282 



ANIMAL LIFE 



hand of man has carried the golden trout to other little 

 mountain torrents, where it thrives as well as in the one 

 where its peculiarities were first acquired. 



Other cases of this nature are found among the blind 

 fishes of the caves in different parts of the world (Fig. 172). 



In general, caves are 

 formed by the ero- 

 sion or wearing of 

 underground rivers. 

 These streams are 

 either clear and cold, 

 and when they issue 

 to the surface those 

 fishes which like cold 

 and shaded waters 

 are likely to enter 

 them. But to have 

 eyes in absolute dark- 

 ness, in which no use 

 can be made of them, 

 is a disadvantage in 

 the struggle for life. 

 Hence the eyed species die or withdraw, while those in which 

 the eye grows less from generation to generation, until its 

 function is finally lost, are the ones which survive. By such 

 processes the blind fishes in the limestone caves of Ken- 

 tucky, Indiana, Tennessee, and Missouri have been formed. 



FIG. 172. Fishes showing stages in the loss of eyes 

 and color. A, Dismal Swamp fish ( Chologaster 

 avetus), ancestor of the blind fish ; B, Agassiz's 

 cave fish ( Chologaster agassizi) ; C, cave blind 

 fish ( Typhlichthys subterraneus). 



rather when specimens showing intergradation of characters are known, 

 the word sub-species is used. The word variety has much the same 

 meaning when used for a subdivision of a species, but it is a term 

 denned with less exactness. Thus the common fox ( Vulpes pennsyl- 

 vanicus) is a distinct species, being separate from the arctic fox or the 

 gray fox or the fox of Europe. The cross fox ( Vulpes pennsylvanicus 

 decussatus) is called a sub-species, as is the silver fox ( Vulpes pennsyl- 

 vanicus argentatus), because these intergrade perfectly with the common 

 red fox. 



