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 {138} 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 Schioedte 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 Schioedte 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 {139} Professor Dana; and some of
the European cave-insects are very closely allied to those of the
surrounding country