188 ADAPTATION 



caves, or the ability of ichneumon flies to detect 

 and lay their eggs in deeply hidden grubs, we do 

 not really need to account for the extreme adap- 

 tation to the extreme environment. The envi- 

 ronment and the adaptation may have developed 

 together, as armor-piercing projectiles and armor 

 have so developed together. An illustration is 

 found in the origin of the cave fishes of Ken- 

 tucky, and still more of those of Cuba. 



The cave fauna of Kentucky, so highly 

 adapted it would be hopelessly lost if removed 

 from its peculiar environment, is the result of 

 (Selective emigration, immigration, and local 

 adaptation. It has become adjusted with the 

 development of the environment it inhabits. At 

 Horse Cave, Ky., a wide valley extends north 

 and south. Tributary valleys come from the 

 east and west. The hills bordering these valleys 

 are limestone capped with sandstone. The 

 north-and-south valley was formed by the Horse 

 Cave River, which originally flowed over sand- 

 stone like that capping the bordering hills. No 

 doubt it had a fauna as varied as that of any 

 surface stream. The stream first cut through 

 the sandstone, then into the hmestone, in which 

 it gradually dissolved an underground channel. 

 To-day not a sign can be seen on the surface of 

 the streams that are responsible for the valleys 

 about Horse Cave. At least one of them rushes 

 through lofty chambers one hundred and eighty- 

 five feet beneath the streets of Horse Cave. 



