The colour was, therefore, gradually changed
by the process of variation and natural selection; and as the birds
obtained ample shelter among the heather which clothes so many of our
moorlands, it became useful for them to assimilate with its brown and
dusky stems and withered flowers rather than with the snow of the higher
mountains. An interesting confirmation of this change having really
occurred is afforded by the occasional occurrence in Scotland of birds
with a considerable amount of white in the winter plumage. This is
considered to be a case of reversion to the ancestral type, just as the
slaty colours and banded wings of the wild rock-pigeon sometimes
reappear in our fancy breeds of domestic pigeons.[38]
The principle of "divergence of character" pervades all nature from the
lowest groups to the highest, as may be well seen in the class of birds.
Among our native species we see it well marked in the different species
of titmice, pipits, and chats. The great titmouse (Parus major) by its
larger size and stronger bill is adapted to feed on larger insects, and
is even said sometimes to kill small and weak birds.
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