ef that the
amount of possible variation is a strictly limited quantity, is likewise
a simple assumption.

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, 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 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.

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