EFFECTS OF USE AND DISUSE 149 



in many parts of the world are frequently blown to sea and 

 perish; that the beetles in ^Madeira, as observed by Mr. Wol- 

 laston. lie much concealed, until the wind lulls and the sun 

 shines; that the proportion of wingless beetles is larger on 

 the exposed Desertas than in Madeira itself; and especially 

 the extraordinary fact, so strongly insisted on by Mr. Wol- 

 laston, that certain large groups of beetles, elsewhere ex- 

 cessively num.erous, which absolutely require the use of their 

 wings, are here almost entirely absent; — these several con- 

 siderations make me believe that the wingless condition of 

 so many Madeira beetles is mainly due to the action of 

 natural selection, combined probably with disuse. For dur- 

 ing many successive generations each individual beetle which 

 flew least, either 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 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 re- 

 duced, 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 v,^ould 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 

 rudimentary 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 



