88 ON HEREDITY. [II. 



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. 



But it is unnecessary to depend upon this assumption alone, 

 for when a useless organ degenerates, there are also other 

 factors w^hich demand consideration, namely, the higher 

 development of other organs which compensate for the loss 

 of the degenerating structure, or the increase in size of ad- 

 jacent parts. If these newer developments 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 ^ 

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

 important part. Cases of atrophy, follow^ing 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 b^^ 

 a drain upon others, and this tendency will increase, from 

 generation to generation, in proportion to the development 

 of the growing organ, which is favoured by natural selection 

 in its increased blood-supply, etc. ; while the operation of 

 natural selection has also determined the organ which can 

 bear a corresponding loss without detriment to the organism 

 as a whole. 



Without the operation of natural selection upon different 

 individuals, the struggle between the organs of a single in- 

 dividual would be unable to encourage a predisposition in 

 the germ towards the degeneration or non-development of 

 a useless organ, and it could only limit and degrade the de- 

 velopment of an organ in the lifetime of the individual. If, 

 therefore, acquired characters are not transmitted, the dis- 

 position to develope such an organ would be present in 



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



