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 ^Wnter 

 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 snoAV 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 

 "svinter 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 oiu* fancy 

 breeds of domestic pigeons.^ 



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 Ave 

 see it well marked in the different species of titmice, pijjits, 

 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 

 ^ Yarrell's British Birds, fourtli edition, vol. iii. p. 77. 



