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.