USE OR DISUSE OF ORGANS. 131 



means adopted to bring about these differ- 

 ences are only applicable to man, or to 

 animals under his control. 



No specific modification arises in the 

 organs of wild animals, when brought under 

 what seem to be more favourable conditions 

 of existence, and such trifling modifications 

 in expression of type as do arise, come within 

 a limited space of time. When an animal 

 is first exposed to an adverse climate, its 

 protection from the weather will develop ; 

 but the development will probably be as 

 much in the first season as it ever will be : 

 it is not indefinite, and there is no tendency 

 to specific variation. 



Darwin attempts to prove the existence 

 of a general law of Nature, that use and 

 disuse of parts are attended by modifica- 

 tions that lead to specific variation ; but 

 although use and disuse do produce slight 

 differences in man, and in animals under 

 his influence, the cases among wild animals 

 where use has not caused development, nor 

 disuse degeneration, are sufficiently numerous 

 to prove that there is no general law of 

 Nature such as he seeks to establish. 



Certain fishes found in caves are blind, 

 and in some cases have only fragments of 

 eyes ; but then reptiles, and even rats, living 



