1064 FRESH-WATER BIOLOGY 



weakened, so that obstacles on the way to the spawning ground 

 are less Uable to be surmounted than if the fish were in full vigor. 

 In South America small catfishes Uve in the gills of larger catfishes. 



Origin of Adapted Faunas. — It has been shown that the major 

 adaptations of fresh-water fishes were acquired by their ancestors 

 before they were ehgible to a fresh-water existence. 



The origin and modification of the cave fauna gives us a con- 

 crete example of the change of location, resulting from predestined 

 adaptation and of subsequent minor adaptations. Caves are at the 

 present time being colonized by the immigration of salamanders of 

 the genus Sprier pes and other animals that have become adapted 

 to a cave existence through their habit of living in the dark under 

 rocks, bark, and other similar places. The adaptation to the con- 

 ditions of cave existence in this case determines the change of 

 location when the opportunity arises. 



That minor adaptations will occur in these after they have 

 become exclusively cave forms, is shown by the structure of the 

 permanent cave salamanders of Missouri and Texas. These have 

 in large measure lost their color and have degenerate eyes. 



A somewhat more complex example is furnished by the history of 

 the Horse Cave River. At Horse Cave, Kentucky, 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 that originally flowed over sandstone like that capping 

 the bordering hills. No doubt it had a fauna as varied as that of 

 any surface stream. The stream cut first through the sandstone, 

 then through the limestone. When it had reached the easily dis- 

 solving limestone of the Kentucky caves and Green River had cut 

 some distance below the surface. of this, some part of its water, 

 later more and more, found its way to the Green River by under- 

 ground channels. To-day not a sign is seen on the surface of the 

 streams that are responsible for the valley about Horse Cave. At 

 least one of them rushes through lofty chambers one hundred 

 eighty-five feet beneath the streets of Horse Cave City. With 

 this change in the environment, with the disappearance of Horse 



