142 EFFECTS OF USE AND DISUSE. Chap. V, 



such case be an advantage ; and if so, natural selection would 

 conslanlly aid the efl'ects of disuse. 



It is well known that several animals, belonging to the 

 most different classes, which inhabit the caves of Carniola and 

 of Kentucky, are blind. In some of the crabs the foot-stalk for 

 tlie eye 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 imagine that eyes, though use- 

 less, could be in any way injurious to animals living in dark- 

 ness, I attribute their loss wholly to disuse. In one of the 

 bUnd animals, namely, the cave-rat (Ncotoma), two of which 

 were cajotured by Prof. Silliman at above half a mile dis- 

 tance fj-om the moutli 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 Prof. 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 tlian 

 deep limestone caverns imdcr a nearly similar climate ; so that, 

 on the common view of the blind animals haWng been sepa- 

 rately created for the American and European caverns, very 

 close similarity in their organization 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 INIannnoth 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 deeper recesses of the Kentucky 

 caves, as did European animals into the caves of Europe. We 

 have some evidence of this gradation of habit ; for, as Schiodte 

 remarks, " We accordingly look upon the subterranean faunas 

 as small ramifications which have penetrated into the earth from 

 the geograjihically limited faunas of the adjacent tracts, and 

 which, as they extended themselves into darkness, have been 

 accommodated to surrounding circumstances. Animals not far 

 remote from ordinary forms, proiwre the transition from light 

 to darkness. Next follow those that are constructed for twi- 



