102 EFFECTS OF USE AND DISUSE. [CHAP. V. 



indolent habit, will have had the best chance of surviving from 

 not being blown out to sea ; and, on the other hand, those beetles 

 which most readily took to flight would oftenest have been blown 

 to sea, and thus destroyed. 



The insects in Madeira which are not ground-feeders, and 

 which, as certain flower-feeding coleoptera and lepidoptera, must 

 habitually use their wings to gain their subsistence, have, as 

 Mr. Wollaston suspects, their wings not at all reduced, but even 

 enlarged. This is quite compatible with the action of natural 

 selection. For when a new insect first arrived on the island, the 

 tendency of natural selection to enlarge or to reduce the wings, 

 would depend on whether a greater number of individuals were 

 saved by successfully battling with the winds, or by giving up the 

 attempt and rarely or never flying. As with mariners ship- 

 wrecked near a coast, it would have been better for the good 

 swimmers if they had been able to swim still further, whereas it 

 would have been better for the bad swimmers if they had not 

 been able to swim at all and had stuck to the wreck. 



The eyes of moles and of some burrowing rodents are rudi- 

 mentary in size, and in some cases are quite covered by skin and 

 fur. This state of the eyes is probably due to gradual reduction 

 from disuse, but aided perhaps by natural selection. In South 

 America, a burrowing rodent, the tuco-tuco, or Ctenomys, is even 

 more subterranean in its habits than the mole ; and I was assured 

 by a Spaniard, who had often caught them, that they were 

 frequently blind. One which I kept alive was certainly in this 

 condition, the cause, as appeared on dissection, having been in- 

 flammation of the nictitating membrane. As frequent inflam- 

 mation of the eyes must be injurious to any animal, and as eyes 

 are certainly not necessary to animals having subterranean habits, 

 a reduction in their size, with the adhesion of the eyelids and 

 growth of fur over them, might in such case be an advantage; 

 and if so, natural selection would aid the effects of disuse. 



It is well known that several animals, belonging to the most 

 different classes, which inhabit the caves of Carniola and of 

 Kentucky, are blind. In some of the crabs the foot-stalk for the 

 eye remains, though the eye is gone ; the stand for the telescope 

 is there, though the telescope with its glasses has been lost. As 

 it is difficult to imagine that eyes, though useless, could be in any 

 way injurious to animals living in darkness, their loss may be 

 attributed to disuse. In one of the blind animals, namely, the 

 cave-rat (Neotoma), two of which were captured by Professor 

 Silliman at above half a mile distance from the mouth of the 

 cave, and therefore not in the profoundest depths, the eyes were 

 lustrous and of large size ; and these animals, as I am informed 

 by Professor Silliman, after having been exposed for about a 



