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 Cinclidae 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 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.