We see
nothing of these slow changes in progress, until the hand of time has
marked the long lapse of ages, and then so imperfect is our view into
long past geological ages, that we only see that the forms of life are
now different from what they formerly were.
Although natural selection can act only through and for the good of
each being, yet characters and structures, which we are apt to
consider as of very trifling importance, may thus be acted on. When we
see leaf-eating insects green, and bark-feeders mottled-grey; the
alpine ptarmigan white in winter, the red-grouse the colour of
heather, and the black-grouse that of peaty earth, we must believe
that these tints are of service to these birds and insects in
preserving them from danger. 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 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.
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