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." ("More
Letters", I. page 12.) In the "Journal of Researches" (1876, page 97.)
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 Naturalist in Nicaragua" (2nd edition)
London, 1888, page 321.)
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."
("Descent of Man", page 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, pages 117-122.) Darwin's original letter
exists ("Life and Letters", III. pages 93, 94.), 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 ("Life and Letters", III. pages 94, 95.) is extremely
interesting both for its enthusiasm at the brilliancy of the hypothesis
and its caution in acceptance without full confirmation:
"Bates was quite right; you are the man to apply to in a difficulty. I
never heard anything more ingenious than your suggestion, and I hope you
may be able to prove it true. That is a splendid fact about the white
moths (A single white moth which was rejected by young turkeys, while
other moths were greedily devoured: "Natural Selection", 1875, page
78.); it warms one's very blood to see a theory thus almost proved to be
true."
Two years later the hypothesis was proved to hold for caterpillars of
many kinds by J. Jenner Weir and A.G. Butler, whose observations have
since been abundantly confirmed by many naturalists. Darwin wrote to
Weir, May 13, 1869: "Your verification of Wallace's suggestion seems
to me to amount to quite a discovery." ("More Letters", II. page 71
(footnote).)