Philosophers and scientific observers have defined the distinction between human and animal love as consisting in the fact that man can love at all times, the animal periodically only; but this distinction certainly does not apply to the beginnings of human development; it originates beyond question with the =first appearance of the spiritual element in love=. This alone makes man capable of enduring love, this alone frees him from dependence upon periodic rutting seasons. The =prolongation= of love by the introduction of the spiritual element was already pointed out by Kant, whose writings (especially the lesser ones) are rich in valuable observations of a similar kind. In his treatise published in 1786, “The Probable Beginning of Human History,” he says regarding the sexual instinct: “Reason, as soon as it had become active, did not delay to exert its influence also in the sexual sphere. Man soon discovered that the stimulus of sex, which in animals depended merely on a transient and for the most part periodic impulse, was in his own case =capable of prolongation, and indeed of increase, by the force of imagination=. This influence works more moderately, it is true, but with more persistence and more evenness the more the affair is withdrawn from the dominion of the senses, so that the satiety produced by the gratification of a purely animal passion is avoided.” This important question regarding the origin of the love of human beings as contrasted with the periodic instinct of the lower animals and primitive man has hitherto, strangely enough, hardly received any attention, notwithstanding the fact that it is one of the most important evolutionary problems in the history of human civilization, and represents to a certain extent the only problem in the primitive history of love. The =principal= cause of the perennial nature of human love, as contrasted with the periodic character of the sexual impulse of the lower animals, must, as Kant says, be sought in the appearance of these psychical relations between the sexes. Hypotheses such as that put forward by Dr. W. Rheinhard in his book, “Man considered as an Animal Species, and his Impulses,” according to which the prolonged =separation= of the sexes, consequent on the increased difficulty in the provision of sufficient nutriment (more especially in the Ice Age), led to an incomplete satisfaction of the sexual impulse during the rutting season, and thus gave rise to an =enduring= sexual excitement, cannot be treated seriously. The same author suggests that the excessive =consumption of meat= of the Ice Age, owing to the absence of vegetable food, was responsible for the stronger stimulation of the sexual impulse, and for its prolongation beyond the rutting season.