Obviously the sexual adaptations of a species are definitely related to the conditions of its life, and, though Nature's endeavour to prevent inbreeding and to secure cross-fertilization is evidenced by the occurrence of separate sexes in such a multitude of forms yet in many cases gonochorism has been relinquished, and always where this was necessitated by the conditions of life to which the group concerned was subject. In such a case inbreeding is regulated as far as possible, for instance, by an arrangement which ensures that individuals shall be crossed at least from time to time. But cases of exclusive and constant self-fertilization do also seem to occur, and even these may be brought into harmony with our conception, according to which cross-fertilization is an advantage, but only an advantage which must be weighed against others, and which may eventually be given up in favour of greater advantages. This occurrence of persistent autogamy can no more be reconciled with the rejuvenation theory than can continuous parthenogenesis, because, according to this theory, the mingling of different individuals is a _sine qua non_, for the continued life of the species. It is impossible for me here to discuss in detail all the deviations from pure gonochorism or bi-sexuality which occur in nature, but I must at least attempt to take a general survey, and to arrange the chief phenomena of these various modes of 'sexual reproduction' in an orderly scheme. I must take a survey of both plants and animals, but I shall give the precedence to animals, as being to me more familiar ground. Where do we find, in the animal kingdom, that Nature has departed from gonochorism, from the separation of the sexes, and for what reasons was this departure necessary? And further, what means does Nature take to compensate for this renunciation of the simplest method of securing the continual cross-fertilization of individuals?