oduced by the arts of man there
is no one instance of sterility. There is not much difficulty in this,
for the races produced by the natural means above explained will be
slowly but steadily selected; will be adapted to various and diverse
conditions, and to these conditions they will be rigidly confined for
immense periods of time; hence we may suppose that they would acquire
different constitutional peculiarities adapted to the stations they
occupy; and on the constitutional differences between species their
sterility, according to the best authorities, depends. On the other hand
man selects by external appearance{257}; from his ignorance, and from
not having any test at least comparable in delicacy to the natural
struggle for food, continued at intervals through the life of each
individual, he cannot eliminate fine shades of constitution, dependent
on invisible differences in the fluids or solids of the body; again,
from the value which he attaches to each individual, he asserts his
utmost power in contravening the natural tendency of the most vigorous
to survive. Man, moreover, especially in the earlier ages, cannot have
kept his conditions of life constant, and in later ages his stock pure.
Until man selects two varieties from the same stock, adapted to two
climates or to other different external conditions, and confines each
rigidly for one or several thousand years to such conditions, always
selecting the individuals best adapted to them, he cannot be said to
have even commenced the experiment. Moreover, the organic beings which
man has longest had under domestication have been those which were of
the greatest use to him, and one chief element of their usefulness,
especially in the earlier ages, must have been their capacity to undergo
sudden transportals into various climates, and at the same time to
retain their fertility, which in itself implies that in such respects
their constitutional peculiarities were not closely limited. If the
opinion already mentioned be correct, that most of the domestic animals
in their present state have descended from the fertile commixture of
wild races or species, we have indeed little reason now to expect
infertility between any cross of stock thus descended.

{256} _Origin_, Ed. i. p. 268, vi. p. 398.

{257} <Notes in original.> Mere difference of structure no guide to
what will or will not cross. First step gained by races keeping
apart. <It is not clear w