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.

In the first place, varieties, even strongly-marked ones, though having somewhat of the character of species--as is shown by the hopeless doubts in many cases how to rank them--yet certainly differ far less from each other than do good and distinct species.

Nevertheless according to my view, varieties are species in the process of formation, or are, as I have called them, incipient species.