Page:The Negro a menace to American civilization.djvu/81

Rh to produce variability in form by a change of environment and by crossing. But in doing this he cannot alter in the slightest degree any of the existing conditions, as climate, or any of the existing laws, as the laws controlling variability. Man may expose living forms of animals and plants to these existing conditions and laws, but beyond that his power ceases. He may cause, within certain limits, various forms of animals and plants to cross and reproduce other forms that differ in certain particulars from the parent ones, and so on down through successive generations. But his limitations here are equally exacting, for, in the first place, all animal forms, for example, are either not fertile inter se, or the copulatory act is impossible, as would be the case in a giraffe and a bat, or even when possible the two species cannot be induced to copulate, as would be the case in all probability in a wolf and a peccary. Let us consider a few examples to make my meaning still clearer. In the group of domestic pigeons we may select and cross certain varieties with the view of producing particular changes either in plumage or form, or both, and this selection and crossing may be persisted in until we have accomplished our object and produced the kind of birds we desired to manufacture. But we should have been absolutely powerless in our efforts here had it not been for the ceaseless operation of the natural law of variability and heredity to insure the gradual changes. I know of a variety of domestic pigeon which had been produced by artificial selection extending through a number of generations with the view, among other characters, of having the birds have as short bills as possible. So successful was the breeder that he obtained