86 ON HEREDITY. [II. 



Darwin long ago called attention to the fact that the de- 

 generation 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 maybe of advantage 

 to many beetles inhabiting oceanic islands. The individuals 

 with imperfectly developed or atrophied wings have an advan- 

 tage, 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 ad- 

 vantage to animals which creep through narrow holes and 

 clefts ; and the degeneration 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 e3^es of animals which inhabit 

 dark caves (such as insects, crabs, fish. Amphibia, etc.) have 

 undergone degeneration ; yet this can hardly be of direct ad- 

 vantage 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 natural selection, viz. 

 the power of conservation exerted by it. Not only does the 

 survival of the fittest select the best, but it also maintains it ^ 

 The struggle for existence does not cease with the foundation 

 of a new specific type, or with some perfect adaptation to 

 the external or internal conditions of life, but it becomes, on 

 the contrary, even more severe, so that the most minute 

 differences of structure determine the issue between life and 

 death. 



The sharpest sight possessed by birds is found in birds of 

 prey, but if one of them entered the world with ej^es rather 

 below the average in this respect, it could not, in the long run, 

 escape death from hunger, because it would always be at a 

 disadvantage as compared with others. 



Hence the sharp sight of these birds is maintained by means 



^ This principle was, I believe, first pointed out by Seidlitz. Compare 

 Seidlitz, 'Die Darwin'sche Theorie,' Leipzig, 1875, p. 198. 



