" USE" INCLUDES USEFULNESS. 91 



A SOURCE OF MENTAL CONFUSION. 



Of course in a certain sense this thickening 

 of the sole has resulted from use. In one sense 

 or other, most or perhaps all of the results 

 of natural selection are inherited effects of use 

 or disuse. Natural selection preserves that 

 which is of use and which is used, while it 

 eliminates that which is useless and .is not used. 

 The most confident assertions of the effects of use 

 and disuse in modifying the heritable type, appear 

 to rest on this indefeasible basis. Darwin's state- 

 ments concerning the effects of use and disuse in p 

 evolution can frequently be read in two senses.^ 

 ^They often command assent as undeniable truisms 

 as they stand, but are of course written in another 

 and more debatable sense.) Thus in the case of the 

 shortened wings and thickened legs of the domestic 

 duck, I believe equally with Darwin and Spencer 

 that " no one will dispute that they have resulted 



