122 ON THE ORIGIN OF SPECIES 



and natural selection will then accumulate all profitable 

 variations, however slight, until they become plainly 

 developed and appreciable by us. 



Effects of Use and Disuse. — From the facts alluded to 

 in the first chapter, I think there can be little doubt 

 that use in our domestic animals strengthens and en- 

 larges certain parts, and disuse diminishes them ; and 

 that such modifications are inherited. Under free 

 nature, we can 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 have structures which can be 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. As the 

 larger ground-feeding birds seldom take flight except to 

 escape danger, I believe that the nearly wingless condi- 

 tion of several birds, which now inhabit or have lately 

 inhabited several oceanic islands, tenanted by no beast 

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

 inhabits continents and is exposed to danger from which 

 it cannot escape by flight, but by kicking it can defend 

 itself from enemies, as well as any of the smaller quad- 

 rupeds. We may imagine that the early progenitor 

 of the ostrich had habits like those of a bustard, and 

 that as natural selection increased in successive genera- 

 tions the size and weight of its body, its legs were used 

 more, and its wings less, until they became incapable 

 of flight. 



Kirby has remarked (and I have observed the same 

 fact) that the anterior tarsi, or feet, of many male 

 dung- feeding beetles are very often broken off; he 

 examined seventeen specimens in his own collection, 

 and not one had even a relic left. In the Onites 

 apelles the tarsi are so habitually lost, that the insect 

 has been described as not having them. In some 



