EFFECTS OF tfSE AND DISUSE. 121 



In one sense the conditions of life may be said, not only 

 to cause variability, either directly or indirectly, but like- 

 wise to include natural selection, for the conditions deter- 

 mine whether this or that variety shall survive. But when 

 man is the selecting agent, we clearly see that the two ele- 

 ments of change are distinct ; variability is in some manner 

 excited, but it is the will of man which accumulates the 

 variations in certain direction ; 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 dimin- 

 ished 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 dis- 

 use. 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 beasts 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 progenitor of the ostrich genus had 

 habits like those of the bustard, and that, as the size and 

 weight of its body were increased during successive genera- 

 tions, 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 specie 



