the nature of this extraordinary persistent force of heredity which acts on the egg of a Human Being, which Human Being has for thousands of years lost to a great extent his resemblance to unhuman-like ancestors. The early stages of pre-natal development, were these mechanical in nature, would be more easily understood, because the embryos of many animals are then almost indistinguishable, and might, so to speak, be cast in the same form of mould. But, with regard to the later stages, where the mechanical notion is quite impossible to entertain, we ask how does heredity act in evolving a generalized fore-limbed-embryo into the special form of its parent? It is true that aberrant types do arise, but these are so exceedingly rare[2] that their occurrence does not seem to affect the question. We ask if an embryo, say of a dog, is during its stages of development recapitulating its genealogical tree, why is it not sometimes born unlike a dog, and like some more or less remote vertebrate ancestor? For, after all, when due reflection is made with regard to the wonderful transformations in later embryonic existence which go on, it is remarkable with what surety the offspring reaches the goal and structurally is born an exact miniature of its parents. This marvellous hereditary conservation which permits of _like begetting_

[Sidenote: _The Force of Heredity in Ontogeny_]

_like_ seems to depend upon _long-associated habits of the cellular elements of the embryo itself_. This is made more clear when we remember that, as Sir Francis Darwin[3] has put it, the characteristic of habit is, _par excellence_, a capacity acquired by repetition of reacting to a fraction of the original environment. Thus, when a series of actions are compelled to follow each other by applying a series of stimuli, the actions become organically tied together, or associated, and follow each other automatically even when the whole series of stimuli are not acting. And further light is thrown on the subject when we take into consideration the fact that stimuli (here represented by a series of stages of cell-division and growth, each stage apparently serving as a stimulus to the next) are not momentary in effect, but leave a trace of themselves on the organism constituting thereby the physical basis of the phenomena grouped under _memory_ in

[Sidenote: _Physical basis of Memory_]

its widest sense. Indeed, there is reason to believe that _memory_ has its place in the morphological or structural as well as in the temporary reactions of living things. And finally, with regard to the memory-faculty in connection with the development of the Human Embryo from its initial stage as a simple egg into the perfect organism, in referring to the wonderful series of ancestor-like changes which take place and which resemble those that arose in the long process of stem-evolution, here Sir Francis Darwin draws a striking analogy in saying: “This is precisely paralleled by our own experience of memory, for it often happens that we cannot reproduce the last-learned verse of the poem without repeating the earlier part: each verse is suggested by the previous one and acts as a stimulus for the next. The blurred and imperfect character of the ontogenetic version of the phylogenetic series may at least remind us of the tendency to abbreviate by omission what we have learnt by heart.” It is a matter of profound interest to

[Sidenote: _The Existence of Memory in plants_]

know that the basis of _memory_ by association exists in very low forms of animal as well as in plant organisms. In the latter this factor has been illustrated by the power of movement, which power, though

## acting to stimuli, can be seen to take place in the absence of such.

That a simple form of associated action implies consciousness, as we understand that phenomenon, is a point I am unable to enter upon; and yet it is impossible to know whether or not plants or the simplest

[Sidenote: _Psychic element pervades organic Nature_]

forms of animal-life are conscious; “but it is consistent with the doctrine of continuity that in all living things there is something psychic, and, if we accept this point of view, we must believe that in plants there exists a faint copy of what we know as consciousness in ourselves” (Sir Francis Darwin).

[2] Such must be distinguished from the _monsters_ of medical science, which include many forms of arrest of development, and plural fusions. One genuine aberrant form of kitten has come under my notice, in which the face was long and pointed and the eyes open at birth.

[3] _Presidential Address._ Brit. Assoc. Dublin, 1908.

##