Page:Popular Science Monthly Volume 57.djvu/66

56, 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: "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."

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.

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.

a. The prime requisite for a candidate to underground existence is a negative reaction to light. We found that even the epigæan Chologaster is negatively heliotropic.

b. It must also be evident that a fish depending on its sight