The nature of Life and its power of reproduction—The stuff of which Life is made—The Emotions—The simplest living things—Where is neither Birth nor Death yet the Population increases—The First Marriage—The beginning of sex—The two dominating instincts—The conditions of survival—The Oyster’s narrow world—“Fiddling work”—Amorousness—The superior Male—Where Death begins—“Germ-plasm” and what it means—Sex and “Secondary sexual Characters”—Some theories—“Hormones” what are they?

The nature of life is generally regarded as affording a theme which possesses no more than an academic interest: but there is one aspect of this great subject which must attract us all, and that is its power of reproducing itself. Life begets Life, as Love is said to beget Love. The nature of this mysterious power we can only dimly realize, and the forces which underlie its manifestations few even suspect, save perhaps in a vague way. Yet the tree of Knowledge bears no fruit more vitally important to our well-being, than that which will make us “as Gods, knowing good and evil” in all that concerns the processes of reproduction. But curiously enough, this is a forbidden fruit, and those who eat thereof are expected to maintain a discreet silence on the subject. These enlightened ones, however, cannot remain altogether dumb. But they speak, in the veiled language, of Art and Poetry, Literature and the Drama. They talk round the subject rather than of it. Love, Hate, Jealousy, and Envy, are but attributes thereof. We profess to believe that “Knowledge is Power” and to desire to increase its force among us by raising the standard of our system of education. But education which does not, of set purpose, reveal the sources of our being and of our emotions, good and evil, is no more than a travesty of education; and they who seek to foist upon the community Knowledge thus emasculated, are unworthy to wield the power which has been placed in their hands. If social well-being be the aim of the high-priests of Education, then something more than copybook maxims like “Be good and you will be happy” must henceforth be preached. Of what avail is it to exalt the name of Knowledge, while the straightest road thereto is barred across and marked “No thoroughfare!” These blind leaders of the blind seem to imagine that the social well-being they profess to desire can only be attained by side roads, leading anywhere, save in the direction of this Pool of Siloam.

The stuff of which living things are made is called “Protoplasm.” Text-books of Physiology give its chemical constituents with fearsome accuracy, and each of these constituents can be isolated in the laboratory, but “all the king’s horses and all the king’s men” cannot build these up again into living matter. Its consistent inconsistency defies us; every statement we make of it has to be qualified by reservations and saving clauses. Its permanency is attested by the fact that it has endured through millions of years, yet we are daily reminded of its evanescent nature. Its power of reproducing itself according to type, none can doubt, yet no two individuals are exactly alike.

The purely physical phenomena of life, to be rightly appreciated, must always be considered in relation to the psychical phenomena which are the soul of life. These subtle and intangible forces cannot be experimented with in the laboratory, or expressed in formulÆ; we cannot denote their strength in horse-power. Just as the physical manifestations of life begin with lowly types, so the psychical begin, and they gather strength and complexity with the bodies they pervade. These manifestations we call behaviour, and in their more intense developments, “emotions.”

These emotions present an infinite range of variety in the higher animals, and they attain their maximum of intensity wherever the reproductive activities are concerned. The part which these activities play in controlling behaviour is by no means always apparent, and is commonly not even suspected. Even man himself is subject to this control. And it is this fact which lifts the “Courtship” of the lower animals out of the category of merely curious phenomena. For the springs of his conduct, his behaviour and “emotions” under varying circumstances, can only be understood, and even then but imperfectly, by comparison with other creatures lower in the scale, so far, of course, as comparison is possible.

This line of inquiry, then, takes one back to the simplest living things, among which there is neither marrying nor giving in marriage, neither birth nor death. Life is reduced to its simplest terms—a speck of animated jelly is all that confronts one, and this is only to be seen under a high power of the microscope. It has neither mouth nor organs of digestion, no visible means of locomotion are traceable, and the special senses of sight and hearing are wanting; but taste and smell, of a nebulous kind, are there. Shape it cannot be said to have, for its bodily outline is constantly changing, thereby it moves. A long tongue of its jelly-like substance, or “protoplasm” as it is called, is thrust forwards, and the rest of the body is, as it were, dragged after it. Whatever animal, or vegetable, matter it passes over, in the course of its wanderings, is drawn up into the semi-fluid substance of this diaphanous body, and its juices extracted, the undigestible residue is left behind in the course of the morning’s walk! In due time it becomes adult; further growth is impossible. When this stage is attained a strange thing happens. A certain minute, more solid portion of this body, which lies in the very centre of the mass and is known as the “nucleus,” begins to assume an hour-glass shape. Speedily the constriction becomes apparent across the whole body and rapidly increasing, cuts it in two, as if by the tightening of some invisible thread. Here Death is cheated, and records of births are unknown! And just as there are no parents so there are no children. But a foreshadowing of what is to be occurs even here. For every now and then two individuals, to all appearances identical, meet and promptly begin to merge the one into the other till they twain become one flesh in very truth. Here is the most primitive form of marriage in Nature. And here, in this union, or fusion, of separate entities of Germ-plasm, we have the beginning of sex. Such unions are common among these primeval forms of life. In many cases this “marriage” takes place between two particles of Protoplasm of which one is rather larger than the other. In such case the smaller is regarded as male, the larger as female. Here we have the first sign of “sexual differentiation” or the evolution of “male” and “female” individuals.

