ADAPTATION 187 



The warm-water species are warm-water species 

 not because their individuals are incapable of 

 entering cold water, for they do, but because 

 their eggs will not develop in anything but water 

 much warmer than that in which the eggs of cold- 

 water species develop. Their eggs are quickly 

 developed, they are adjusted to fluctuations in 

 temperature, and they respond to such fluctua- 

 tions in temperature by hastening or slowing 

 their rate of development. 



The origin and nlodification of the cave fauna 

 give a concrete example of the change of loca- 

 tion resulting from predestined major adapta- 

 tions and subsequent minor adjustments. Caves, 

 at the present time, are being colonized by immi- 

 gration of salamanders of the genus Spelerpes 

 and other animals that have become adapted to 

 a cave existence while living in the dark under 

 rocks, bark, and in other similar places. The 

 adaptation to the conditions of cave existence in 

 this case determines the change of location when- 

 ever a cave presents itself. 



That minor adaptations occur in these after 

 they have become exclusively cave forms is shown 

 by the structure of the permanent cave salaman- 

 ders of Missouri and Texas. These have, in 

 large measure, lost their color, and have degen- 

 erate eyes. 



Not infrequently where wehave extreme adap- 

 tations to a particular and a peculiar environ- 

 ment, such as are found in the blind fishes to the 



