We see the same process of extermination among our domesticated productions, through the selection of improved forms by man.

Many curious instances could be given showing how quickly new breeds of cattle, sheep and other animals, and varieties of flowers, take the place of older and inferior kinds.

In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these "were swept away by the short-horns" (I quote the words of an agricultural writer) "as if by some murderous pestilence.

" The principle, which I have designated by this term, is of high importance, and explains, as I believe, several important facts.