CHAP V.] EFFECTS OF USE AND DISUSE. 103 



month to a graduated light, acquired a dim perception of 

 objects. 



It is difficult to imagine 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 animals having been 

 separately created for the American and European caverns, very 

 close similarity in their organisation and affinities might have 

 oeen 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 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 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 geographically limited 

 faunas of the adjacent tracts, and which, as they extended them- 

 selves into darkness, have been accommodated to surrounding 

 circumstances. 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 darkness, and whose formation is quite peculiar." These 

 remarks of Schiodte's, it should be understood, apply not to the 

 same, but to distinct species. By the time that an animal had 

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

 pensation for blindness. Notwithstanding such modifications, we 

 might expect still to see in the cave-animals of America, affinities 

 to the other inhabitants of that continent, 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 

 Professor Dana ; and some of the European cave-insects are very 

 closely allied to those of the surrounding country. It would be 

 difficult to give any rational explanation of the affinities of the 

 blind cave-animals to the other inhabitants of the two continents 

 on the ordinary view of their independent creation. That several 

 of the inhabitants of the caves of the Old and New Worlds should 



