Page:The British Warblers A History with Problems of Their Lives - 1 of 9.djvu/62

 instance, the females of the present species are occasionally as rich in colouring as the finest males. Here, then, the impotency 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.

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 Grasshopper-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 versâ.

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