and why blue flowers should be specially adapted for fertilization by their aid.  For Mr. A.R.  Wallace has shown that color is most apt to appear or to vary in those parts of plants or animals which have undergone the highest amount of modification.  The markings of the peacock and the argus pheasant come out upon their immensely developed secondary tail-feathers or wing-plumes; the metallic hues of sun-birds, or humming-birds, show themselves upon their highly specialized crests, gorgets, or lappets.  It is the same with the hackles of fowls, the head ornaments of fruit-pigeons, and the bills of toucans.  The most exquisite colors in the insect world are those which are developed on the greatly expanded and delicately feathered wings of butterflies; and the eye-spots which adorn a few species are usually found on their very highly modified swallow-tail appendages.  So too with flowers:  those which have undergone most modification have their colors most profoundly altered.  In this way, we may put it down as a general rule (to be tested hereafter) that the least developed flowers are usually yellow or white; those which have undergone a little more modification are usually pink or red; and those which have been most highly specialized of any are usually purple, lilac, or blue.  Absolute deep ultramarine probably marks the highest level of all.

On the other hand, Mr. Wallace’s principle also explains why the bees and butterflies should prefer these specialized colors to all others, and should therefore select those flowers which display them by preference over any less developed types; for bees and butterflies are the most highly adapted of all insects to honey-seeking and flower-feeding.  They have themselves on their side undergone the largest amount of specialization for that particular function.  And if the more specialized and modified flowers, which gradually fitted their forms and the position of their honey-glands to the forms of the bees or butterflies, showed a natural tendency to pass from yellow through pink and red to purple and blue, it would follow that the insects which were being evolved side by side with them, and which were aiding at the same time in their evolution, would grow to recognize these developed colors as the visible symbols of those flowers from which they could obtain the largest amount of honey with the least possible trouble.  Thus it would finally result that the ordinary unspecialized flowers, which depended upon small insect riff-raff, would be mostly left yellow or white; those which appealed to rather higher insects would become pink or red; and those which laid themselves out for bees or butterflies, the aristocrats of the arthropodous world, would grow for the most part to be purple or blue.