THE AQUATIC VERTEBRATES 1065 



Cave River from the surface, its inhabitants had to migrate. 

 They moved in two directions to adapted environments. The 

 shore fishes and channel fishes moved out to the Green River 

 where their descendants live to the present day. The negatively 

 heliotropic, nocturnal, or stereotropic fishes moved into the holes 

 dissolved in the bottom of the river. Their descendants live, at 

 the present time, in the stream below or within the valley. They 

 are colorless and all but eyeless, and have, no doubt, acquired this 

 exaggerated adaptation to their present abode since their immi- 

 gration. But the major adaptation to the cave existence they 

 possessed before the formation of the caves, and it was responsible 

 for their migration to their present habitat. 



As was pointed out in the opening paragraphs, the fauna of the 

 glaciated region of North America has similarly been derived by 

 immigration from the south and possibly the ocean and Siberia to 

 the north and west. The Great Lake Basin has but twenty-seven 

 of its one hundred fifty- two species peculiar to itself; five are but 

 varieties of more southern species and the remaining twenty-one 

 more than represent the extent to which its fauna has become 

 adapted in this area for some of them (eight Salmonidae and eight 

 Cottidae) are cold-water species that may have been crowded out of 

 the region south of the basin by the encroaching heat after the 

 passing of the glacial epoch. 



The selective migration to adapted locations must be added to 

 the factors contributing to the origin of adapted faunas. This 

 factor, "change of location," is as important to the origin of 

 adapted faunas as the "change of function" to the origin of 

 adaptive structures. Innumerable minor adaptations to heat, 

 sediment, light, food, and to the peculiar combinations found in 

 each selected locality have no doubt arisen in such localities. 



