" 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 themselves 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.