Some such union, some such process of “rejuvenation” by the importation of “fresh blood” seems to be imperative for the continuance of existence throughout the whole animal world, even though it may take place at rare intervals of time. Why should this be? Is this strange meeting and commingling a matter of chance, or is the one seeking the other possessed by a ravenous mate-hunger?

As we ascend higher in the scale it becomes apparent that life has gathered force. That primitive speck of jelly, the Amoeba, with which we started, gave but two signs of animation—the power of movement, and hunger. Whether these responses to internal stimuli can be called instinctive is open to argument. But there can be no question about the instinctive nature of the behaviour of these higher animals. After the instinct to feed the two most powerful are the desire for self-preservation—the avoidance of danger—and the desire to mate. These two are the dominating instincts throughout the rest of the animal world, not even excepting man himself.

The tremendous power of “mate-hunger” has been overlooked by a strange confusion between cause and effect. Almost universally its sequel, the production of offspring, has been regarded as the dominant instinct in the higher animals. This view has no foundation in fact. “Desire” for the sake of the pleasure it affords, and not its consequences, is the only hold on life which any race possesses. And this is true both in the case of man himself and of the beasts that perish. Wherever this instinct becomes weak, or defective, extinction speedily and inevitably follows. This “Amorousness” is the motive power of “Courtship” wherever it is met with; manifesting itself in the eccentric, and often grotesque posturings, or in the loud and often musical cries which constitute the study of courtship. Intensity of desire is indispensable to survival.

Only the lowly and sedentary types, of which the Oyster may be taken as an example, lack this fire; and here because it is unnecessary. For the reproductive germs of this animal are discharged into the water, to take their chance of attaining their object. They are liberated unconsciously, discharged like the undigested residue of the food, without effort, and without cognizance of the act. This must be so, for the Oyster merely lives—vegetates. Sightless, and without power of movement, after its larval wanderings are over, it lives merely to eat. And even in this, choice is denied it. The currents of water mechanically brought to afford the necessary oxygen for the maintenance of life, bring with them the food which is to restore the slowly wasting tissues. To such a creature there can be no “outer-world,” no consciousness of the existence of individuality other than its own.

The desire for sexual intercourse is met with only where the co-operation of two individuals is necessary to ensure the production of offspring. Such individuals being free to roam, must have some incentive to seek one another at the time when their germ-cells have attained maturity. And this incentive is furnished by the glands in which these elements are produced: supplemented by the secretions of certain ancillary glands. These stimulating juices, known as the “Hormones,” will be presently described.

But if we owe our existence to the gratification of what may be called our lower instincts, it is no less certain that all that is best in us we owe to our offspring. We meet with the beginnings of altruism, which the begetting of offspring entails, far down in the animal kingdom, and it attains to its full perfection in the human race. Here only, in its best and truest sense, Love begins: though affection may be found, and in a high degree, in many of the lower animals.

Living things are as clay in the hands of the potter. But it is as if they made themselves, for the designer and the guiding hand are alike invisible. No vessel is exactly like its neighbour, either in the quality of its substance or in the details of its construction. And this because the clay of which it is made possesses that mysterious property we call life. A property which endows each new feature as it appears, with an individuality of its own, whose survival, or suppression, depends entirely on its relationship to surrounding parts; on its harmony with its environment, in short. Colour, size, shape, temperament, behaviour, may each be regarded as so many entities depending for survival on whether or not they can exist in harmony with their environment—the several parts which make up what we call the individual.

