Sex as a Phenomenon of Adaptation Of what advantage is it to have the individuals of many species separated into males and females? It is obviously a disadvantage from the point of view of propagation to have half of the individuals incapable of producing young, and the other half also incapable of doing so, as a rule, unless the eggs are fertilized by the other sex. Is there any compensation gained because each new individual arises from two parents instead of from one? Many answers have been attempted to these questions. At the outset it should be recognized that we are by no means forced to assume, as is so often done, that because there is this separation of the sexes it must have arisen on account of its advantage to the species. Whether the result may be of some benefit regardless of how it arose, may be an entirely different question. It would be extremely difficult to weigh the relative advantages (if there are any) and disadvantages (that are obvious as pointed out above), nor is it probable that in this way we can hope to get a final answer to our problem. We may begin by examining some of the modern hypotheses that have been advanced in this connection. Darwin has brought together a large number of facts which appear to show the beneficial effects of the union of germ-cells from two different individuals. Conversely, it is very generally believed, both by breeders and by some experimenters, that self-fertilization in the case of hermaphroditic forms leads, in many cases, though apparently not in all, to the production of less vigorous offspring. Darwin’s general position is that it is an advantage to the offspring to have been derived from two parents rather than to have come from the union of the germ-cells of the same individual, and he sees, in the manifold contrivances in hermaphroditic animals and plants to insure cross-fertilization, an adaptation for this purpose. This question of whether self-fertilization is less advantageous than cross-fertilization is, however, a different question from that of whether _non-sexual_ methods of reproduction are less advantageous than sexual ones. Since some plants, like the banana, have been propagated for a very long time solely by non-sexual methods without any obvious detriment to them, it is at first sight not easy to see what other advantage could be gained by the sexual method. The case of the banana shows that some forms do not require a sexual method of propagation. Other forms, however, are so constituted, as we find them, that they cannot reproduce at the present time except by the sexual method. In other words, the latter are now adapted, as it were, to the sexual method, and there is no longer any choice between the two methods. The question of whether a non-sexual form might do better if it had another method of propagation is not, perhaps, a profitable question to discuss.