252 SECRETS OF ANIMAL LIFE 



blindness arose independently of the presence or 

 absence of light, and Loeb finds an analogy in that 

 hereditary kind of human blindness which is called 

 glaucoma and is associated with atrophy of the 

 optic nerve. 



If the variations in the direction of blindness 

 occurred long ago within the cave itself, then, if 

 the creature was otherwise adaptable, a blind race 

 might arise. But what if the variations occurred 

 in the open? The first half of the answer is that 

 they would be speedily wiped out unless they took 

 to a cryptozoic mode of life; for blind members 

 of an eyed race would be severely handicapped in 

 ihe search for food and mates. The other half of 

 the answer is that there would often be a chance 

 that blindish variants, arising in the open, might 

 find their way into the asylum of caves, where 

 blindness is no disadvantage. In this connection 

 there is great interest in Eigenmann's observation 

 that the open- world relatives of cave-animals are 

 constitutionally light-avoiders (i.e. negatively helio- 

 tropic) and given to keeping in contact with solid 

 bodies (i.e. positively stereotropic). Thus, if the 

 variation in the direction of blindness did not 

 change the constitution, the variants would have a 

 tendency to move into caves. 



These constitutional proclivities probably count 

 for much, but we cannot believe that they exhaust 

 the situation, for we see all around us in the realm 

 of organisms illustrations of a kind of behavior 

 which spells endeavor after well-being. Even the 



