134 LAWS OF VARIATION. Chap. V. 



the conditions of life must act. Again, innumerable 

 instances are known to every naturalist of species 

 keeping true, or not varying at all, although living 

 under the most opposite climates. Such considerations 

 as these incline me to lay very little weight on the 

 direct action of the conditions of life. Indirectly, as 

 already remarked, they seem to play an important part 

 in affecting the reproductive system, and in thus in- 

 ducing variability ; and natural selection will then accu- 

 mulate 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 



