We can not suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; indeed, in many cases, we know that this has not been their history.

The key is man's power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him.

In this sense he may be said to have made for himself useful breeds.

The great power of this principle of selection is not hypothetical.

It is certain that several of our eminent breeders have, even within a single lifetime, modified to a large extent their breeds of cattle and sheep.