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- 



