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 antenn 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 are very closely
allied to those of the surrounding country. It would be most 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 be closely
related, we might expect from the well-known relationship of most of
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