V NATURAL SELECTION 117 



etc. 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 mnters. 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, AATens, etc., 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 hairy 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 the winter ; and as such places afforded them much 

 protection from enemies and ample shelter for their nests and 

 young, further adaptations would occur, till the wonderful 

 power of diving and flying under water was acquired by a 

 true land-bird. 



That such habits might be acquired under stress of need 

 is rendered highly probable by the facts stated by the well- 

 known American naturalist. Dr. Abbott. He says that "the 

 water -thrushes (Seiurus sp.) all wade in water, a-nd often, 

 seeing minute mollusca on the bottom of the stream, plunge 

 both head and neck beneath the surface, so that often, for 



