THE COLORATION OF FLOWERS
From 'The Colors of Flowers'
The different hues assumed by petals are all thus, as it were, laid up beforehand in the tissues of the plant, ready to be brought out at a moment's notice. And all flowers, as we know, easily sport a little in color. But the question is, Do their changes tend to follow any regular and definite order? Is there any reason to believe that the modification runs from any one color toward any other? Apparently there is. The general conclusion to be set forth in this work is the statement of such a tendency. All flowers, it would seem, were in their earliest form yellow; then some of them became white; after that, a few of them grew to be red or purple; and finally, a comparatively small number acquired various shades of lilac, mauve, violet, or blue. So that if this principle be true, such a flower as the harebell will represent one of the most highly developed lines of descent; and its ancestors will have passed successively through all the intermediate stages. Let us see what grounds can be given for such a belief.
Some hints of a progressive law in the direction of a color-change from yellow to blue are sometimes afforded to us even by the successive stages of a single flower. For example, one of our common little English forget-me-nots, Myosotis versicolor, is pale yellow when it first opens; but as it grows older, it becomes faintly pinkish, and ends by being blue, like the others of its race. Now, this sort of color-change is by no means uncommon; and in almost all known cases it is always in the same direction, from yellow or white, through pink, orange, or red, to purple or blue. For example, one of the wall-flowers, Cheiranthus chamoeleo, has at first a whitish flower, then a citron-yellow, and finally emerges into red or violet. The petals of Stytidium fructicosum are pale yellow to begin with, and afterward become light rose-colored. An evening primrose, Oenothera tetraptera, has white flowers in its first stage, and red ones at a later period of development. Cobea scandens goes from white to violet; Hibiscus mutabilis from white through flesh-colored to red. The common Virginia stock of our gardens (Malcolmia) often opens of a pale yellowish green, then becomes faintly pink; afterward deepens into bright red; and fades away at the last into mauve or blue. Fritz Müller's Lantana is yellow on its first day, orange on its second, and purple on the third. The whole family of Boraginaceae begin by being pink and end with being blue. The garden convolvulus opens a blushing white and passes into full purple. In all these and many other cases the general direction of the changes is the same. They are usually set down as due to varying degrees of oxidation in the pigmentary matter. If this be so, there is a good reason why bees should be specially fond of blue, 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.
Now, this is very much what we actually find to be the case in nature. The simplest and earliest flowers are those with regular, symmetrical open cups, like the Ranunculus genus, the Potentillas, and the Alsine or chickweeds, which can be visited by any insects whatsoever; and these are in large part yellow or white. A little higher are flowers like the Campions or Sileneoe, and the stocks (Matthiola), with more or less closed cups, whose honey can only be reached by more specialized insects; and these are oftener pink or reddish. More profoundly modified are those irregular one-sided flowers, like the violets, peas, and orchids, which have assumed special shapes to accommodate bees and other specific honey-seekers; and these are often purple and not unfrequently blue. Highly specialized in another way are the flowers like harebells (Campanulaceoe), scabious (Dipsaceoe), and heaths (Ericaceoe), whose petals have all coalesced into a tubular corolla; and these might almost be said to be usually purple or blue. And finally, highest of all are the flowers like labiates (rosemary, Salvia, etc.) and speedwells (Veronica), whose tubular corolla has been turned to one side, thus combining the united petals with the irregular shape; and these are almost invariably purple or blue.
AMONG THE HEATHER
From 'The Evolutionist at Large'
I suppose even that apocryphal person, the general reader, would be insulted at being told at this hour of the day that all bright-colored flowers are fertilized by the visits of insects, whose attentions they are specially designed to solicit. Everybody has heard over and over again that roses, orchids, and columbines have acquired their honey to allure the friendly bee, their gaudy petals to advertise the honey, and their divers shapes to insure the proper fertilization by the correct type of insect. But everybody does not know how specifically certain blossoms have laid themselves out for a particular species of fly, beetle, or tiny moth. Here on the higher downs, for instance, most flowers are exceptionally large and brilliant; while all Alpine climbers must have noticed that the most gorgeous masses of bloom in Switzerland occur just below the snow-line. The reason is, that such blossoms must be fertilized by butterflies alone. Bees, their great rivals in honey-sucking, frequent only the lower meadows and slopes, where flowers are many and small: they seldom venture far from the hive or the nest among the high peaks and chilly nooks where we find those great patches of blue gentian or purple anemone, which hang like monstrous breadths of tapestry upon the mountain sides. This heather here, now fully opening in the warmer sun of the southern counties--it is still but in the bud among the Scotch hills, I doubt not--specially lays itself out for the humble-bee, and its masses form almost his highest pasture-grounds; but the butterflies--insect vagrants that they are--have no fixed home, and they therefore stray far above the level at which bee-blossoms altogether cease to grow. Now, the butterfly differs greatly from the bee in his mode of honey-hunting: he does not bustle about in a business-like manner from one buttercup or dead-nettle to its nearest fellow; but he flits joyously, like a sauntering straggler that he is, from a great patch of color here to another great patch at a distance, whose gleam happens to strike his roving eye by its size and brilliancy. Hence, as that indefatigable observer, Dr. Hermann Müller, has noticed, all Alpine or hill-top flowers have very large and conspicuous blossoms, generally grouped together in big clusters so as to catch a passing glance of the butterfly's eye. As soon as the insect spies such a cluster, the color seems to act as a stimulant to his broad wings, just as the candle-light does to those of his cousin the moth. Off he sails at once, as if by automatic action, towards the distant patch, and there both robs the plant of its honey, and at the same time carries to it on his legs and head fertilizing pollen from the last of its congeners which he favored with a call. For of course both bees and butterflies stick on the whole to a single species at a time; or else the flowers would only get uselessly hybridized, instead of being impregnated with pollen from other plants of their own kind. For this purpose it is that most plants lay themselves out to secure the attention of only two or three varieties among their insect allies, while they make their nectaries either too deep or too shallow for the convenience of all other kinds.