In like manner the individual—the complex bundle of parts and qualities—must attain, and maintain, a certain harmony with its environment—the outer world. The process of change, both in quality and quantity, which is for ever going on among the several parts of every separate individual, brings about the elimination of unfavourable variations; and “selects” those which vary in the right direction: that is to say, which serve to maintain a place in the sun for the individual in which these momentous changes are going on. But it is not enough that the individual should be in “working order”; it must be in harmony with all the conditions on which existence depends. And the standard of this harmony is set by that very exacting arbiter of life and death, “Natural Selection.” It is not enough that the instincts in regard to this or that habit should be keen, or that this or that particular organ of the body should be efficient—a certain minimum, all-round, standard of efficiency is demanded, or elimination follows. It is through this instability of “temperament,” this tendency to vary in infinite directions, that the balance between the individual and the environment is maintained. Evolution follows the line of least resistance.

The little boy who remarked that it must be “fiddling work, making flies,” was more sage than he knew. The complex web of factors which even a fly represents are beyond the grasp of human understanding. But it is clear that the reproductive instincts, and the emotions they beget, have played, and play, a tremendous part in the evolution of the higher animals.

Those whose business it is, for one reason or another, to study these emotions know well that “mate-hunger” may be as ravenous as food-hunger, and that, exceptions apart, it is immensely more insistent in the males than in the females. But for this, reproduction in many species could not take place: for the sexes often live far apart, and mates are only to be won after desperate conflict with powerful rivals no less inflamed. Thus it is idle to speak of an equality between the sexes in this matter, in regard to the human race. Dogmatism, and the frequent repetition of pretty platitudes, will not alter what Nature has ordained. The failure to realize this is painfully obvious in the utterances of many who speak in the name of the newly-founded “Eugenics” society, which seeks the means to ensure the well-being of the race by the spread of a more intimate knowledge of this all-important subject. The existence of what Mr. Heape has recently called a “sex-antagonism” is beyond dispute, for the instincts of the male and female are fundamentally different. The male is dominated by the desire to gratify the sexual appetite; in the female this is counteracted by the stimulation of other instincts concerned with the cares of offspring.

Amorousness, then, is the dominant feature of the males among all animals: and this sex presents yet another characteristic which is to be borne in mind. In all that concerns the evolution of ornamental characters the male leads. In him we can trace the trend which evolution is taking; the female and young afford us the measure of the advance along the new line which has been taken. Why this should be is inexplicable. But sooner or later the females assume, or will assume, all the features originally possessed by their lords; and finally the young also follow suit. That is to say, the females and young tend to retain the ancestral characters. In the course of time the ability to develop new features by the male loses its impetus, and not till then, apparently, do the females, and still later, the young, begin to share his glory. These remarkable features are strikingly illustrated among the birds, as these pages will show.

Nature is nothing, if not perverse. And hence it happens that there are many exceptions to every rule which one formulates. Among the birds, for example, there are species wherein the rule that the female follows her mate in the acquisition of new characters is, so to speak, set aside. She follows a line of her own. This is true, at any rate, of superficial characters, such as coloration. By some curious change in her “metabolism,” as the conversion into living tissue of the substances taken as food is called, this coloration may attain a brilliance in no way inferior to that of the male, but strikingly different. The beautiful Orange Fruit-pigeon (Chrysoenas victor) furnishes a case in point, the male being of a gorgeous orange-yellow, the female of a no less vivid green. But the differences are not so great as they appear at first sight. For the male was originally green, and the female has thus but intensified the ancestral livery. Green, it should be remarked, of a more or less olive shade, always precedes yellow in development; and yellow may yield to red, but this order is never reversed. A no less striking case is that of the Upland Goose (Cloephaga magellanica), the male of which is pure white, while the female wears a livery of chestnut and brown. But so sharply are the colours defined that it would be difficult to say that one was of a higher order of coloration than the other. To what causes or factors are these departures due?

Reproduction in the simplest living things takes place by a simple division of the body into two as soon as its maximum size or adult condition has been attained. In such simple types the body consists only of a single “blob,” or particle, of jelly. But a new era began when large numbers of such particles, or “cells,” began to form coherent masses, different parts of the mass performing different work for the mutual benefit of the community. Some have come to form what we call the body, which is born, and in due course dies. Others are alone concerned with the task of reproduction. They are nourished by the body, and on attaining maturity, give rise to new bodies. These reproductive cells are excessively small. The male, or “sperm” cell, can only be distinguished under the highest powers of the microscope. The female cell, or “Ovum,” is always larger than the male, because, in addition to the germinal matter which it contains, it is furnished with a store of food in the shape of yolk. This accounts for the relatively enormous size of the egg of the hen. Within the hardened shell the germ develops into the chick, deriving food for its growth from this generous store. Where this yolk is limited in quantity the growing body is hastily fashioned, and launched forth into the world in the form of a “larva,” when it must forage for itself till it has attained its adult form. Or it is retained within the body of the mother until development is complete.

