134 ARE THE EFFECTS OF USE INHERITED? 



natural selection has had to counteract this increase 

 so far as length is concerned, and to effect 8 per 

 cent, of shortening besides. If use-inheritance 

 thickens bones without proportionally lengthening 

 them, it would hinder rather than help the evolution 

 of such structures as the long light wings of birds, 

 or the long legs and neck of the giraffe or crane. 



VARIED EFFECTS OF USE AND DISUSE. 



The changes which we somewhat roughly 

 and empirically group together as the effects of 

 " use and disuse " are of widely diverse character. 

 Thus bone, as the physiological fact, thickens un- 

 der alternations of pressure (and the consequent 

 increased flow of nourishment), but atrophies 

 under a steadily continued pressure ; so that if 

 the use of a bone involved continuous pressure, 

 the effect of such use would be a partial or total 

 absorption of that bone. Darwin shows that 



