jurious 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.

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 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 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 twilight; and, last
of all, those destined for total darkness." 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 compensation 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