II.] ON HEREDITY. 87 



of the continued operation of natural selection, by which the 

 individuals with the weakest sight are being continually'- exter- 

 minated. 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 gradually 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 recon- 

 cile 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 nerve is divided ; 

 and that great degeneration of the eye may be brought about 

 by the artificial destruction of the optic centre in the brain. 

 Since, therefore, the effects of disuse are so striking in a single 

 life, we should certainly expect, if such effects can be trans- 

 mitted, that all traces of an eye would soon disappear from a 

 species which lives in the dark. 



The caverns in Carniola and Carinthia, in which the blind 

 Proteus and so many other blind animals live, belong geo- 

 logically to the Jurassic formation ; and although we do not 

 exactly know when for example the Proteus first entered them, 

 the low organization of this amphibian certainly indicates that 

 it has been sheltered there for a very long period of time, and 

 tliat thousands of generations of this species have succeeded 

 one another in the caves. 



