USE AND DISUSE OF ORGANS. 183 
its environment ; and an important guarantee of the 
correctness of this interpretation of facts is afforded by 
the other observation, that most female birds with gaily 
coloured or speckled plumage sit in covered and con- 
cealed nests. It must be added, that the construction 
of nests is not determined by the absolute rules of a 
blind instinct, but is modified by the experience of the 
animals, an experience of which we are indeed scarcely 
able to perceive the development, except with the age of 
the individual, but which, at least in several cases, has 
been proved to be the progress of the species. 
Natural selection has an important accessory in the 
modifications produced by the use or disuse of organs. 
Compulsion to more diligent use, inducements to dis- 
use, are involved in the varying conditions of life. In 
both cases it is therefore a question of adaptation. 
Looking at nature, profound modifications are most 
readily demonstrated as the consequence of disuse; but 
artificial selection gives numerous examples of both 
sorts, especially where disproportionate use of certain 
organs is combined with simultaneous disuse of others. 
Such products of selection with disproportionate use are 
the racer and the dray-horse. 
The blindness of cave animals admits of no explana- 
tion, but that, with the increasing uselessness of the eyes 
during accommodation to cave life, the exchange of 
material in the less active organs gradually diminished, 
and atrophy was initiated. The accuracy of these 
theoretical observations is enforced by the observation 
that the nearest kin of many blind cave animals, espe- 
cially of insects and spiders, reside in the vicinity of 
the cave, and that those cave animals which inhabit pas- 
9 . 
