AN OULLINE OF THE THEORY: 19 
to hold on to stones at the bottom, and thus to retain 
their position while picking up insects, shells, etc. As 
they frequent chiefly the most rapid and boisterous tor- 
rents, 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 struc- 
ture, 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 generalized an- 
cestor 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 mollusks as 
they could reach in shallow water. When food becomes 
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 pos- 
sessed 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 
* Note characteristic phrase ‘‘We may suppose that,—.”. G. 
