138 
LAWS OF VAEIATION. 
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 bhnd 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 
