ifferent parts of
the great Europaeo-Asiatic continent, and thence spread to other countries.
This fact of the gradual increase in size of our domestic animals is all
the more striking as certain wild or half-wild animals, such as red-deer,
aurochs, park-cattle, and boars,[937] have within nearly the same period
decreased in size.

The conditions favourable to selection by man are,--the closest attention
being paid to every character,--long-continued perseverance,--facility in
matching or separating animals,--and especially a large number being kept,
so that the inferior individuals may be freely rejected or destroyed, and
the better ones preserved. When many are kept there will also be a {428}
greater chance of the occurrence of well-marked deviations of structure.
Length of time is all-important; for as each character, in order to become
strongly pronounced, has to be augmented by the selection of successive
variations of the same nature, this can only be effected during a long
series of generations. Length of time will, also, allow any new feature to
become fixed by the continued rejection of those individuals which revert
or vary, and the preservation of those which inherit the new character.
Hence, although some few animals have varied rapidly in certain respects
under new conditions of life, as dogs in India and sheep in the West
Indies, yet all the animals and plants which have produced strongly marked
races were domesticated at an extremely remote epoch, often before the dawn
of history. As a consequence of this, no record has been preserved of the
origin of our chief domestic breeds. Even at the present day new strains or
sub-breeds are formed so slowly that their first appearance passes
unnoticed. A man attends to some particular character, or merely matches
his animals with unusual care, and after a time a slight difference is
perceived by his neighbours;--the difference goes on being augmented by
unconscious and methodical selection, until at last a new sub-breed is
formed, receives a local name, and spreads; but, by this time, its history
is almost forgotten. When the new breed has spread widely, it gives rise to
new strains and sub-breeds, and the best of these succeed and spread,
supplanting other and older breeds; and so always onwards in the march of
improvement.

When a well-marked breed has once been established, if not supplanted by
still improving sub-breeds, and if not exposed to greatly changed
c