Several eminent breeders, during a single lifetime, have so largely modified some of the higher animals, which propagate their kind much more slowly than most of the lower animals, that they have formed what well deserves to be called a new sub-breed.

Few men have attended with due care to any one strain for more than half a century, so that a hundred years represents the work of two breeders in succession.

It is not to be supposed that species in a state of nature ever change so quickly as domestic animals under the guidance of methodical selection.