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 (Phryniscus 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!” In the Journal of Researches? 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’, 
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*.” Darwin’s original letter exists®, 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® is extremely interesting both for its 
1 More Letters, 1. p. 12. 2 1876, 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, 111. pp. 93, 94. 
8 Life and Letters, 111. pp. 94, 95. 
