Warning Colours 281 



advanced stage of feather pigmentation a brilliant iridescent bronze 

 or green tint made its appearance on those areas where iridescence 

 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 environment. 



Warning or 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 (JPhryniscus nigricans). He described 

 it in a letter to Henslow, written from Monte Video, Nov. 24, 1832 : 

 "As for one little toad, I hope it may be new, that it may be 

 christened ' diabolicus.' Milton must allude to this very individual 

 when he talks of ' squat like a toad ' ; its colours are by Werner 

 [Nomenclature of Colours, 1821] ink black, vermilion red and buff 

 orange 1 ." In the Journal of Researches 2 its colours are described as 

 follows : "If we imagine, first, that it had been steeped in the blackest 

 ink, and then, when dry, allowed to crawl over a board, freshly 

 painted with the brightest vermilion, so as to colour the soles of 

 its feet and parts of its stomach, a good idea of its appearance will 

 be gained." "Instead of being nocturnal in its habits, as other toads 

 are, and living in damp obscure recesses, it crawls during the heat of 

 the day about the dry sand-hillocks and arid plains,. . .." The appearance 

 and habits recall T. Belt's well-known description of the conspicuous 

 little Nicaraguan frog which he found to be distasteful to a 

 duck 3 . 



The recognition of the Warning Colours of caterpillars is due 

 in the first instance to Darwin, who, reflecting on Sexual Selection, 

 was puzzled by the splendid colours of sexually immature organisms. 

 He applied to Wallace " who has an innate genius for solving 

 difficulties 4 ." Darwin's original letter exists 5 , and in it we are told 

 that he had taken the advice given by Bates : "You had better ask 

 Wallace." After some consideration Wallace replied that he believed 

 the colours of conspicuous caterpillars and perfect insects were a 

 warning of distastefulness and that such forms would be refused 

 by birds. Darwin's reply 6 is extremely interesting both for its 



1 More Letters, i. p. 12. 2 187G, p. 97. 



3 The Naturalist in Nicaragua (2nd edit.) London, 1888, p. 321. 



4 Descent of Man, p. 325. On this and the following page an excellent account 

 of the discovery will be found, as well as in Wallace's Natural Selection, London, 1875, 

 pp. 117—122. 



5 Life and Letters, in. pp. 93, 94. 



6 Life and Letters, in. pp. 94, 95. 