The reproductive cells are the bearers of the Germ-plasm, the stuff of which man and the beasts of the field alike are fashioned. Only a portion of this germ-plasm gives rise to a new body; the rest is, as it were, held over and stored within the new body to give rise to another in due course. That which produces the body we call the “Somatoplasm,” because it is the “plasm” or stuff of which the “Soma,” or body, is made. As to the nature of this Germ-plasm and its mysterious properties, a wide divergency of opinion exists among savants. But the views which find most favour to-day are those of the veteran Professor August Weissmann, as set forth in his work on the “Germ-Plasm, a Theory of Heredity.”

The excessively minute quantity of this germ-plasm which suffices to form a new body is incredible. By what miracle of miracles is the essence of a man distilled? His body arises from the union or commingling of two particles of living matter so minute as to be invisible to the naked eye. One of these particles is the “sperm”—cell furnished by the male parent; the other, the “ovum,” furnished by the mother. True the ovum may measure as much as the one-hundred and fiftieth part of an inch, but the bulk of this is yolk-food necessary to furnish the tender germ with life and energy till it shall have attached itself to the walls of the womb, whence all its future nourishment is derived.

By no process of analysis known to us could the germ-plasm of man be distinguished from that of, say, a jelly-fish; and in the matter of quantity there is no more difference. Yet, identical to our senses, in potentiality how amazingly different are these two particles of jelly! In the lowliest animals, such as jelly-fish, one cannot distinguish male and female at sight. The appearance of separate male and female individuals begins somewhat high in the scale marking an epoch in the history of animal life. For the birth of sex inaugurated not merely individuals producing distinctive “male” and “female” germs, but individuals which, by virtue of their sex, developed differences of behaviour and mentality which were to be followed by tremendous consequences. Certain aspects of this behaviour are to furnish the theme of these pages; others, and no less important, those who will may discover in Professor Arthur Thomson’s “Evolution of Sex.”

We are far, indeed, from being able to explain the attributes of sex. At most, we can but endeavour to interpret the behaviour associated therewith. This was the task which Darwin set himself to achieve in his theory of sexual selection. He was influenced in the train of thought which he followed up with such brilliant success by what he had observed in the behaviour of highly-ornamented species, such as the Peacock and the Birds of Paradise. The strange antics of these birds when under the influence of sexual excitement persuaded him that they were at least dimly conscious of their splendour, and of its power to fascinate. The female, on the other hand, was supposed to be coy, and to bestow her person on the finest performer. In this way the dullest birds and the poorest performers were gradually eliminated. Here, indeed, was sexual selection. The frills thus begotten he called “Secondary Sexual Characters,” a term which is also used, and was used, by him, to include any feature whereby the sexes can be distinguished apart from the character of the genital organs.

Horns, tusks, and spurs are other forms of secondary sexual characters. And these stand for another form of sexual selection—that of selection by battle. Herein victory falls to the strongest and most pugnacious male who, as the spoils of victory, annexes the females which formed the subject of the duel. This theory, which must be discussed at greater length in the course of these pages, has had many critics, and among them men of mark. But whatever modifications may be deemed necessary, they will be such as are demanded by the results of later discoveries rather than to the force and subtlety of the arguments of his opponents.

One of the most formidable of the opponents of the Sexual Selection theory was Wallace. But his arguments were far from convincing, and often inconsistent. He attributed the more frequent occurrence in male animals of brilliant coloration and exaggerations of growth such as give rise to manes, beards, long plumes, and so on, to a “surplus of strength, vitality and growth-power which is able to expend itself in this way without injury,” or, as he sometimes expresses it, to superabundant vitality. He was evidently striving to find words for the faith that was in him, and he was nearer the truth than he knew or than his critics supposed. He was seeking facts which only the physiologist could furnish. And these made their appearance long years after with Professor Starling’s discovery of Hormones. We are far from understanding the origin of these mysterious juices which must be so frequently alluded to in these pages, but they are evidently intimately associated with the expenditure of energy. This may sometimes find an outlet in increased stature, sometimes in pure luxuries of growth. The force of Wallace’s arguments was crushed out by the weight of detail they were made to bear.

