66 JACQUES LOEB. 



manders a hereditary disturbance in the circulation or nutrition 

 of the eye or its surroundings is the cause of the degeneration of 

 the eyes. There remains then the one difficulty mentioned in 

 the beginning, namely to account for the fact that the relative 

 number of blind species is greater in caves than in the open. 



Eigenmann has shown that all those forms which live in caves 

 were adapted to life in the dark before they entered the cave. 

 These animals are all negatively heliotropic and positively 

 stereotropic and with these tropisms they would be forced to 

 enter the cave whenever they are put at the entrance to the cave. 

 Even those among the Amblyopsidae which live in the open have 

 those tropisms of the cave dweller. This eliminates the idea 

 that the cave adapted the animals for the life in the dark. 



Eigenmann 's observation makes it also clear that blind animals 

 are comparatively rare in the open and that animals with normal 

 eyes are not in the majority in the caves. Only those animals 

 can thrive in the caves which for their feeding and mating do not 

 depend upon visual mechanisms; and conversely animals which 

 are not provided with visual mechanisms can only under excep- 

 tional conditions hold their own in the open where they meet 

 the competition of animals which can see. This would account 

 for the fact that in caves blind species are comparatively more 

 prevalent than in the open. 



In spite of all this, Eigenmann is inclined to assume that the 

 darkness of the caves was a factor in promoting the blindness of 

 the cave fauna. While those Amblyopsidae which live in the 

 open have already abnormal eyes those species of the family 

 which are found in the caves have more degenerated eyes than 

 those in the open and Eigenmann is inclined to ascribe this 

 fact to an accumulated influence of the darkness. This would, 

 however, compel us to account in a similar way for the incipient 

 degeneracy of the eyes of those Amblyopsidae that have never 

 been found in caves and for the complete blindness of Typhlo- 

 gobius; and would leave us at a loss to account for the presence 

 of salamanders with perfectly normal eyes in the caves. It seems 

 to the writer that consistency would demand to consider a com- 

 mon mode of origin of all these blind forms, namely as mutations, 

 i. e., as the result of some factorial change in the germ the cause 

 and nature of which we are not yet able to define. Among the 



