ORIGIN OF CAVE LIFE. 101 



Nevertheless, there is a real cave fauna in the sense that in caves 

 these dark-seeking species are accumulated in relatively greater abun- 

 dance than elsewhere. Furthermore, in caves species have become more 

 highly adapted to cave conditions than elsewhere. In this limited 

 sense there is a real cave fauna hedged about and shut off from the 

 upper world of light. 



Whether cave faunas originate by the accumulation of species 

 highly adapted or by the gradual colonization of species which become 

 gradually adapted to cave life does not seem an essential point. It is 

 certain that the adjustment of the cave fauna occurred in dark and 

 sheltered places. 



The question of where the adaptation to caves has arisen has an 

 important bearing, however, upon the determination of the age of the 

 present cave fauna. Could one be sure that Amhlyopsis spelseus, for 

 example, entered Indiana caves as they were being formed as a very 

 slightly modified or perhaps nearly typical shade-loving fish of open 

 streams, one would have a possible index of the rapidity of modifica- 

 tions in the development of specially numerous and large sense-papillas 

 and of the rate of production of retrograde modifications in the color 

 and the eye. Such a means of getting at the age of the cave fauna 

 would be of doubtful validity unless it could be shown that conditions 

 before the formation of the caves were such that there would be little 

 or no suitable opportunity for the production of shade-inhabiting forms, 

 and to establish this point seems to me well-nigh impossible. 



HOW HAVE CAVE ANIMALS REACHED THEIR PRESENT CONDITION? 



This question necessitates a statement of the peculiarities of cave 

 species before the causes leading to the peculiarities can be satisfac- 

 torily disposed of. 



THE MODIFICATIONS OF CAVE ANIMALS. 



The modifications of cave animals in general need only be referred 

 to. They consist in the possession of more slender bodies, longer 

 appendages, more abundant or more highly developed sense-organs, 

 together with a retrograde development in pigment and organs of 

 vision. 



It is immaterial to the discussion whether these adaptations were 

 acquired in the cave or before the species entered the cave. 



Amhlyopsis is white, has eyes entirely functionless, and has the 

 sense-papillae on the head more highly developed than in its near rela- 

 tives within or outside of caves. Cxcidotea stygia has a more slender 

 body, more slender appendages, a greater number of sensory setae, is 

 white in color, and has mere rudiments of eyes as compared with its 



