60 ARE THE EFFECTS OF USE INHERITED ? 



bones of the wild duck long and light. The 

 used leg-bones and the disused wing-bones have 

 .alike been shortened and thickened, though in 

 different proportions* Natural or artificial selection 

 might easily thicken legs without lengthening 

 them, or shorten wings without eliminating strong 

 heavy bones, but it can hardly be contended that 

 use-inheritance has acted in such conflicting ways. 

 The thickening of the wing-bones has actually 

 more than kept pace with any increase of weight 

 in the skeleton, in spite of the effect of indi- 

 vidual disuse and of the alleged cumulative effect 

 of ancestral disuse for hundreds of generations. 

 The case of the duck deserves special attention 

 as a crucial one, if only from the fact that in 

 this instance, and in this instance only, has Darwin 

 given the weights of the skeletons, thus furnishing 

 the means for a closer examination of his details 

 than is usually possible. 



If we ignore such factors as selection, pan- 



