138 LAWS OF VARIATION. Chap. V. 



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. 



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 ani- 

 mals having been separately created for the American 

 and European caverns, close similarity in their organisa- 

 tion and affinities might have been expected ; 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 gra- 

 dation of habit ; for, as Schiodte remarks, " animals 

 not far remote from ordinary forms, prepare the transi- 

 tion from light to darkness. Next follow those that are 

 constructed for twilight ; and, last of all, those destined 

 for total darkness." By the time that an animal had 

 reached, after numberless generations, the deepest re- 

 cesses, disuse will on this view have more or less per- 

 fectly obliterated its eyes, and natural selection will 

 often have effected other changes, such as an increase 

 in the length of the antennae or palpi, as a compensa- 

 tion for blindness. Notwithstanding such modifications, 

 we might expect still to see in the cave-animals of 

 America, affinities to the other inhabitants of that con- 

 tinent, and in those of Europe, to the inhabitants of 

 the European continent. And this is the case with 

 some of the American cave-animals, as I hear from 



