ON HEREDITY. 87 



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 liv&, belong geologically 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 that thousands of generations of 

 this species have succeeded one another in the caves. 



Hence there is no reason to wonder at the extent to which the 

 degeneration of the eye has been already carried in the Proteus ; 

 even if we assume that it is merely due to the cessation of the 

 conserving influence of natural selection. 1 



But it is unnecessary to depend upon this assumption alone, for 

 when a useless organ degenerates, there are also other factors which 

 demand consideration, namely, the higher development of other 

 organs which compensate for the loss of the degenerating structure, I 

 or the increase in size of adjacent parts. If these newer develop- 

 ments are of advantage to the species, they finally come to take 

 the place of the organ which natural selection has failed to 

 preserve at its point of highest perfection. 



In the first place, a certain form of correlation, which Roux 1 

 calls ' the struggle of the parts in the organism/ plays a most 

 important part. Cases of atrophy, following disuse, appear to be 

 always attended by a corresponding increase of other organs : blind 

 animals always possess very strongly developed organs of touch, 

 hearing, and smell, and the degeneration of the wing-muscles of 

 the ostrich is accompanied by a great increase in the strength of 

 the muscles of the leg. If the average amount of food which an 

 animal can assimilate every day remains constant for a considerable 

 time, it follows that a strong influx towards one organ must be 

 accompanied by a drain upon others, and this tendency will increase, 

 from generation to generation, in proportion to the development of 



1 W. Roux, ' Der Kampf der Theile im Organismus,' Leipzig, 1881. 



