In his reply to  CD questions the significance of the supposed likeness of the bee, spider, and fly orchids to their presumed namesakes.
He thinks that the beauty of shells is altogether incidental and of no use to the animals.
Down. | Beckenham | Kent. S.E.
My dear Sir
I have often reflected over the two cases to which you refer.When working on Orchids, I have often & long watched the speciesof Ophrys, because R. Brown thought that their supposed likenessto insects served to keep them away.f2 This might possibly beadvantageous to the Bee Ophrys; but certainly not to theothers, as they absolutely require the aid of insects forfertilisation, though they get this aid but rarely or ratherrarely.—
I believe (& Dr Hookerf3 who happens to be here agrees with me)that the resemblance is fanciful;f4 the flowers are odd looking &insects are the most natural standard of comparison. Who ever sawa Bee with violet wings like the petals of the Bee Ophrys? TheFly Ophrys is more like. Hooker believes that the Spider ophrys isso called simply from the curved marks on the Labellum like themarks on the backs of some Epeiræ. The Butterfly orchis hashardly any resemblance to a butterfly, & so with some foreignorchids which have received all kinds of fanciful names.—f5
Your second case seems to me much more difficult: for my ownpart I believe the beauty of shells is altogether incidental onother causes & of no service whatever to the animals. The formdepending on manner & symmetry of growth, & the colour beingincidental on the chemical nature of the constituent element ofthe shell.f6 To explain what I mean,—nothing is more beautifulthan arterial blood, yet no one supposes that the colour has beengained for the sake of colour or beauty, though it gives beautyto the cheeks of Caucasian maidens. Or again, & this is a betterillustration; the extreme beauty of the Eolidæ is due to theirbiliary secretions & organs being seen through their transparenttissues; but no one will suppose that their bile has beencoloured for beauty, but is incidental on its chemical nature, asmuch as the recently discovered aniline colours.f7 Nothing in thisnote can, I think, be of the least service to you, but you arewelcome to use it as [illeg] from me, if you think fit. I haveintroduced these illustrations in my M.S. on sexualselection.—f8
Pray believe me | My dear Sir | Yours sincerely | Ch. Darwin
Pray forgive this untidy & ill-expressed letter from want oftime.—
Bearing in mind the magnificent colours of many artificialorganic substances made by elements, it wd be wonderful if suchhad not been produced under nature, without being of any specialuse: if there had been no such beautiful colours, this fact wd.have had been to have been accounted for—
I think of the beauty & splendour of the withering leaves in anAmerican forest.—