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 
the 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 
