All the facts with which we are acquainted confirm this conclusion, for whole groups of purely parthenogenetic species or genera are never met with, as would certainly be the case if parthenogenesis had been the only method of reproduction through a successional series of species. We always find it in isolated instances, and under conditions which compel the conclusion that it has become predominant in the species in question, and has not been transmitted from any preceding species. There still remains a very different class of facts which, so far as we can judge, are in accordance with my theory as to the significance of sexual reproduction, and which may be quoted in its support. I refer to the condition of functionless organs in species with parthenogenetic reproduction. Under the supposition that acquired characters cannot be transmitted—and this forms the foundation of the views here set forth—organs which are of no further use cannot become rudimentary in the direct and simple manner in which it has been hitherto imagined that degeneration takes place. It is true that an organ which does not perform any function exhibits a marked decrease of strength and perfection in the individual which possesses it, but such acquired degradation is not transmitted to its descendants, and we must therefore look for some other explanation of the firmly established fact that organs do become rudimentary through a series of generations. In seeking this explanation, we shall have to start from the supposition that new forms are not only created by natural selection, but are also preserved by its means. In order that any part of the body of an individual of any species may be kept at the maximum degree of development, it is necessary that all individuals possessing it in a less perfect form must be prevented from propagation—they must succumb in the struggle for existence. I will illustrate this by a special instance. In species which, like the birds of prey[193], depend for food upon the acuteness of their vision, all individuals with relatively weak eyesight must be exterminated, because they will fail in the competition for food. Such birds will perish before they have reproduced themselves, and their imperfect vision is not further transmitted. In this way the keen eyesight of birds of prey is kept up to its maximum.