so
much so, that on parts of the Continent persons are warned not to keep
white pigeons, as being the most liable to destruction. Hence Natural
Selection might be effective in giving the proper color to each kind of
grouse, and in keeping that color, when once acquired, true and
constant. Nor ought we to think that the occasional destruction of an
animal of any particular color would produce little effect: we should
remember how essential it is in a flock of white sheep to destroy a lamb
with the faintest trace of black. We have seen how the color of hogs
which feed on the "paint-root" in Virginia, determines whether they
shall live or die. In plants, the down on the fruit and the color of the
flesh are considered by botanists as characters of the most trifling
importance; yet we hear from an excellent horticulturist, Downing, that
in the United States smooth-skinned fruits suffer far more from a
beetle, a curculio, than those with down; that purple plums suffer far
more from a certain disease than yellow plums; whereas another disease
attacks yellow-fleshed peaches far more than those with other colored
flesh. If with all the aids of art, these slight differences make a
great difference in cultivating the several varieties, assuredly, in a
state of nature, where the trees would have to struggle with other trees
and with a host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or a purple-fleshed
fruit, should succeed.

In looking at many small points of difference between species, which, as
far as our ignorance permits us to judge, seem quite unimportant, we
must not forget that climate, food, etc., have no doubt produced some
direct effect. It is also necessary to bear in mind that owing to the
law of correlation, when one part varies, and the variations are
accumulated through Natural Selection, other modifications, often of the
most unexpected nature, will ensue.

As we see that those variations which under domestication appear at any
particular period of life, tend to reappear in the offspring at the same
period;--for instance, in the shape, size, and flavor of the seeds of
the many varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the
eggs of poultry, and in the color of the down of their chickens; in the
horns of our sheep and cattle when nearly adult; so in a state of nature
Natural