THE PHENOMENA OF MUTATION 191 



into them when frightened, while the blind species is 

 found only at Point Loma, and never leaves the 

 burrows of the shrimp. It would appear, therefore, 

 that Typhlogobius lives in almost if not quite com- 

 plete darkness, instead of being, as Loeb states, 

 ' blind in spite of exposure to light,' while the 

 closely allied forms which are exposed to light are 

 not blind. 



Loeb states, on the authority of Eigenmann, that 

 all those forms which live in caves were adapted to 

 life in the dark before they entered the cave, because 

 they are all negatively heliotropic and positively 

 stereotropic, and with these tropisms would be 

 forced to enter a cave whenever they were put at the 

 entrance. Even those among the Amblyopsidae 

 which live in the open have the tropisms of the cave 

 dweller. But these latter are not blind, and the 

 argument only tends to show that the blind fish 

 Amblyopsis entered the caves before it was blind. 

 Nocturnal animals generally must be said to be 

 negatively heliotropic, but these usually have larger 

 and more sensitive eyes than the diurnal. 



It is said, however, that Chologaster agassizii, 

 which is not blind, lives in the underground streams 

 of Kentucky and Tennessee, but I think it is open 

 to doubt whether it is a species entirely confined to 

 darkness. 



Another point which Loeb omits to mention is 

 the absence of pigment in cave animals, especially 

 Vertebrates such as Amblyopsis and Proteus. If 

 absence of light is not the cause of blindness in these 

 cases, how is it that the blindness is always as- 

 sociated with absence of pigment, since we know that 

 the latter in Fishes and Amphibia is due to the 



