Darwin fully recognised that the colours of living beings are not
necessarily of value as colours, but that they may be an incidental
result of chemical or physical structure. Thus he wrote to T. Meehan,
Oct. 9, 1874: "I am glad that you are attending to the colours of
dioecious flowers; but it is well to remember that their colours may be
as unimportant to them as those of a gall, or, indeed, as the colour
of an amethyst or ruby is to these gems." ("More Letters of Charles
Darwin", Vol. I. pages 354, 355. See also the admirable account of
incidental colours in "Descent of Man" (2nd edition), 1874, pages 261,
262.)
Incidental colours remain as available assets of the organism ready to
be turned to account by natural selection. It is a probable speculation
that all pigmentary colours were originally incidental; but now and for
immense periods of time the visible tints of animals have been modified
and arranged so as to assist in the struggle with other organisms or in
courtship. The dominant colouring of plants, on the other hand, is
an essential element in the paramount physiological activity of
chlorophyll. In exceptional instances, however, the shapes and visible
colours of plants may be modified in order to promote concealment.