44 MODERN IDEAS OF EVOLUTION 



plumage retaining so much air that the water is prevented 

 from touching their bodies, or even from wetting their feathers 

 to any great extent. Their powerful feet and long curved 

 claws enable them to hold on to stones at the bottom, and 

 thus to retain their position while picking up insects, shells, 

 &c. As they frequent chiefly the most rapid and boisterous 

 torrents, among rocks, waterfalls, and huge boulders, the 

 water is never frozen over, and they are thus able to live 

 during the severest winters. Only a very few species of 

 dipper are known, all those of the old world being so 

 closely allied to our British bird that some ornithologists 

 consider them to be merely local races of one species ; 

 while in North America and the Northern Andes there are 

 two other species. 



Here, then, we have a bird, which, in its whole structure, 

 shows a close affinity to the smaller typical perching birds, 

 but which has departed from all its allies in its habits and 

 mode of life, and has secured for itself a place in nature 

 where it has few competitors and few enemies. We may 

 well suppose that, at some remote period, a bird which was 

 perhaps the common and more generalised ancestor of most 

 of our thrushes, warblers, wrens, &c. had spread widely 

 over the great northern continent, and had given rise to 

 numerous varieties adapted to special conditions of life. 

 Among these some took to feeding on the borders of clear 

 streams, picking out such larvae and molluscs as they could 

 reach in shallow water. When food became scarce they 

 would attempt to pick them out of deeper and deeper water, 

 and while doing this in cold weather many would become 

 frozen and starved. But any which possessed denser and 

 more heavy plumage than usual, which was able to keep 

 out the water, would survive ; and thus a race would be 

 formed which would depend more and more on this kind of 

 food. Then, following up the frozen streams into the 

 mountains, they would be able to live there during winter ; 



