Rousseau suggested that primitive man embraced primitive woman only in the fugitive moments of domination by his instinctive impulse. It is no doubt very probable that primæval man shared with other animals the periodicity of the sexual impulse; this periodicity disappeared only in the subsequent course of human development, and traces of it yet remain. It is probable that this periodicity of the sexual impulse was associated with variations in the supply of nutriment, and was thus, as Darwin assumes, a kind of natural obstacle to too rapid an increase in the population. Later, in consequence of an increase in individual security, and of a more enduring supply of abundant nutriment, such periodic rutting ceased to occur, or was preserved only in the form of menstruation (ovulation) in women, in whom at this period there is a perceptible increase in sexual excitability. =Among savage races this periodicity of the sexual impulse, its increase at definite seasons of the year, is still clearly manifested even in the male.= Heape and Havelock Ellis have carefully studied this primitive phenomenon, and have adduced numerous proofs of its truth.[5] Only the human female experiences true “menstruation”; that is to say, only in women is the maturation of the ovum accompanied by a monthly discharge of blood from the genital passage. The so-called menstruation of female apes is limited to a periodic swelling of the external genital organs, with a mucous discharge therefrom. According to Metchnikoff, the menstruation of apes constitutes the intermediate stage between the rutting of the lower animals and the menstruation of the human female. This latter is a new acquisition, the purpose of which is perhaps the limitation of fertility and the prevention of the excessively early marriage of girls. With the advanced development of the brain, the old periodic rutting, of which rudiments still persist, became more and more subordinate to the conscious will, was transformed more and more into enduring love. Charles Letourneau writes: “If we go to the root of the matter, we find that human love is in its essence merely the rutting season in a reasoning being; it increases all the vital forces of the human being, just as rutting increases those of the lower animals. If love apparently differs enormously from rutting, this is merely due to the fact that the reproductive impulse, the most primitive of all impulses, becomes in developed nerve centres more diffuse in its sphere of operations, and thus in man awakens and excites a whole province of psychical life which is entirely unknown to the lower animals.”