DISUSE OF ORGANS. 239 
strongly developed than in wild ducks. In ostriches and 
other running birds which have become completely unac- 
customed to fly, the consequence is that their wings are 
entirely crippled and degenerate into mere “rudimentary 
organs” (p. 12). In many domestic animals, especially in 
many races of dogs and rabbits, we find that in the 
cultivated state they have acquired pendulous ears. This 
is simply a consequence of a diminished use of the auri- 
cular muscles. In a wild state these animals have to exert 
their ears very much in order to discover an approaching 
foe, and this is accompanied by a strong development of 
the muscular apparatus, which keeps the outer ears in an 
upright position, and by which they can turn them in all 
directions. In a domestic state the same animals no longer 
require to listen so attentively, they prick up or turn their 
ears only a little; the auricular muscles cease to be used, 
gradually become weakened, and the ears hang down 
flabbily, or become rudimentary. 
As in these cases the function, and consequently the form 
also, of the organ becomes degenerated through disuse, so, 
on the other hand, it becomes more developed by greater 
use. This is particularly striking if we compare the brain, 
and the mental activity belonging to it, in wild animals 
and those domestic animals which are descended from 
them. The dog and horse, which are so vastly improved 
by cultivation, show an extraordinary degree of mental 
development, in comparison with their wild original 
ancestors, and evidently the change in the bulk of the 
brain, which is connected with it, is mainly determined by 
persistent exercise. It is also well known how quickly 
and powerfully muscles grow and change their form by con- 
