USE AND DISUSE OF ORGANS. 183 



ered and concealed 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. Look- 

 ing at nature, profound modifications are most readily 

 demonstrated as the consequence of disuse; but artificial 

 selection gives numerous examples of both sorts, espe- 

 cially where disproportionate use of certain organs is 

 combined with simultaneous disuse of 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 ma- 

 terial in the less active organs gradually diminished, and 

 atrophy was initiated. The accuracy of these theoretical 

 observations is enforced by the observation that the near- 

 est kin of many blind cave animals, especially of insects 

 and spiders, reside in the vicinity of the cave, and that 

 those cave animals which inhabit passages only partially 

 obscure, possess less atrophied optic apparatus. A 

 singular gradation occurs among the burrowing mam- 

 mals, and Darwin "^ cites an example admirably illus- 

 tating the loss of sight in consequence of the mode of 

 ^3 



