ll freshwater shells. These birds,
although not far removed in structure from thrushes and wrens, have the
extraordinary power of flying under water; for such, according to the
best observers, is their process of diving in search of their prey,
their dense and somewhat fibrous 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, 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 acq