USE AND DISUSE OF ORGANS. 183 



Its environment ; and an innportant guarantee of the 

 correctness of this interpretation of facts is afforded by 

 the other observation, that most female birds with gaily 

 coloured or speckled plumage sit in covered and con- 

 cealed nests. It must be added, that the construction 

 of nests is not determined by the absolute rules of a 

 blind instinct, but is modified by the experience of the 

 animals, an experience of which we are indeed scarcely 

 able to perceive the development, except with the age of 

 the individual, but which, at least in several cases, has 

 been proved to be the progress of the species. 



Natural selection has an important accessory in the 

 modifications produced by the use or disuse of organs. 

 Compulsion to more diligent use, inducements to dis- 

 use, are involved in the varying conditions of life. In 

 both cases it is therefore a question of adaptation. 

 Looking at nature, profound modifications are most 

 readily demonstrated as the consequence of disuse ; but 

 artificial selection gives numerous examples of both 

 sorts, especially where disproportionate use of certain 

 organs is combined with simultaneous disuse ot others. 

 Such products of selection with disproportionate use are 

 the racer and the dray-horse. 



The blindness of cave animals admits of no explana- 

 tion, but that, with the increasing uselessness of the eyes 

 during accommodation to cave life, the exchange of 

 material in the less active organs gradually diminished, 

 and atrophy was initiated. The accuracy of these 

 theoretical observations is enforced by the observation 

 that the nearest kin of many blind cave animals, espe- 

 cially of insects and spiders, reside in the vicinity of 

 the cave, and that those cave animals which inhabit pas- 



