COMPARATIVE ANATOMY 



95 



reduced to a mere vestige of 

 what we see in the apes, the 

 nearest relatives we have (Plate 



37)- 



The eyes of some cave- 

 dwelling animals are among 

 our best examples of vestigial 

 structures. In Mammoth Cave, 

 for example, there is an under- 

 ground river of considerable 

 size in which are found fish 

 and Crustacea whose eyes are 

 in different stages of degenera- 

 tion (Fig. 18). Of course, liv- 

 ing in total darkness as these 



animals do, they can have no use for eyes. The presence 

 of eyes in a vestigial condition is an indication of the fact 



that these cave -dwelling 

 species are descended from 

 forms which once lived in 

 the outer world. As eyes 

 are useless to animals living 

 in the dark, natural selection 

 of course no longer will keep 

 the eyes perfect, and the 

 degeneration begun by the 

 withdrawal of natural selec- 

 tion will still further, 



FIG. 17. Muscles of the human ear. 

 From Gray's Anatoniv. 



FIG. 18. Three fishes, showing stages in the 

 loss of eyes and color. A. Dismal Swamp fish 



(ChologiUter avetus), thought to be the ancestor ^ "i. ' nnSltlVP dlS- 



of the blind fish. B. Agassiz's cave fish (Choi*- Cal 



of the blind fish. B. Agassiz's 



faster agassui). C. Cave blind fish ( Typklich- _ r l vnntno .p f- n nnv c n pripS to 



thys sitbterraneus). From Jordan and Kellogg's advantage ail\ bpt 



Animal Life, bv the courtesy of the authors and i 



waste nutriment on useless 



