This post updates my post of 17 Feb 2009.
I have recently found that the search engine at darwin-online.org is not perfect. If you search for “helic” on the whole darwin-online.org site, you’ll draw a blank. I had then also forgotten something I had known for a long time: that Darwin did discuss Heliconius and other mimetic butterflies in the Descent of Man and Selection in Relation to Sex.
Darwin was trying to explain bright coloration, and birds and butterflies were among his main empirical examples. Bright colours in many butterflies are characteristic of males, and can be explained by sexual selection.
However, heliconians and danaines are more or less sexually monomorphic. So why are these butterflies brightly coloured? Warning colours and mimicry provide an alternative explanation of bright coloration not involved with sexual selection. See: http://darwin-online.org.uk/content/frameset?itemID=F944&viewtype=text&pageseq=1 and search for “helic”.
This is also where Darwin recounts how Wallace in the 1860s solved the problem for him of bright colours in caterpillars (which have no sex, and so couldn’t be sexually selected!). They were warningly coloured.