184 ADAPTATION 



The latter is a comparatively simple question. 

 No one, for a moment, would claim that the 

 entire fauna of any particular area of land, or 

 river, or ocean arose where it resides, — ^became 

 adapted in its present habitat. Adapted faunas 

 are only in small part autochthonous; in large 

 part they are made up by selective migration. 



III. ORIGIN OF ADAPTED FAUNAS 



For a consideration of the origin of adapted 

 faunas I would invite attention to the fresh water 

 and cave fish-faunas. 



The major conditions distinguishing fresh 

 waters from the ocean as an environment for 

 fishes are these: (1) The fresh water contains a 

 very much smaller per cent of salts in solution 

 than sea water. (2) It is, with few exceptions, 

 in continuous locomotion in one direction. (3) 

 It contains sediment. Minor characters distin- 

 guishing fresh water differ in different locaUties. 



Fresh-water fishes are not a group different 

 from salt-water fishes. Many salt-water fishes 

 can enter fresh water, and we may for present 

 purposes assume that all candidates for fresh- 

 "water existence are adapted or readily adaptable 

 to the fresh water. Adaptations to the second 

 and third of the fresh-water conditions imply pe- 

 culiarities in habit or structure not possessed by 

 all fishes, and these must, in the main, have been 

 acquired by the marine fishes before they could 

 enter and maintain themselves in fresh water. 



