being the most liable to destruction. Hence I can see no reason to
doubt that natural selection might be most effective in giving the
proper colour to each kind of grouse, and in keeping that colour, when
once acquired, true and constant. Nor ought we to think that the
occasional destruction of an animal of any particular colour would
produce little effect: we should remember how essential it is in a
flock of white sheep to destroy every lamb with the faintest trace of
black. In plants the down on the fruit and the colour 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 coloured 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 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 to be quite
unimportant, we must not forget that climate, food, etc., probably
produce some slight and direct effect. It is, however, far more
necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is
modified through variation, and the modifications are accumulated by
natural selection for
the good of the being, will cause other modifications, often of the
most unexpected nature.
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 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
colour of the down of their chickens; in the horns of our sheep and
cattle when nearly adult; so in a state of nature, natural selection
will be enabled to act on and modify organic beings at any age, by the
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