192 ORIGIN OF NON-SEXUAL CHARACTERS 



absence of light ? It has been shown that Proteus 

 when kept in the light develops some amount of 

 pigment, although it does not become pigmented to 

 the same degree as ordinary Amphibia. We have 

 here, I think, an example of the essential difference 

 between mutations and somatic modifications. 

 Absence of the gametic factor or factors for pig- 

 mentation results in albinism, and no amount of 

 exposure to light produces pigmentation in albinos, 

 e.g. albino Axolotls which are well known in 

 captivity. Absence of light, on the other hand, 

 prevents the development of pigment. The question 

 therefore is whether the somatic modification is 

 inherited. The fact that Proteus does not rapidly 

 become as deeply coloured when exposed to light 

 as ordinary Amphibia shows that the gametic 

 factors for pigmentation have been modified as well 

 as the somatic tissues. 



Loeb attributes the blindness of cave fishes to a 

 disturbance in the circulation and mutation of the 

 eyes originally occurring as a mutation. But how 

 could an explanation of this kind be applied to the 

 case of Anableps tetrophthalmus, in which each eye is 

 divided by a partition of the cornea and lens into an 

 upper half adapted for vision in air and a lower half 

 for vision in water ? This fish lives in the smooth 

 water of estuaries in Central America, and swims 

 habitually with the horizontal partition of the lens 

 level with the surface of the water. It is impossible 

 to understand in this case, firstly, how a mutation 

 could cause the eyes to be divided and doubly 

 adapted to two different optic conditions, and, 

 secondly, how at the same time a convenient 

 4 tropism ' should occur which caused the animal 



