194 A^iimal Life and Intelligence, 



buried beneath the skin, through a reversal of selection. 

 The tuco-tuco {Ctenomys), a burrowing rodent of South 

 America, is frequently blind. One which Darwin kept 

 alive was in this condition, the immediate cause being 

 inflammation of the nictitating membrane. ''As frequent 

 inflammation of the eyes," says Darwin, "must be in- 

 jurious to any animal, and as eyes are certainly not 

 necessary to animals having subterranean habits, a reduc- 

 tion in their size, with the adhesion of the eyelids and 

 growth of fur over them, might in such cases be an 

 advantage ; and, if so, natural selection would aid the effect 

 of disuse.* Granting that the inflammation of the eyes is 

 a sufficient disadvantage to lead to elimination, such cases 

 may be assigned to the effects of a reversal of selection. 



Perhaps the best instances of the reversal of selection 

 are to be found in the insects of wind-swept islands, in 

 which, as we have already seen (p. 81), the power of flight 

 has been gradually reduced or even done away with. 

 Such instances are, however, exceptional. And one can 

 hardly suppose that such reversal of selection can be very 

 far-reaching in its effects, at least, through any direct 

 disadvantage from the presence of the organ. One can 

 hardly suppose that the presence of an eye in a cave- 

 dwelling fish could be of such direct disadvantage as to 

 lead to the elimination of those members which still possess 

 this structure. 



But may it not be of indirect disadvantage ? May not 

 this structure be absorbing nutriment which would be more 

 advantageously utilized elsewhere ? This is Darwin's 

 principle of economy. Granting its occurrence, is it effec- 

 tive ? We may put the matter in this way : The Crustacea 

 which have been swept into a dark cave may be divided 

 into three classes so far as fortuitous variations of eyes 



* " Origin of Species," p. 110. 



t With regard to blind cave-fish, Professor Kay Lankester has suggested 

 that some selection has been effected. Those animals whose sight-sensitive- 

 ness enabled them to detect a glimmer of light would escape to the exterior, 

 leaving those with congenitally weak sight to remain and procreate in the 

 darkness of the cave. 



