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POPULAR SCIENCE MONTHLY. 



opsidse, 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 



Fig. 8.— Lateral view of Chologaster papilliferus, showing the location of the tactile ridges. 



for eyes, enumerating burrowing animals and parasitic animals, 

 concluding that " the origin of the cave species (nonaquatic espe- 

 cially) 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 Amblyopsidse. 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, (6) 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 exist- 

 ence is a negative reaction to light. We found that even the 

 epigsean Chologaster is negatively heliotropic. 



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



* Popular Science Monthly, vol. xliii, pp. 487, 488. 



