250 SECRETS OF ANIMAL LIFE 
from the fact that it finds no bodily expression in an 
unnatural environment! 
But without excluding the possibility that the 
darkness and also the short commons of caves may 
have some effect on the development of the eye in 
the individual lifetime, we cannot believe that this 
is more than a side-issue. We must remember, for 
instance, that mammals begin their life in an 
environment of complete darkness and dwell there 
often for many months—in the case of the elephant 
for more than a year. Yet this has no prejudicial 
effect on the development of the eyes. Not long 
ago Professor Jacques Loeb made the simple 
experiment of rearing embryos of a minnow-like 
fish (a species of Fundulus) in an absolutely dark 
room, but no trace of blindness was observed after 
a month. This result is the more interesting 
because, as we shall see, it is very easy in the case 
of this fish to produce blind embryos by experi- 
mental methods. But not by darkness! There 
are many other difficulties in the way of the theory 
(which Darwin accepted) that the cave-blindness is 
the hereditary result of atrophy of the eye, incident 
on disuse and dwelling in darkness. Professor 
Eigenmann, who has made a special study of cave- 
fishes, thinks that living in darkness tends at least 
to increase the optic degeneration, but he calls 
attention to the difficulty that of the four kinds of 
salamander living habitually in North American 
caves, two have quite degenerate eyes and two have 
them normal. But what is sauce for the goose 
