110 THE VALUE OF COLOUR 



was reached that the colour differences on the 

 two sides neutralize the differences in illumination, 

 and remove the appearance of solidity. 1 



It is also necessary to direct attention to 

 C. W. Beebe's 2 recent discovery that the pig- 

 mentation of the plumage of certain birds is 

 increased by confinement in a superhumid atmo- 

 sphere. In Scardafella inca, on which the most 

 complete series of experiments was made, the 

 changes took place only at the moults, whether 

 normal and annual or artificially induced at 

 shorter periods. There was a corresponding 

 increase in the choroidal pigment of the eye. 

 At a certain advanced stage of feather pigment- 

 ation a brilliant iridescent bronze or green tint 

 made its appearance on those areas where iri- 

 descence most often occurs in allied genera. 

 Thus in birds no less than in insects, characters 

 previously regarded as of taxonomic value, can 

 be evoked or withheld by the forces of the en- 

 vironment. 



WAENING OK APOSEMATIC COLOURS 



From Darwin's description of the colours and 

 habits it is evident that he observed, in 1833, 

 an excellent example of warning colouring in a 

 little South American toad (Phryniscus nigricans). 

 He described it in a letter to Henslow, written 



1 Zool. Journ. Linn. Soc., xxx. 45. 



8 Zoologica: N.Y. Zool. Soc., i. No. 1, Sept. 25, 1907: Geographic 

 variation in birds with especial reference to the effects of humidity. 



