LAWS OF VARIATION 126 



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

 crabs the foot-stalk for the 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 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 in- 

 creased the size of the eyes ; whereas with all the 

 other inhabitants of the caves, disuse by itself seems to 

 have done its work. 



It is difficult to imagine conditions of life more 

 similar than deep limestone caverns under a nearly 

 similar climate ; so that on the common view of the 

 blind animals having been separately created for the 

 American and European caverns, close similarity in 

 their organisation and affinities might have been ex- 

 pected ; but, as Schiodte and others have remarked, 

 this is not the case, and the cave-insects of the two 

 continents are not more closely allied than might have 

 been anticipated from the general resemblance of the 

 other inhabitants of North America and Europe. 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. We 

 have some evidence of this gradation of habit ; for, 

 as Schiodte remarks, f animals not far remote from 

 ordinary forms, prepare the transition from light to 

 darkness. Next follow those that are constructed for 

 twilight ; and, last of all, those destined for total dark- 



