ADAPTATION 327 



in the skin, but it requires light for its development. In surface- 

 dwelling salamanders, pigment develops in the skin even in the 

 dark. Therefore, something in the skin of Proteus was lost as a 

 result, presumably, of residence in caves. The eyes of Proteus 

 under cave conditions develop to a certain stage, stop, and then 

 degenerate. If Proteus is reared in an alternating combination 

 of white and red light, the eyes develop almost to a normal state. 



The American blind salamander, Typhhtriton spelaeus, spends 

 its larval (tadpole) period near the mouths of caves and during 

 this period has functional eyes. During metamorphosis (when 

 the gills are lost and other changes take place) the animal 

 migrates into caves and as a result, the eyelids become partially 

 fused and the rods and cones, the sensory cells of the retina, 

 degenerate completely. That the degenerative effect on the eyes 

 is the result of cave life is indicated by the fact that the eyes fail 

 to degenerate when the animals are raised in the light. In the 

 case of both Proteus and Typhlotriton the reactions of the animals 

 to external conditions must be due to the cumulative effect of 

 cave life on the ancestors, induced by darkness. It does not seem 

 to be a direct and gradual effect as Lamarck pictured it, but a 

 complex and indirect effect involving the loss of factors that are 

 necessary for the full development of the pigmentation of the skin 

 and the complete development of the eyes in the absence of light. 



It should be clear from what has been said that there is a 

 difference of opinion as to the significance of adaptation. Assum- 

 ing that structural adaptation is a fact and not an illusion, it is 

 entirely reasonable to admit that in some cases preadaptation 

 has played an important part in bringing about what are called 

 adaptive relations in organisms ; but it must also be admitted that 

 environmental influences have also played a part in modifying 

 preadapted structures and, in some cases at least, have been a 

 potent influence in shaping the course of adaptation inde- 

 pendently of a preadapted origin. That an interplay of internal 

 protoplasmic factors and external environmental stimuli, though 

 not clearly understood and difficult to demonstrate, should exist, 

 is a logical conclusion; but to translate this interplay into terms 

 of structural adaptation is another matter. It must be borne in 

 mind that environment determines the conditions under which 

 organisms live and that organisms must be adapted to these 

 conditions. 



