lobes or optic nerves; if they are wanting at all, they are totally. abolished; there is not a series of individuals with these organs in different degrees of development corresponding to the rudimentary conditions of the eye. The atrophy is comparatively rapid, sudden, and wholesale. On the other hand, we have series, as in Cæcidotæa or Chthonius, where there is but a single, or two or three, or several crystalline lenses, partially enveloped in pigment.
These varying degrees of development in the peripheral parts of the eye prove that the animals entered the caves at different periods, and have been exposed for different lengths of time to the loss of light. For example, those individuals of Chthonius Packardii which live in the Labyrinth of Mammoth Cave are eyeless, or have merely pigment spots; those collected in the Rotunda (which is much nearer the entrance to the cave) have eyes, or at least lenses and a retina. While most individuals of the Cæcidotæa are eyeless, a few have rudimentary eyes. Thus, in the differing conditions of the eyes in different individuals, we have an epitome of the developmental history of the genus Cæcidotæa and its species. Certain Aselli borne into caves or introduced into subterranean streams feeding deep, dark wells, losing the stimulus of the light, begin to lose their eyes and the power of sight. The first step is the decrease in the number of facets and corresponding lenses and retina; after a few generations—perhaps in four or five—the facets become reduced to only four or five; the eye is then useless; then all at once, perhaps after only two or three generations, as a result of disuse, there is a failure in forming images on the retina, and those complicated, elaborate structures, the optic ganglion and optic nerve, suddenly break down and are absorbed, though the external eye still exists in a rudimentary state. These imperfect lenses and retinæ, like all rudimentary organs throughout the animal world, are like ancient, decayed sign-posts, pointing out some nearly obliterated path now unworn and disused. The result of change of environment, with disuse and atrophy of the organs of vision, together with the inheritance of these defects and their establishment as fixed specific and even generic characters, results in the creation of a new natural genus with its assemblage of species, and, if we include all the cave animals thus produced, the creation of a new fauna. It would be a thorough test of the theory of descent if we could keep these creatures in confinement, exposed first to twilight and then to the full light of day, and endeavor to breed a few generations of these blind animals and ascertain whether their descendants would not revert to the original ancestral eyed forms. The Cæcidotæa would perhaps be the best subject for such an experiment; it is so abundant and easy to breed. That the Cæcidotæa has been evolved
