Chap, v.] EFFECTS OP USE AND DISUSE. 167 



ditions 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 dis- 

 tinct; variability is in some manner excited, but it is 

 the will of man which accumulates the variations in 

 certain directions; and it is this 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 in- 

 herited. 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 sev- 

 eral 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 es- 

 cape 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 

 13 



