1. The explanation of Lankester seems either a pleasantry or the most unwarranted speculation. He says: “Supposing a number of some species of Arthropod or fish to be swept into a cavern or to be carried from less to greater depths in the sea, those individuals with perfect eyes would follow the glimmer of light, and eventually escape to the outer air or the shallower depths, leaving behind those with imperfect eyes to breed in the dark place. A natural selection would thus be effected. In every succeeding generation this would be the case, and even those with weak but still seeing eyes would in the course of time escape, until only a pure race of eyeless or blind animals would be left in the cavern or deep sea.”
Fig. 7.—Lateral view of Amblyopsis, showing the location of the tactile ridges.
This process does not, of course, account for the degeneration of the eye beyond blindness. But, aside from this objection, the humor of his “glimmer of light” impresses itself very forcibly on one after spending a day in following the devious windings of a living cave, not to mention his tendency in cave animals, which are negatively heliotropic, to follow it. There are other objections. Fishes are annually swept into the caves, but they are not able to establish themselves in them. To do this they must have peculiar habits, special methods of feeding and mating before a successful colonization of caverns can become successful. Further, if the origin of the cave fauna is due to accident, the accident must have happened to four species out of six of the Amblyopsidæ, while none of the numerous other species of fishes about the caves met with the same accident.
2. The second explanation is that of Herbert Spencer:[E] “The existence of these blind cave animals can be accounted for only by supposing their remote ancestors began making excursions into the caves, and, finding it profitable, extended them, generation after generation, farther in, undergoing the required adaptations little by little.”
[E] Popular Science Monthly, vol. xliii, pp. 487, 488.
This second view has been modified by H. Garman in so far as he supposes the adaptations to do without eyes and consequent degeneration of eyes to occur anywhere where a species has no use for eyes, enumerating burrowing animals and parasitic animals, concluding that “the origin of the cave species (nonaquatic especially) of Kentucky were probably already adjusted to a life in the earth before the caves were formed.” In this modified sense, Spencer’s view is directly applicable to the Amblyopsidæ. Hamann goes so far as to suppose that darkness itself is not the primary cause of degeneration, but unknown factors in the animal itself.
Fig. 8.—Lateral view of Chologaster papilliferus, showing the location of the tactile ridges.
The three things to be considered in this connection are (a) the habit of the cave form, (b) the modifications to enable the form to do without the use of light, and (c) the structure of the eye as a result of a and b.