Cave-Insects 287 



their underground home. The most striking char- 

 acters of cave-insects are their pale colour and their 

 blindness. The development of pigment in the skin 

 of insects is known to be largely due to the action 

 of light ; in the total darkness of the caves, there- 

 fore, little or no pigment is developed, and the in- 

 habitants — both insects and other animals — tend to 

 assume a bleached appearance. As regards the 

 blindness of cave-insects it has been found that, in 

 the most typical cave-beetles, not only is there no 

 trace of a cornea on the outer skin of the head, but 

 the optic tracts of the brain have entirely disappeared 

 also. Many naturalists see in this atrophy of the 

 eyes a direct result of the life of many generations 

 in total darkness, the absence of light — the stimulus 

 to which eyes and their associated nerve-centres re- 

 spond — is supposed to have led to the withering of the 

 organs by disuse (167). This view, however, implies 

 belief in the transmission from generation to genera- 

 tion of characters acquired during the life-time of the 

 individual. On this account many students hesitate 

 to ascribe the blindness of cave-insects to the direct 

 effects of darkness and disuse, and prefer to regard 

 it as resulting from "cessation of selection" (57a). 

 Those organs of animals, which are in constant use, 

 are believed to be kept up to the mark by the rigid 

 action of natural selection ; for example, an insect 

 living above ground with defective eyesight would 

 quickly perish. But should a race of insects become 

 transferred to a totally dark cave, individuals with 

 good sight would cease to have any advantage over 

 those with defective eyes ; natural selection would 

 cease so far as eyes were concerned, and these organs, 

 it is believed, would degenerate and finally disappear. 

 The weakness of this theory lies in the unlikelihood 

 that mere cessation of selection could so overcome the 