Mr. J. T. Cunningham a few years ago entered the lists and failed to achieve his purpose no less completely. His was a theory which assumed too much. In the first place it was based on the transmissibility of acquired characters, of the truth of which there is at present no evidence.

He contends, for example, that the vivid hues of scarlet, blue, yellow and violet which colour the naked skin of the neck of the cassowaries and of both sexes, and the curious horny casque which surmounts the head, are the outcome of the constant laceration of the skin inflicted by the males during their conflicts for the possession of the females. He assumes that such conflicts take place, and he assumes that such “acquired characters” are transmitted. Now, as a matter of fact, these birds do not fight with their beaks, but with their feet. And to this end the claw of the inner toe is enlarged to form a great spur. But there is no evidence that the skin of the neck is ever damaged in such conflicts as they may engage in. No scars are ever found, at any rate, to lend support to this theory. The casque, which is similarly supposed to be a mark of honourable conflict, is an “ornament” of great frailty, for it is composed of a delicate filigree-work of bone covered with a thin sheath of horn. In like manner, the long plumes which surmount the heads of birds like the Peacock, and many Birds of Paradise, and the wattle which surmounts the beak of the Turkey, are supposed to have had their origin in similar pugilistic encounters in the past. Mr. Cunningham is surely pushing the theory of the transmission of acquired characters a little far. For what has been transmitted in these cases is not a number of scarred surfaces, but a series of hypertrophied structures. An amazing array of ornamental characters, symmetrically disposed, and often vividly coloured, in short, has been produced from lacerated tissues which in kind and extent can have varied but little.

Evidence has been accumulating during the last few years which would have rejoiced the heart of Darwin. Had he known that birds of sober hues “display” with the same animation and with as much elaboration of posture as the Peacock and the Pheasant, his theory of “Sexual Selection “would probably have left little for those who came after him to criticize. Since his time it has been discovered that both permanent and recurrent secondary sexual characters, such as the antlers of deer and the temporary nuptial plumage of birds, such as the Ruff for example, are controlled as to their growth by the stimulating action of the “secretions or juices formed by certain of the ductless glands “; that is to say, of glands having no apparent connection with their surrounding tissues. We owe much of our knowledge of this subject to Professor Starling, who has called these secretions “Hormones.”

Darwin knew that the essential sexual glands, the testes and the ovaries, in some mysterious way controlled, in a large degree, the development of these “hall-marks” of sex, for it was known in his time that castrated stags failed to produce antlers, and that hen pheasants, for example, in extreme old age, or when the ovaries were damaged by disease or injury, at once assumed the plumage of the cock; but the part played by these ductless glands was quite unsuspected. They are the Thyroid, and the Thymus glands, which are attached to the outer walls of the trachea or windpipe. The Pituitary body, which forms part of the brain, and the Suprarenal bodies, attached to the kidneys. It would be foreign to the purpose of these pages to enter into the functions of these glands; suffice it to say, that the juices formed therein are taken up by the blood, and distributed over the system. Their action is only very imperfectly understood. We know that any derangement in their efficiency results in disease, and that they play a very important part in the reproductive system, as will become abundantly evident in the course of these pages. Much hitherto attributed to the action of “Sexual Selection” alone, it is now evident is largely due to their action.

The all-sufficiency of the “Sexual Selection” theory to account for the development of armature, such as horns, antlers, and the huge spine-like outgrowths which form so conspicuous a feature of many of the extinct Land-dragons, or Dinosaurs, has been by no means universally accepted. Some authorities like Dr. A. Smith Woodward and Professor Osborne interpret these after another fashion. They hold that these are the “expression points” of inherent growth forces, a process of concentration marking the final stages of evolution prior to extinction. From which it may be inferred that there is a term to the life of a species as there is to the life of the individual. In many cases it is suggested the very exuberance of growth has been the exterminating factor, as in the case of the huge antlers of the Irish “elk,” whose enormous weapons hampered his endeavours to escape his enemies. This is the theory of “Orthogenesis,” or direct development. According to this, new structures, arising in the germ-plasm as “variations,” will of their own inherent vitality go on increasing in each generation unless, and until, checked by “Natural Selection.” Changes in the character of the “Hormones” might very well bring about these excesses of growth. It is well known that the exuberance of growth which produces giants among the human race is due to a derangement of the secretions or hormones of the pituitary body which largely control growth.

