629] remarks with respect to the
former, "There is another principle, namely, that the more a type has
entered into a state of variation, the greater is its tendency to continue
doing so; and the more it has varied from the original type, the more it is
disposed to vary still farther." We have, indeed, already discussed this
latter point when treating of the power which man possesses, through
selection, of continually augmenting in the same direction each
modification; for this power depends on continued variability of the same
general kind. The most celebrated horticulturist in France, namely,
Vilmorin,[630] even maintains that, when any particular variation is
desired, the first step is to get the plant to vary in any manner whatever,
and to go on selecting the most variable individuals, even though they vary
in the wrong direction; for the fixed character of the species being once
broken, the desired variation will sooner or later appear.

As nearly all our animals were domesticated at an extremely remote epoch,
we cannot, of course, say whether they varied quickly or slowly when first
subjected to new conditions. But Dr. Bachman[631] states that he has seen
turkeys raised from the eggs of the wild species lose their metallic tints
and become spotted with white in the third generation. Mr. Yarrell many
years ago informed me that the wild ducks bred on the ponds in St. James's
Park, which had never been crossed, as it is believed, with domestic ducks,
lost their true plumage after a few generations. An excellent
observer,[632] who has often reared birds from the eggs of the wild duck,
and who took precautions {263} that there should be no crossing with
domestic breeds, has given, as previously stated, full details on the
changes which they gradually undergo. He found that he could not breed
these wild ducks true for more than five or six generations, "as they then
proved so much less beautiful. The white collar round the neck of the
mallard became much broader and more irregular, and white feathers appeared
in the ducklings' wings." They increased also in size of body; their legs
became less fine, and they lost their elegant carriage. Fresh eggs were
then procured from wild birds; but again the same result followed. In these
cases of the duck and turkey we see that animals, like plants, do not
depart from their primitive type until they have been subjected during
several generations to domestication. On the other hand,