86 ON HEREDITY. 



''natural selection, viz. the power of conservation exerted by it. 

 Not only does the survival of the fittest, select the best, bnf, it also . 

 jnaintains it 1 . 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 eyes 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 of 

 I the continued operation of natural selection, by which the indi- 

 viduals with the weakest sight are being continually exterminated. 

 But all this would be changed at once, if a bird of prey of a certain 

 species were compelled to live in absolute darkness. The quality of 

 the eyes would then be immaterial, for it could make no difference 

 to the existence of the individual, or the maintenance of the species. 

 The sharp sight might, perhaps, be transmitted through numerous 

 generations ; but when weaker eyes arose from time to time, these 

 would also be transmitted, for even very short-sighted or imperfect 

 eyes would bring no disadvantage to their owner. Hence, by con- 

 tinual crossing between individuals with the most varied degrees 

 of perfection in this respect, the average of perfection would gradu- 

 ally decline from the point attained before the species lived in the 

 dark. 



We do not at present know of any bird living in perfect 

 darkness, and it is improbable that such a bird will ever be found ; 

 but we are acquainted with blind fish and Amphibia, and among 

 these the eyes are present it is true, but they are small and hidden 

 under the skin. I think it is difficult to reconcile the facts of the 

 case with the ordinary theory that the eyes of these animals have 

 simply degenerated through disuse. If disuse were able to bring 

 about the complete atrophy of an organ, it follows that every trace 

 of it would be effaced. We know that, as a matter of fact, the 

 olfactory organ of the frog completely degenerates when the olfactory 



1 This principle was, I believe, firat pointed out by Seidlitz. Compare Seidlitz, 

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



