ORIGIN OF CAVE LIFE. 105 



The occurrence of animals in caves everywhere and of similarly mod- 

 ified animals in every conceivable situation in which such animals are 

 capable of living suggests that these modifications, like those of the teeth 

 of mammals, may be congenital, and due to the inherent predisposition of 

 protoplasm to produce variations in a given direction, these variations 

 in the case of the animals under consideration forcing them into secluded 

 places, caves, and similar situations, where further modifications may 

 have occurred through similar causes, natural selection serving to elimi- 

 nate individuals modified in an unfavorable way. 



There is a serious difficulty, however, in the way of offering such an 

 explanation for the degeneration of the eyes of the cave fishes, in which 

 it has been conclusively shown (Eigenmann, 1899) that the more active 

 elements, the muscles, retinal elements, etc. , degenerate most, and the 

 passive scleral cartilages little or not at all. But, on the other hand, all 

 cave animals do not suffer a like degeneration of the eye, some of those 

 living under a typical cave environment possessing eyes fairly well de- 

 veloped. These modifications of cave animals are hard to explain on 

 the hypothesis that they are the direct result of the environment when 

 a cave environment is so absolutely uniform. 



To take an illustration, Chologaster agassizii, possessing eyes, lives 

 in Mammoth Cave with Typhlichthys and Amblyopsis, both practically 

 eyeless. It is hard to explain, if degeneration in the two cases is pro- 

 duced by the effects of disuse becoming hereditary, why the same 

 result has not occurred in the other case.* 



The case of Chologaster, a species really highly suited to cave con- 

 ditions, is readily understood if looked at from the standpoint of deter- 

 minate adaptations which suited the animal to cave life but did not 

 involve an extensive degeneration of the eyes. 



There is possibly equally good reason to suppose that the Theridiidss 

 about the mouth of Mayfield's Cave are becoming cave inhabitants 

 through determinate congenital variations which cause them to seek a 

 favorable habitat farther arid farther within the cave as there is to sup- 

 pose that they entered the cave because they were somewhat adapted 

 for cave life and that they are being gradually further modified by the 

 influence of the environment. 



In conclusion, it seems clear that cave animals have originated from 

 forms living outside of caves and that their occurrence in caves is not 

 due to accident but to their finding, in their ordinary movements, an 



*I am aware that to this very point the answer has been made that Chologaster 

 agassizii has not been in caves as long as Typhlichthys and hence has been less mod- 

 ified. To establish this point is, however, as difficult as to establish the heredity of 

 acquired characters. 



