V 
NATURAL SELECTION 
107 
naturalists, is entirely confined to the British Isles. It is, 
however, very closely allied to the willow grouse (Lagopus 
albus), a bird which ranges all over Europe, Northern Asia, 
and North America, but which, unlike our species, changes to 
white in winter. No difference in form or structure can be 
detected between the two birds, but as they differ so decidedly 
in colour — our species being usually rather darker in winter 
than in summer, while there are also slight differences in the 
call-note and in habits, — the two species are generally con¬ 
sidered to be distinct. The differences, however, are so 
clearly adaptations to changed conditions that we can hardly 
doubt that, during the early part of the glacial period, when 
our islands were united to the continent, our grouse was 
identical with that of the rest of Europe. But when the cold 
passed away and our islands became permanently separated 
from the mainland, with a mild and equable climate and very 
little snow in winter, the change to white at that season 
became hurtful, rendering the birds more conspicuous instead 
of serving as a means of concealment. The colour was, there¬ 
fore, 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 . 1 
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. The 
smaller and weaker coal titmouse (Parus ater) has adopted a 
1 Yarrell’s British Birds, fourth edition, vol. iii. p. 77. 
