VARIABLE RESEMBLANCE IN VERTEBRATA, ETC. 91 



and that other blind animals living in dark caverns 

 are often white. In the majority of cases this result 

 is undoubtedly due to the gradual disappearance of 

 the useless pigment, and not to the excessive con- 

 traction of the structures in which it is usually con- 

 tained. Just as the useless eye has become rudi- 

 mentary in these animals, so has the useless colour 

 gradually disappeared from the skin. The energy 

 necessary for the production and maintenance of such 

 structures has been diverted, either wholly or in 

 part, to other and more useful ends. In the Proteus, 

 however, the degeneration is as yet incomplete, for the 

 skin still retains pigment cells. An individual now in 

 my possession has gradually become much darker since 

 its removal from the cave at Adelsberg. It is probable 

 that this result has followed from the direct effect of 

 light upon the skin ; for it is known that superficial 

 pigment cells are sensitive to light, although the 

 changes of colour thus induced differ from those caused 

 indirectly through the nervous system, in the absence 

 of any harmony with the colours of the environment. 

 The skin of the Proteus is probably extremely sensitive 

 to light. By day the animal in my laboratory always 

 lies concealed beneath a plate at the bottom of the 

 aquarium, while ife comes out every night and swims 

 freely about. As the eyes are very degenerate and 

 buried beneath the surface, it appears certain that the 

 difference between light and darkness is appreciated 

 by the skin. W. Bateson has shown that blind 



