"In all living creatures there are two great lines of variation, primarily determined by the very nature of protoplasmic change (metabolism); for the ratio of the constructive (anabolic) changes to the disruptive (katabolic) ones, that is of income to outlay, of gains to losses, is a variable one. In one sex, the female, the balance of debtor and creditor is the more favourable one; the anabolic processes tend to preponderate, and this profit may be at first devoted to growth, but later towards offspring, of which she hence can afford to bear the larger share. To put it more precisely, the life-ratio of anabolic to katabolic changes, A/K, in the female is normally greater than the corresponding life-ratio, a/k, in the male. This for us, is the fundamental, the physiological, the constitutional difference between the sexes; and it becomes expressed from the very outset in the contrast between their essential reproductive elements, and may be traced on into the more superficial sexual characters." A little further on (p. 17), the authors say:-- "Without multiplying instances, a review of the animal kingdom, or a perusal of Darwin's pages, will amply confirm the conclusion that on an average the females incline to passivity, the males to activity. In higher animals, it is true that the contrast shows itself rather in many little ways than in any one striking difference of habit, but even in the human species the difference is recognized. Every one will admit that strenuous spasmodic bursts of activity characterize men, especially in youth, and among the less civilized races; while patient continuance, with less violent expenditure of energy, is as generally associated with the work of women." We must shortly proceed to study the origin and determination of sex, and more especially of femaleness, in the individual, and here we shall be entirely concerned with the new knowledge commonly called Mendelism, to which there is no allusion in our authors' pages. Meanwhile it must be insisted that the reader who will either read their pages for a survey of the evidence in detail, or who will for a moment consider the evident necessities imposed by the facts of parenthood, cannot possibly fail to satisfy himself that the main contention, as stated in the foregoing quotations, is correct. A further point of the greatest importance to us requires to be made.