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.