Chap. V.] EFFECTS OF USE AND DISUSE. 167 



conditions determine whether this or that variety shall 

 survive. But when man is the selecting agent, we 

 clearly see that the two elements of change are 

 distinct; variability is in some manner excited, but it 

 is the will of man which accumulates the variations in 

 certain directions; and it ij^his latter agency which 

 answers to the survival of the fittest under nature. 



Effects of the increased Use and Disuse of Parts, as 

 controlled by Natural Selection. 



From the facts alluded to in the first chapter, I think 

 there can be no doubt that use in our domestic animals 

 has strengthened and enlarged certain parts, and disuse 

 diminished them ; and that such modifications are 

 inherited. Under free nature, we have no standard 

 of comparison, by which to judge of the effects of long- 

 continued use or disuse, for we know not the parent- 

 forms ; but many animals possess structures which can 

 be best explained by the effects of disuse. As Professor 

 Owen has remarked, there is no greater anomaly in 

 nature than a bird that cannot fly ; yet there are 

 several in this state. The logger-headed duck of South 

 America can only flap along the surface of the water, 

 and has its wings in nearly the same condition as the 

 domestic Aylesbury duck : it is a remarkable fact that 

 the young birds, according to Mr. Cunningham, can fly, 

 while the adults have lost this power. As the larger 

 ground-feeding birds seldom take flight except to 

 escape danger, it is probable that the nearly wingless 

 condition of several birds, now inhabiting or which 

 lately inhabited several oceanic islands, tenanted by no 

 beast of prey, has been caused by disuse. The ostrich 

 9 



