CHAPTER VI 



CRUSTACEA 



Dahwin states that in surveying the classes of animals from 

 the lower upwards this is the first class in which undoubted 

 secondary sexual characters occur. He has previously 

 mentioned that among the class Annelida, or marine worms, 

 the sexes when separate sometimes differ very greatly, but 

 considers that their differences are not of the kind which 

 can be safely attributed to sexual selection. But if sexual 

 differences of great magnitude can exist where sexual selection 

 does not and cannot occur, what proof is there that other 

 sexual differences are due to sexual selection ? Doubtless 

 Darwin's view was that in certain cases where courtship, 

 rivalry, and the choice of the opposite sex did not exist, 

 natural selection acted differently on males and females. 

 But as has been already pointed out, the facts prove that 

 the difference is not merely in the selection but in the 

 variation, in the modifications. In Darwin's treatment of 

 the subject it is merely taken as a fact that variations occur 

 which are limited to one sex, in other words which are trans- 

 mitted only to individuals of that sex in which they occur. 

 No attempt is made to explain this. On the other hand, 

 on the theory that different conditions cause different modi- 



