The impression that infra-human mammals more or less confine themselves to heterosexual activities is a distortion of the fact which appears to have originated in a man-made philosophy, rather than in specific observations of mammalian behavior. Biologists and psychologists who have accepted the doctrine that the only natural function of sex is reproduction, have simply ignored the existence of sexual activity which is not reproductive. They have assumed that heterosexual responses are a part of an animal's innate, 'instinctive' equipment, and that all other types of sexual activity represent 'perversions' of the 'normal instincts'. Such interpretations are, however, mystical. They do not originate in our knowledge of the physiology of sexual response (Chapter 15), and can be maintained only if one assumes that sexual function is in some fashion divorced from the physiologic processes which control other functions of the animal body.