116 DARWINISM chap. 



are the species that Avould 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 Avould especially be the 

 case when the change of conditions was rather rapid, and Avhen 

 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 afibrded by the curious 

 little birds called dippers or water -ouzels, forming the genus 

 Cinclus and the family Cinclidte of naturalists. These birds 

 are something like small thrushes, with very short Avings 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 flying 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 uj) insects, shells. 



