The consideration of this case throws some light on the significance of the non-occurrence of natural hybrids and of the very remarkable and curious fact that hybrids are so usually sterile. When we come to think of it, the natural preliminary assumption should be (as is that of unsophisticated humanity) that any animal or plant might, so far as possibilities go, breed with any other; and the questions to be answered are: (1) What advantage to a species is it not to be able to hybridize with other species, and (2) how–that is to say, by what structure or by what subtle chemical differences or other features in their make-up and habit–are they prevented from so hybridizing? Then we come on further to the question, Why should a hybrid, once produced, fail to bear healthy eggs or sperms according to its sex, although it grows up to full size and is to all appearances mature? And why should hybrids between parents of origin locally remote from one another not show this failure, but behave like ordinary healthy organisms? In the full solution of these inquiries we should get very near to some of the most important secrets of the living body which have still to be searched out. But a reply to these questions which is probably in large measure true, and serves to help us in the further collection and examination of facts, is as follows: First, the production and maintenance of "species" of plants and of animals by survival of favourable variations in the struggle for existence (Darwin and Wallace's theory of the origin of species) requires the maintenance of the purity of the favourable stock which survives in the struggle. If it were continually liable to hybridization by other species it would never establish its own distinctive features. It would deteriorate by departing from those characteristics which have been "naturally selected" and have rendered it a successful "species." Thus the breeder, when he has selected a stock for propagation which approaches the standard at which he is aiming, keeps it apart, and does not allow it to be "crossed" by other stock. One of the qualities "naturally selected" in "the wild" is the power of resistance to fertilization by neighbouring species.