From T. H. Farrer 5 May 1869
3 Gloucester Terrace | Regents Park
5 May /69
My dear Mr Darwin
You asked me last year what I meant by the “degraded” structure of Viola Odorata.—1 I think now, looking at the flowers again, the word was justified. In “Viola Tricolor” admirably described in Hildebrand, there is an apparatus for cross fertilization by insects almost as wonderful and complete as that of the Orchises.2 In Viola Cornuta, it is even still more elaborate.
In Viola Canina there is the same structure: but in almost every particular, less complete.— The flower less conspicuous: less open: the lower petal less coloured & less displayed: the anthers less syngenesious: good pollen less universal & smaller in quantity: and above all the style & stigma less adapted either in position or form to catch the pollen from the proboscis of the bee. Consequently Viola Canina does sets some seeds from open flowers—but not constantly or frequently as in Viola Tricolor.
In Viola Odorata the deficiencies of V. Can. are much increased. The nature & position of the footstalks: of the sepals: the colour & position & shape of the petals make the flower one of the most inconspicuous there is—and the way to the nectary is no “royal road”— The anthers seldom have good pollen. And the form &c of style & stigma though similar in generally to that of V. tricolor has lost the peculiarities & use which give it a meaning. Consequently it is very seldom that open flowers of Viola odorata set their seed.
Is there not here a gradation or degradation, shewing how an elaborate structure originally full of function & purpose becomes comparatively meaningless & useless.
Very truly yours | T H Farrer
Justifies his use of term "degraded" by comparing contrivances for cross-fertilisation in different species of Viola.