Chap. V.] EFFECTS OF USE AND DISUSE. 171 



is gone ; — the stand for the telescope is there, though 

 the telescope with its glasses has been lost. As it is 

 difficult to imagine that eyes, though useless, could be 

 in any way injurious to animals living in darkness, 

 their loss may be attributed to disuse. In one of the 

 blind animals, namely, the cave-rat (Neotoma), two of 

 which were captured by Professor Silliman at above 

 half a mile distance from the mouth of the cave, and 

 therefore not in the profoundest depths, the eyes were 

 lustrous and of large size ; and these animals, as I am 

 informed by Professor Silliman, after having been 

 exposed for about a month to a graduated light, 

 acquired a dim perception of objects. 



It is difficult to imagine conditions of life more 

 similar than deep limestone caverns under a nearly 

 similar climate; so that, in accordance with the old 

 view of the blind animals having been separately 

 created for the American and European caverns, very 

 close similarity in their organisation and affinities 

 might have been expected. This is certainly not the 

 case if we look at the two whole faunas ; and with 

 respect to the insects alone, Schiodte has remarked, 

 "We are accordingly prevented from considering the 

 entire phenomenon in any other light than something 

 purely local, and the similarity which is exhibited in a 

 few forms between the Mammoth cave (in Kentucky) 

 and the caves in Carniola, otherwise than as a very 

 plain expression of that analogy which subsists 

 generally between the fauna of Europe and of North 

 America." On my view we must suppose that 

 American animals, having in most cases ordinary 

 powers of vision, slowly migrated by successive 

 generations from the outer world into the deeper and 



