CiiAP. V. EFFECTS OF USE AND DISUSE. 141 



alino.st entire abs'jnco of certain lar^jo groups of beetles, else- 

 where excessively numerous, and which groups have ha])its of 

 life almost necessitating frequent flight — tliese several con- 

 siderations have made me believe that the wingless condition 

 of so many Madeira beetles is mainly due to the action of 

 natural selection, but combined probably with disuse. For 

 during many successive generations each individual beetle 

 which Hew least, cither from its wings having been ever so 

 little less perfectly developed or from indolent habit, will have 

 had the best chance of surviving from not being blown out to 

 sea ; and, on the other hand, those beetles whicli most readily 

 took to flight would oftenest have been blown to sea and thus 

 have been destroyed. 



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

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

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

 Mr. Wuilaslon suspects, their wings not at all reduced, but 

 even elilarged. This is quite compatible with the action of 

 natural selection. For when a new insect first arrived on the 

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

 the wings, would depend on whether a greater number of in- 

 dividuals were saved by successfully battling with the winds, 

 or by giving up the attempt and rarely or never flying. As 

 wilh mariners shipwrecked near a coast, it would liave been 

 bcttiT for the good swimmers if they had been able to swim 

 still farther, whereas it would have been better for the bad 

 swinnners 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 arc 

 rudimentary in size, and in some cases are quite covered up ])y 

 skin and fur. Tliis state of the eyes is probably due to gradual 

 reduction from disuse, but aided perhaps by natural selection. 

 In Soutli America, a burrowing rodent, the tuco-tuco, or Ctcn- 

 omys, is even more subt(«rranean in its habits than the mole; 

 .and 1 was assured by a Spaniard, who had often caught them, 

 that th(»y were frequently blind; one which I kept alive was 

 certainly in this condition, the cause, as appeared on dissection, 

 having been inllammation of the nictitating membrane. As 

 frefjuent inflammation of the eyes nnist be injurious to any 

 animal, and as eyes are certaiidy not necessary to aiiimals 

 having su])terranean habits, a reduction in their size, with the 

 adhesion of the eyelids and growth of fur over them, might in 



