Anatomy and physiology are not the only sciences concerned in this question; for if the existence of these superposed centres, connected with each other, though distinct in function, were thoroughly established, this would give us certain landmarks, and lay down fixed conditions, or stages, in the development of sexual emotion, which may result from a state of the organs (instinctive form) or from an external perception, or from a pure representation (imaginative love). In the absence of an anatomical basis which might serve for the normal psychology of the subject, and still better for the comprehension of the pathological facts, let us follow the evolution of sexual love so far as observation enables us to do so. It has already been sketched in the Introduction, but too briefly for our present purpose. 2. We have distinguished, in this psychological evolution, three principal periods: the instinctive, the emotional, and the intellectualised. Taking the question at its remotest origin, some naturalists and philosophers assert that the equivalents of sexual attraction exist in living beings devoid of nervous systems, in vegetable or animal micro-organisms. “It is curious,” says Balbiani, “to find, in beings who from their small size, and the external simplicity of their organisation, have been placed by all zoologists at the furthest limit of the animal world, actions denoting the existence of phenomena analogous to those by which the sex-instinct manifests itself in a great number of metazoa.... Thus, with the paramæcids, at the moment of propagation ... a higher instinct seems to govern these little animals; they seek and pursue each other, they go from one to another, feeling each other with their cilia, cling to one another for some instants in the attitude of sexual approach, and then let go in order to seize each other again. These singular games, by which these animalcules seem in turn to provoke one another to the act of copulation, often last for several days before the act becomes definitive.” Other facts of the same nature have been cited. Finally, it has been said that “the coupling of the two sexual elements is analogous to the coupling of the two animals whence these elements are derived: the spermatozoid and the ovule do on a small scale what the two individuals do on a large one; the spermatic element, in directing itself towards the ovule which it is to fertilise, is animated by the same sexual instinct which guides the complete being towards the female of the same species.”[158] ----- Footnote 158: