NATURAL SELECTION SINCE DARWIN 79 



be useful. Among the characters which differen- 

 tiate one species from another, very few serve any ^ 

 useful purpose, most of them being absolutely neu- 

 tral. Darwin admits this point and adds that, in 

 certain cases, usefulness is not apparent and that, in 

 certain others, those neutral characters are due to the 

 influence of the environment or to certain correlations 

 in accordance with what he calls the law of growth. 

 This explanation, however, oversteps the limits of 

 natural selection and rather exposes its shortcomings. 

 Many examples of those neutral characters (which 

 happen to be the most immutable ones in every spe- 

 cies) have been cited: The opposite leaves of the 

 Labiatse, the vorticellate leaves of the Boraginacae 

 (Naegeli) ; the various markings on the wings of cer- 

 tain insects, so fine that they cannot be detected with- ^ 

 out a magnifying glass, although they serve to differ- 

 entiate one species from another (Kellogg and 

 Bell) ; callouses on the legs of all the equines, four 

 on the horse's, two on the ass's legs (Conn) ; the spiral 

 whorls of certain mollusc's shells rolled one way or 

 another (also a specific character) ; the colouring of 

 concealed parts of birds' bodies, etc. The last case 

 was mentioned by Romanes, whose testimony is most 

 valuable, as he was one of the most authoritative and 

 one of the earliest Darwinians. 



The extreme partisans of the selection theory 

 (Wallace and the Neo-Darwinians) defend it from 



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