ON HEREDITY. 85 



the same extent. But the consideration of a definite example 

 clearly shows that this must be the case. When the wild duck 

 became domesticated, and lived in a farm-yard, all the individuals 

 were compelled to walk and stand more than they had done 

 previously, and the muscles of the legs were used to a correspond- 

 ingly greater degree. The same thing happens in the wild state, 

 when any change in the conditions of life compels an organ to be 

 more largely used. No individual will be able to entirely avoid 

 this extra use, and each will endeavour to accommodate itself to 

 the new conditions according to its power. The amount of this 

 power depends upon the predisposition of the germ ; and natural 

 selection, while it apparently decides between individuals of various 

 degrees of strength, is in truth operating upon the stronger and 

 weaker germs. 



But the very conclusions which have been drawn from the 

 increase of activity which has arisen from exercise, must also be 

 drawn from the instances of atrophy or degeneration following from 

 the disuse of organs. 



Darwin long ago called attention to the fact that the degeneration 

 of an organ may, under certain circumstances, be beneficial to the 

 species. For example, he first proved in the instance of Madeira, 

 that the loss of wings may be of advantage to many beetles inhabit- 

 ing oceanic islands. The individuals with imperfectly developed or 

 atrophied wings have an advantage, because they are not carried 

 out to sea by the frequent winds. The small eyes, buried in fur, 

 possessed by moles and other subterranean mammals, can be similarly 

 explained by means of natural selection. So also, the complete dis- 

 appearance of the limbs of snakes is evidently a real advantage to 

 animals which creep through narrow holes and clefts ; and the de- 

 generation of the wings in the ostrich and penguin is, in part, 

 explicable as a favourable modification of the organ of flight into 

 an organ for striking air or water respectively. 



But when the degeneration of disused organs confers no benefits 

 upon the individual, the explanation becomes less simple. Thus 

 we find that the eyes of animals which inhabit dark caves (such 

 as insects, crabs, fish, Amphibia, etc.) have undergone degeneration ; 

 yet this can hardly be of direct advantage to the animals, for they 

 could live quite as well in the dark with well-developed eyes. But 

 we are here brought into contact with a very important aspect of 



