134 
LAWS OF VARIATION. 
Chap. V. 
the conditions of life 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 inducing variability; 
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 
