IN artificial selection, through which, with or without conscious intention, our domesticated animals and cultivated plants have arisen, there must obviously be three kinds of co-operative factors: first, the _variability_ of the species; second, the capacity of the organism for _transmitting_ its particular characters to its progeny; and third, the _breeder_ who selects particular qualities for breeding. No one of the factors can be dispensed with; the breeder could effect nothing, were there not presented to him the variations of parts in the particular direction in which he wishes them to vary; an indefinite variation, that is, a variation not guided by selection, would never lead to the formation of new breeds; the species would probably become in time a motley mixture of all sorts of variations, but a breed with definite characters, transmissible in their purity to its descendants, could never be formed. Finally, every process of selective breeding would be futile, if the variations which appeared could not be transmitted. Darwin assumes that processes of transformation quite similar to those which take place under the guidance of Man occur also in nature, and that it is mainly these which have brought about and guided the transformations of species which have taken place in the course of the earth's history. This process he calls _natural selection_. It will readily be admitted that two out of the three factors necessary to a process of selective breeding are present also in the natural conditions of the life of species. Variability in some degree or other is absent from no species of animal or plant, though it may be greater in one than in another, and it cannot be doubted that the inborn differences which distinguish one individual from another are capable of transmission. It is only to untrained observers that all the individuals of a species appear alike; for instance, all garden whites, or all the individuals of the small tortoiseshell butterfly (_Vanessa urticæ_), or all the chaffinches. If the individuals are carefully compared it will be recognized that, even in these relatively constant species, no individual exactly resembles another; that even among butterflies twenty black scales may go to form a particular spot on the wings in one individual and thirty or twenty-five in others; that the length of the body, the legs, the antennæ, the proboscis exhibit minute differences; and it is probable that the same combination of quite similar parts never occurs twice. In many animals this cannot, of course, be proved, because our power of diagnosis is not fine enough to be able to estimate the differences directly, and because a comparison of measurements of all the parts in detail is not practicable. So we may here confine ourselves to the differences in the human race, which we can recognize with ease and certainty. Even as regards the face alone, all men differ from one another, and, numerous and complete as