324 Adaptation to Environment 



fish owe their deficiency not to lack of light but to a 

 condition which interferes with the circulation in the 

 embryonic eye. Such a condition might be brought 

 about by an anomaly in the germ plasm or in one 

 chromosome, the nature and cause of which we are not 

 able to determine at present ; but which, since it occurs 

 in the germ plasm or the chromosomes, must be heredi- 

 tary. This would explain why it is, that animals 

 with perfect eyes may occur in caves and why perfectly 

 blind animals may occur in the open. It leaves, how- 

 ever, one point unexplained; namely, the greater fre- 

 quency of blind species in caves or in the dark and the 

 relative scarcity of such forms 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. 1 These animals are all nega- 

 tively helio tropic and positively stereotropic, and with 

 these tropisms they would be forced to enter a cave 

 whenever they are put at the entrance. Even those 

 among the Amblyopsidae which live in the open have 

 the tropisms of the cave dweller. This eliminates the 

 idea that the cave adapted the animals for the life in 

 the dark. 



Only those animals can thrive in caves which for their 

 feeding and mating do not depend upon visual mechan- 



1 Cunot has proposed the term preadaptation for such cases and 

 this term expresses the situation correctly. Cue"not, L., La Genese des 

 Especes animates. Paris, 1911. 



