ADAPTATION 189 



With this change in the environment, with the 

 disappearance of Horse Cave River from the 

 surface, its inhabitants were compelled to mi- 

 grate. They moved in two directions to adapted 

 environments. The shore-fishes, channel-fishes, 

 etc., depending on light to find their food and 

 mates, moved out to the Green River, where their 

 descendants live to the present day. The fishes 

 negatively heliotropic, nocturnal, or stereotropic, 

 moved into the holes dissolved in the bottom of 

 the river, followed its subterranean development, 

 and their descendants live to-day in the stream 

 which now flows entirely below the valley. They 

 are colorless and all but eyeless, and have, no 

 doubt, acquired this exaggerated adaptation to 

 their present abode since their immigration. The 

 major adaptation to the cave existence, the power 

 of finding their food and mates without the use 

 of light, they possessed before the formation of 

 the caves, and it is responsible for their present 

 habitat. 



Primarily blind fishes do not have degenerate 

 eyes because they live in caves, but they Uve in 

 caves because their ancestors were adjusted to do 

 without the use of eyes. The degeneration and 

 disappearance of their eyes form another matter. 



Wherever in the past environments arose lack- 

 ing light, they became, and still are, the gather- 

 ing place of those not dependent upon light. 



The Cuban blind fishes offer another example 

 of the concomitant development of a peculiar 



