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 



