estication under altered
conditions, facilitates the process of modification. Yet this change
does not seem to be an essential condition, for nowhere has the
production of extreme varieties of plants and flowers been carried
farther than in Japan, where careful selection continued for many
generations must have been the chief factor. The effect of occasional
crosses often results in a great amount of variation, but it also leads
to instability of character, and is therefore very little employed in
the production of fixed and well-marked races. For this purpose, in
fact, it has to be carefully avoided, as it is only by isolation and
pure breeding that any specially desired qualities can be increased by
selection. It is for this reason that among savage peoples, whose
animals run half wild, little improvement takes place; and the
difficulty of isolation also explains why distinct and pure breeds of
cats are so rarely met with. The wide distribution of useful animals and
plants from a very remote epoch has, no doubt, been a powerful cause of
modification, because the particular breed first introduced into each
country has often been kept pure for many years, and has also been
subjected to slight differences of conditions. It will also usually have
been selected for a somewhat different purpose in each locality, and
thus very distinct races would soon originate.

The important physiological effects of crossing breeds or strains, and
the part this plays in the economy of nature, will be explained in a
future chapter.

_Concluding Remarks._

The examples of variation now adduced--and these might have been almost
indefinitely increased--will suffice to show that there is hardly an
organ or a quality in plants or animals which has not been observed to
vary; and further, that whenever any of these variations have been
useful to man he has been able to increase them to a marvellous extent
by the simple process of always preserving the best varieties to breed
from. Along with these larger variations others of smaller amount
occasionally appear, sometimes in external, sometimes in internal
characters, the very bones of the skeleton often changing slightly in
form, size, or number; but as these secondary characters have been of no
use to man, and have not been specially selected by him, they have,
usually, not been developed to any great amount except when they have
been closely dependent on those external characters which he h