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 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, 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 
Avater- thrushes (Seiurus sp.) all wade in water, and often, 
seeing minute mollusca on the bottom of the stream, plunge 
both head and neck beneath the surface, so that often, for 
