During the first half of this century the inneity of instinct was placed in the order of cognition, while recent psychology places it in the order of movements, or, to be more accurate, in a fixed relation between certain states of consciousness and certain movements. According to the first hypothesis, stated in a masterly manner by F. Cuvier, instinct consists in images, or innate and constant sensations, which determine to action in the same manner as ordinary sensations; it is “a sort of vision, a dream, analogous to somnambulism.” According to the second hypothesis, sensations, perceptions, and images excite movements determined by the organisation, as in the case of ducklings when they see the water, the kitten scenting a mouse, the squirrel laying up its winter store. There are no innate representations, or even innate movements, but a pre-established relation between some fortuitous impressions and a group of movements: instinct is the _innate motor reaction_ to an external or internal excitement; it results from the nature of the animal. The impression only pulls the trigger and the shot is fired. Like every other instinct, that of sex consists in a fixed _relation_ between internal sensations coming from the genital organs, or tactile, visual or olfactory perceptions on the one hand, and movements adapted to an end on the other. As far as it is an instinct, it is that and nothing but that. In the immense majority of animals, and frequently in men, it does not rise above this level; in plainer words, it is not accompanied by any tender emotion. The act once accomplished, there is separation and oblivion. More than this, in some cases there is not even indifference, but hostility: the males of the queen bee are put to death as useless, and it is well known that the mate of the female spider often runs the risk of being devoured. Sexual _love_ corresponds to a higher form of evolution. Over and above instinct, it implies the addition of a certain degree of tender feeling. It is not therefore a simple emotion, even in the tolerably numerous species of animals in which it can be studied. In man, more especially in civilised man, its complexity becomes extreme. The analysis made by Herbert Spencer is well known and somewhat lengthy, yet I do not hesitate to transcribe it, since I can find no other to equal it, nor any point which could be added or subtracted:—