320 Adaptation to Environment 



was the cause of the degeneration of the eyes. A closer 

 investigation leads, however, to a different explanation. 

 Eigenmann has shown that of the species of salamanders 

 living habitually in North American caves, two have 

 apparently quite normal eyes. They are Spelerpes 

 maculicauda and Spelerpes stejnegeri. Two others liv- 

 ing in caves have quite degenerate eyes, Typhlotriton 

 spelcsus and Typhlomolge rathbuni. If disuse is the 

 direct cause of blindness we must inquire why Spelerpes 

 is not blind. 



Another difficulty arises from the fact that a blind 

 fish Typhlogobius is found in the open (on the coast of 

 southern California) in shallow water, where it lives 

 under rocks in holes occupied by shrimps. The 

 question must again be raised: How can it happen 

 that in spite of exposure to light Typhlogobius is 

 blind? 



The most important fact is perhaps the one found 

 by Eigenmann in the fishes of the family of Amblyop- 

 sidae. Six species of this group live permanently in 

 caves, are not found in the open, and have abnormal 

 eyes, while one lives permanently in the open, is never 

 found in caves, and one comes from subterranean 

 springs. The one form which is found only in the 

 open, Chologaster cornutus, has a simplified retina as 

 well as a comparatively small eye, in other words, its 

 eye is not normal. This indicates the possibility that 

 the other representatives which are found only in 



