been
rendered swifter, through the selection of the best horses during many
generations; and then the old Eclipse may possibly be eclipsed; but, as Mr.
Wallace has remarked, there must be an ultimate limit to the fleetness of
every animal, whether under nature or domestication; and with the horse
this limit has perhaps been reached. Until our fields are better manured,
it may be impossible for a new variety of wheat to yield a heavier crop.
But in many cases those who are best qualified to judge do not believe that
the extreme point has as yet been reached even with respect to characters
which have already been carried to a high standard of perfection. For
instance, the short-faced tumbler-pigeon has been greatly modified;
nevertheless, according to Mr. Eaton,[596] "the field is still as open for
fresh competitors as it was one hundred years ago." Over and over again it
has been said that {243} perfection had been attained with our flowers, but
a higher standard has soon been reached. Hardly any fruit has been more
improved than the strawberry, yet a great authority remarks,[597] "it must
not be concealed that we are far from the extreme limits at which we may
arrive."

Time is an important element in the formation of our domestic races, as it
permits innumerable individuals to be born, and these when exposed to
diversified conditions are rendered variable. Methodical selection has been
occasionally practised from an ancient period to the present day, even by
semi-civilised people, and during former times will have produced some
effect. Unconscious selection will have been still more effective; for
during a lengthened period the more valuable individual animals will
occasionally have been saved, and the less valuable neglected. In the
course, also, of time, different varieties, especially in the less
civilised countries, will have been more or less modified through natural
selection. It is generally believed, though on this head we have little or
no evidence, that new characters in time become fixed; and after having
long remained fixed it seems possible that under new conditions they might
again be rendered variable.

How great the lapse of time has been since man first domesticated animals
and cultivated plants, we begin dimly to see. When the lake-buildings of
Switzerland were inhabited during the Neolithic period, several animals
were already domesticated and various plants cultivated. If we may judge
from what we