Grouse, if not destroyed at some period of their lives, would
increase in countless numbers; they are known to suffer largely from birds
of prey; and hawks are guided by eyesight to their prey,--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 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 a lamb with the faintest trace of
black. We have seen how the colour 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 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 a
purple-fleshed fruit, should succeed.
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