80 DISCEEPANCIES. 



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, I attribute their 

 loss wholly to disuse. In one of the blind animals, 

 namely, the cave-rat, the eyes are of immense size ; and 

 Professor Silliman thought that it regained, after living 

 some days in the light, some slight power of vision. In 

 the same manner as, in Madeira, the wings of some of 

 the insects have been enlarged, and the wings of others 

 have been reduced, by natural selection aided by use and 

 disuse, so in the case of the cave-rat, natural selection 

 seems to have struggled with the loss of light and to have 

 increased the size of the eyes ; whereas, with all the other 

 inhabitants of the caves, disuse by itself seems to have 

 done its work. 



" .... On my view, we must suppose that American 

 animals, having 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. "VVe have 

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

 remarks, ' animals not far remote from ordinary forms, 

 prepare the transition from light to darkness. Next 

 follow those that are constructed for twUight ; and, last 

 of an, those destined for total darkness. By the time 

 that an animal has reached, after numberless generations, 

 the deepest recesses, disuse will on this view have more 

 or less perfectly obliterated its eyes, and natural selection 

 will often have effected other changes, such as an increase 



