BEITISH WARBLERS 



instance, the females of the present species are occasionally as 

 rich in colouring as the finest males. Here, then, the im- 

 potency of sexual selection becomes apparent, for unless the 

 females that choose the more beautiful males are themselves 

 the more beautiful of the females, they will neutralise the 

 effect of their own selection. 



Although this is a truism which appears to me to be 

 difficult to controvert, yet I must admit that we are here 

 confronted with two difficulties, one an incomplete knowledge 

 of the laws relating to heredity, the other an ignorance of the 

 influence exerted by the female upon her embryo, which is 

 profound. We do not even know whether heredity and this 

 influence are distinct, or inseparable, merging into one another 

 by very gradual stages. Yet this influence is a potent factor 

 in all life, including man. 1 



Little importance need, however, be attached to these 

 difficulties here, since they depend upon the supposition that 

 characters acquired by one sex can be transmitted to that sex 

 only. For instance, if the dull-coloured female G-rasshopper- 

 Warblers in a given area were to pair with the bright-coloured 

 males, and the bright-coloured females with the dull-coloured 

 males, the result in time would not be the continuation of these 

 same conditions, but the gradual annihilation of the bright or 

 the dull colours. It is inconceivable and contrary to the facts 

 in Nature to suppose that the colouring of the female offspring 

 could be in no degree influenced by the male parent, and 

 vice versa. 



It has been suggested that the more vigorous females 

 would be the first to breed, but this is a supposition made 

 only to escape a difficulty, evidence in support of it being 

 completely lacking : and since these same females vary 

 individually very considerably in their colouring, we should 

 at once, providing evidence were forthcoming in proof of 



1 It is indeed difficult to understand the callousness with which this 

 knowledge is treated in human life at the present day. 



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