Page:Popular Science Monthly Volume 36.djvu/408

394 elsewhere, illustrate clearly the theory that certain subterranean forms, living deep in the soil, under stones in the cave regions of both hemispheres, especially in France and Austria, have been carried into caves, have survived the loss of out-of-door conditions, becoming adapted to the new and strange environment, losing their eyes totally or in part from disuse of those organs, and have bred true to the new specific characters thus established, and are now as unchangeable as the physical conditions in which they live.

The cave spiders in all important respects exemplify the same rule. They belong to, or are closely allied to, genera rich in species in the cavernous regions they inhabit, and which live in dark places. Although scarcely necessary in its changed environment, where there are no hydrographic changes, no winter and summer, and few enemies to contend with, the most aberrant form, the completely eyeless Anthrobia of Mammoth Cave, still spins a silk cocoon around its eggs; while in Weyer's Cave Nesticus pellidus Emerton spins a cocoon for its eggs; and either this species or its fellow troglodyte, Linyphia incerta Emerton, or both species, spin a weak, irregular web, consisting of a few threads. Is not this a useless habit, a simple survival of ancestral traits?

It was noticed that the number of individuals of different species was greater in the smaller shallower caves, such as the Weyer and Carter Caverns; each of these groups of caves has three species, while in Mammoth Cave there is but one, and the individuals are less common. Moreover, all are darker than Anthrobia, all have eyes, and the number of eyes is variable. These facts show that Anthrobia and the eyed forms have originated from species living in partial darkness at or near the mouths of the caverns. In Mr. Emerton's description of Linyphia incerta it will be seen how variable are the number of eyes. From this it may be inferred that the specific characters of this form, as regards the eyes at least, have not been firmly established, and hence it has only recently become a true troglodyte.

In the foregoing examples we have as yet not discovered in this country any connecting links between the eyed and blind or eyeless species of cave animals. But in a series of specimens of a cave myriapod, Pseudotremia cavernarum, which is abundant in the Wyandotte and Carter Caves, we have what we regard as good, if not complete, evidence that this cave form has directly originated from a common and widely distributed out-of-door form. The cave Pseudotremia has black eyes, composed of from twelve to fifteen facets arranged in a triangular area; of one hundred and fifty specimens examined none were found to be eyeless. In a large cave like Wyandotte there is little variation in this species as regards size, proportion, or color (being white with a