This view accounts also for the wide distribution of the blind fishes. The ancestry of the Amblyopsidæ we may assume to have had a tendency to seek dark places wherever found, and incipient blind forms would thus arise over their entire distribution. The structural differences between Troglichthys and Typhlichthys argue in favor of this, and certainly the fearless, conspicuous blind fish as at present developed would have no chance of surviving in the open water. Their wide distribution after their present characters had been assumed, except through subterranean waters, would be out of the question entirely. The same would not be true of the incipient cave forms when they had reached the stage at present found in Chologaster. It will be recalled that Chologaster, and even the blind forms, have the habit of hiding underneath boards and in the darker sides of an aquarium. These dark-seeking creatures would, on the other hand, be especially well fitted to become distributed in caves throughout their habitat. S. Garman's able argument for the single origin and dispersal of the blind fishes through epigsean waters was based on the supposition that the cis-Mississippi and trans-Mississippi forms were identical. The differences between these species are such as to warrant not only that they have been independently segregated, but that they are descended from different genera. The external differences between these species are trifling, but this was to be expected in an environment where all the elements that make for external color marking are lacking. The similarity between Typhlichthys and Amblyopsis is so great that the former has been considered to be the young of the latter.
Judging from the structure of the eye and the color of the skin, Troglichthys has been longest established in caves. Amblyopsis came next, and Typhlichthys is a later addition to the blind cave fauna.