Another factor of Sexual Selection which is commonly ignored, but which is of profound importance, is to be found in the part played by the emotions in regard to sexual relationships; the part which the “mind” has played, and plays, in the mating of animals, at any rate of the higher types.

Darwin touched but lightly on this theme. Later writers have almost entirely ignored it. Almost all that is worth knowing on the subject we owe to Professor Lloyd Morgan, who was one of the first to take up this difficult line of investigation, and to Professor Groos. Their researches have shown that there can be no doubt but that the emotions have played and are playing an important part in the phenomena we are striving to analyse. Sexual selection, in short, is concerned not merely with the evolution of the physical characters of the body, but also, and no less, with the psychological attributes thereof. Many new and extremely valuable facts in this regard have been brought to light by Mr. H. Eliot Howard in the course of his remarkable studies on our native warblers. Not until the psychology of sex in the lower orders of creation has been further investigated shall we have a properly balanced account of the part played by sexual selection in the scheme of evolution.

By now it will have become apparent that the study of the “Courtship” of animals is one of alluring interest and full of pitfalls for the unwary. And this because of the apparent difficulty in drawing any hard-and-fast line between the part played by “Natural” and the part played by “Sexual” Selection, at any rate in some cases.

To this aspect of the theme Professor Lloyd Morgan has drawn particular attention. “It is difficult,” he remarks, to accept the view that individual choice has played no part where the sexual instincts are concerned. But supposing that it has played its part ... the effects will be wrought into the congenital tissue of the race if, and only if, there are certain individuals which, through failure to elicit the pairing response, die unmated. Is preferential mating, supposing it to occur, carried to such a degree that some individuals fail to secure a mate? That is the question. If so, sexual selection is a factor in race progress; if not, though it may occur in nature, it is inoperative as a means of evolutionary development. The whole question, in itself a difficult one, is further complicated by the fact that the males which are possessed of the most exuberant vitality, and are therefore by hypothesis rendered the most acceptable through emotional suggestion, are likely to compete with other males of less exuberant vitality by direct combat. Such competition, by which the weakest are excluded from mating through no choice on the part of the female, falls under the head of natural selection, and not of sexual selection, if by that term we understand preferential mating.

“This serves to bring out the difference ... between natural selection through elimination and conscious selection through choice.... Sexual selection by preferential mating begins by selecting the most successful in stimulating the pairing instinct.... The process is determined by conscious choice. It is in and through such choice that consciousness has been a factor in evolution.”

Herein Lloyd Morgan, like Darwin, recognizes the existence of a dual machinery in determining survival, where this depends on the co-operation of two individuals leading separate existences—Natural, and Sexual, Selection—sometimes the one and sometimes the other prevailing. In the former, the females are seized by force; in the latter, won by displays.

But is this really so? In these pages it is contended that a sharp line must be drawn between all those attributes and characters which are necessary to achieve individual survival, the survival of the Ego, and all those which, on the other hand, are necessary to achieve reproduction and the survival of the race. The former are governed, or determined, by Natural, the latter by Sexual, Selection.

The sphere of influence of these two factors may be delimited, if we regard natural selection as the factor accountable only for the qualities necessary for the survival of the individual—necessary to ensure success in the struggle for existence. Then it will become apparent that the qualities and attributes necessary to achieve the survival of the race are of a different kind, and these are the factors which are embraced under the term “Sexual Selection.”

It is a mistake to regard animals in relation to the selection theory as if they were so many tailors’ “mannikins.” Yet a large number of the critics of the selection theory seem to fall into this error, ignoring all but the most superficial characters.

The peculiarities of colour, structure and behaviour, that is to say, the characters and qualities which distinguish the individuals of any given race, are due to inherent qualities of the germ-plasm. Each of such qualities, therefore, may be regarded as entities. Selection determines their survival. Intracellular selection is the first sieve through which they have to pass, natural and sexual selection are others, as circumstances may determine.

As a rule the sex of an individual is attested by more or less conspicuous external features. These are known as the “Secondary Sexual Characters.” But no hard-and-fast line can be established for these, at any rate, so far as colour and ornament are concerned, for such, as will become apparent in the course of these pages, tend to appear first in the male, and then, later, to be acquired by the female, until in many cases the two sexes become again indistinguishable.