EFFECTS OF USE AND DISUSE. 131 



inflammation of the nictitating membrane. As frequent 

 inflammation of the eyes must be injurious to any animal, 

 and as eyes are certainly not necessary to animals having 

 subterranean habits, a reduction in their size, with the 

 adhesion of the eyelids and growth of fur over them, 

 might in such case be an advantage; and if so, natural 

 selection would aid the effects of disuse. 



It is well known that several animals, belonging to the 

 most different classes, which inhabit the caves of Carniola 

 and Kentucky, are blind. In some of the crabs the 

 foot-stalk for the eyes remains, though the eye is gone; 

 the stand for the telescope is there, though the telescope 

 with its glasses has been lost. As it is difficult to imag- 

 ine 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, tlie 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 ex- 

 posed for about a month to a graduated light, acquired a 

 dim perception of objects. 



It is difficult to imasfine 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 ani- 

 mals having been separately created for the American and 

 European caverns, very close similarity in their organiza- 

 tion and affinities might have been expected. This is cer- 

 tainly 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 ordi- 

 nary powers of vision, slowly migrated by successive gener- 

 ations from the outer world into the deeper and deeper re- 

 cesses of the Kentucky caves, as did European animals into 



