SEXUAL SELECTION 349 



the embryo, as is indicated in a passage wherein he alludes to 

 our ignorance of the " influence exerted by the female on her 

 embryo, which is {profound ". On this hypothesis, since she 

 can neither discover nor control the sex of the embryo, both 

 sexes should be coloured alike. 



Finally, he concludes : " Little importance need, however, be 

 attached to these difficulties here, since they depend upon the 

 supposition that characters acquired by one sex can be trans- 

 mitted 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 anni- 

 hilation of the bright or dull colours." The Peacock may be 

 cited as one of a possible hundred examples in disproof of 

 this, and seems to show convincingly enough that this assumed 

 commingling of characters does not take place ; such comming- 

 ling indeed would involve not merely brilliance of colouring but 

 secondary sexual characters of whatever kind. 



What are the factors governing the evolution of brilliant 

 colouring we do not know ; but there is evidence enough to 

 show that this appears first in the males then in the females, 

 and finally in the young birds — if unchecked, as to the females, 

 by natural selection. The case of Pyranga cited in Chapter XVI, 

 p. 275, may serve as a case in point, and numerous other instances 

 are cited in these pages. This much must be conceded to Mr. 

 Howard, that sexual selection is only an indirect factor in the 

 development of resplendent plumage, for he has shown that the 

 peculiar phenomena associated therewith — displays — are just as 

 fully developed in the absence of gaudy hues. Furthermore, he 

 has shown that the females are indifferent to the most vigorous 

 displays until they themselves become "must" so to speak — a 

 point to which we shall refer presently. 



The interpretation of the problem is probably this. Nature 

 secures the mating of the most vigorous birds — whatever their 

 hues — through the stimulus of parental desire which is excited 

 by these " displays ". And this parental desire, it is to be noted, 

 is set in motion by the ripening of the sexual glands, secretions, 

 from which — known as " hormones " — are set free, and pervading 

 the body stimulate the nervous system, and at the same time 



