Biology the starting-point of sociology--The irresistible force of Love--The true place of woman and man in the animal kingdom--Analogy between animal love-matings and our own--The Life-force--Reproduction a process of nutrition--Different modes of Reproduction--Cell-division--Successive stages of growth--Theory of sex--Its nature and origin--Incipient sex among the early forms of life--The true office of sex--The principle of fertilisation--Its use to the species in progressive development--Nutrition as a factor determining sex--Illustration of the _volvox_--The dependence of the male-cell upon the female-cell--The well-nourished female--The hungry male--Relation between food supply and the sexes--Illustrations--Lessons to be learnt--All species are invented and tolerated by Nature for parenthood and its service--The part played by the female--The demand laid upon her heavier than that laid upon the male--The female is mainly responsible for the race--The female led and the male followed in the evolution of life. CHAPTER II THE ORIGIN OF THE SEXES "Before studying the sexual relations, and their more or less regulated form in human societies, it will not be out of place to say a few words on reproduction in general, to sketch briefly its physiology in so far as this is fundamental, and, to show how tyrannical are the instincts whose formation has been determined by physiological causes."--LETOURNEAU.