78 



THE INFLUENCE OF INANIMATE SUKE0UNDIN6S. 



to the law of degeneration, in consequence of their disuse. This 

 explanation, it is obvious, presupposes that such blind animals 

 are descended from a parent form that could 

 see ; and it cannot be denied that many of the 

 facts hitherto ascertained seem to justify this 

 view. Some of the so-called blind animals are 

 not, accurately speaking, sightless; thus the 

 blind Proteus (fig. 20, a), an Amphibian of 

 the caves of Oamiola, has an eye deeply 

 seated in the head and entirely covered by the 

 skin. The structure of this organ is very re- 

 markable ; it possesses all the characteristic 

 parts of the eye, but they have been arrested at 

 an almost embryonic stage, with the exception 

 of the crystalline lens, of which every trace is 

 absent (fig. 20, 6) ; the pigment-layer of the 

 retina is scarcely coherent, and consists of only 



b 



^ret. 



Fig. 20.— a, Protens of the Adelsberg grotto, reduced; §, vertical 

 section of the rndimentary eye ; opt, the optic nerve ; cc, corpus 

 ciliare retinse, the inner portions of which meet in front becaiise 

 ^ the lens k absent ; c«, the iatemal cavity of the eye without 



any vitreous humour. The cell-layers of the retina {ret) are 

 unusually thick ; the pigment-layer, p, very slightly developed. 



a few scattered pigment-cells. We may therefore be very doubt- 

 ful as to whether this Proteus can receive a clear image of the 

 objects that surround it even in a place where there is light ; 



