xxviii A HISTORY OF BIRDS 



distinguishable from those of birds in which such displays 

 are made apparently for the sole purpose of exhibiting 

 to the best advantage some specially modified or beauti- 

 fully coloured patterns, it seems (our author contends) 

 probable that such resplendent dress is to be regarded 

 rather as a by-product of sexual selection — as a varia- 

 tion allowed to assume a more or less extravagant form 

 because unchecked by natural selection. 



That display is the result of sexual selection is not 

 doubted by Mr. Pycraft, for even the most incon- 

 spicuously coloured birds indulge in antics, more or less 

 grotesque, when dominated by sexual desire. Any varia- 

 tion in the direction of more resplendent plumage would 

 thus of course be free to fulfil its potentiality unless, 

 and until, checked by natural selection. The view 

 advanced by Mr. Pycraft is in short this — that the 

 " display "is older than the resplendent plumage, and is 

 not the consequence of its presence. Sexual selection 

 is still regarded as operative, for there must have been 

 degrees of splendour among the ancestors of the now 

 resplendent species, and the brighter coloured would 

 excite the females more effectively. 



Mr. Pycraft considers at some length the coloration 

 of desert-dwelling forms of birds. He emphasises the 

 contention that natural selection is not the agent which 

 has in the first instance determined that coloration. I 

 confess that I should not have supposed that any one 

 would contend that it has done so, any more than that 

 the view could be supported that " natural selection " 

 has primarily started any of the variations of colour or 

 form upon which it operates. Mr. Pycraft holds that 

 there can be no question but that the peculiar physical 

 conditions of a desert environment exercise a direct 

 influence on pigmentation. It is very possible that 



