The attraction of one kind of germinal cell and its bearer for the other must also be more or less mutual. As a rule the bearer of one of the germinal cells becomes active and penetrating; that of the other passive and receptive. However, the latter, who after copulation (when this occurs) becomes the sole bearer of the future individual, is obliged to desire union with the active bearer of the other germinal cell, so that reproduction may become harmonious. This is the basis on which is founded sexual reproduction, and with it the sexual appetite, in plants (as regards cellular conjugation only) as well as in animals, but especially in the latter, in whom the germinal cells are carried by mobile and independent individuals. On the same basis is developed the difference between the sexual appetite in man and woman, as well as that between love and the other irradiations of this appetite in the mental life of both sexes. (Vide Chapters IV and V.) The immense complication of human sexual life makes us regard animals with a certain degree of contempt, and flatter our vanity in qualifying the baser part of our sexual appetite by the term _animal instinct_. But we are really very unjust toward animals. This injustice is partly due to the fact that vocal and written language gives us a means of penetrating into the psychology of our fellow creatures. By the aid of the common symbolism of our thoughts it is easy for us to compare them. Language thus enables us to construct a general human psychology. The absence of language, even in the higher animals, renders it difficult for us to penetrate their mind. Our inductive reasoning in this matter is very uncertain, for we can only judge the mental power of animals by their acts. The brain, and consequently the mind, of the higher mammals being less highly organized than that of man, their sexual psychology is also more primitive, and differs from ours in proportion to the cerebral development of the species. Comparative anatomy confirms this fact in the whole series of organisms which possess a central nervous system. The psychology of the higher apes is thus nearer our own than that of the dog; the psychology of the dog resembles ours more than that of the rabbit, etc. On the other hand, the highly developed cerebral organization of man, although it has complicated the mental irradiations of his sexual appetite, has not always ennobled them; on the contrary, it has often directed them into pernicious paths. We have seen in Chapter VI numerous and striking proofs of the degeneration, brutality and cruelty of the manifestations of the human sexual appetite, and we shall study them further in Chapter VIII. Comparative biology shows us that the sexual appetite is transformed into love in very different ways. In order to avoid the immensity of detail of comparative biology I shall only give a few examples.