108 EFFECTS OF USE AND DISUSE. [Chap. V 



is in some manner excited, but it is the will of man which accumu- 

 lates 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 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. Cunning- 

 ham, 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 indeed 

 inhabits continents, and is exposed to danger from which it cannot 

 escape by flight, but it can defend itself by kicking its enemies, as 

 efficiently as many quadrupeds. We may believe that the proge- 

 nitor of the ostrich genus had habits like those of the bustard, and 

 that, as the size and weight of its body were increased during suc- 

 cessive generations, 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 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 other genera they are present, but in a 

 rudimentary condition. In the Ateuchus or sacred beetle of the 

 Egyptians, they are totally deficient. The evidence that accidental 

 mutilations can be inherited is at present not decisive ; but the 

 remarkable cases observed by Brown-Sequard in guinea-pigs, of the 

 inherited effects of operations, should make us cautious in denying 



