116 
DARWINISM 
CHAP. 
are the species that would be most likely to be so modified, 
while others, not becoming modified, would succumb to the 
changed conditions and become extinct. 
The most important condition of all is, undoubtedly, that 
variations should occur of sufficient amount, of a sufficiently 
diverse character, and in a large number of individuals, so as 
to afford ample materials for natural selection to act upon ; 
and this, we have seen, does occur in most, if not in all, large, 
wide-ranging, and dominant species. From some of these, 
therefore, the new species adapted to the changed conditions 
would usually be derived; and this would especially be the 
case when the change of conditions was rather rapid, and when 
a correspondingly rapid modification could alone save some 
species from extinction. But when the change was very 
gradual, then even less abundant and less widely distributed 
species might become modified into new forms, more especially 
if the extinction of many of the rarer species left vacant 
places in the economy of nature. 
Probable Origin of the Dippers. 
An excellent example of how a limited group of species 
has been able to maintain itself by adaptation to one of 
these “vacant places” in nature, is afforded by the curious 
little birds called dippers or water-ouzels, forming the genus 
Cinclus and the family Cinclidse of naturalists. These birds 
are something like small thrushes, with very short wings and 
tail, and very dense plumage. They frequent, exclusively, 
mountain torrents in the northern hemisphere, and obtain 
their food entirely in the water, consisting, as it does, of water- 
beetles, caddis-worms and other insect-larvae, as well as 
numerous small fresh-water shells. These birds, although not 
far removed in structure from thrushes and wrens, have the 
extraordinary power of Hying under water; for such, ac¬ 
cording 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, 
