Grant Allen: The colors of flowers.

The Living age 1963, 4.2.1882

From The Cornhill Magazine.

Before me, as I write, stands a small specimen vase, containing a little Scotch bluebell, picked upon a bleak, open moorside, yet wonderfully delicate and fragile in stem, and leaf, and bud, and blossom. For the bluebells of Scotland, the bluebells of Walter Scott and of all the old ballad poetry, are not our stiff, thickstemmed English wild hyacinths, but the same dainty, drooping flowers which we in the south call harebells. The word ought really to be heather-bell; but the corruption is quite in accordance with a common law of English phonology, which has similarly degraded several other early words by dropping out the th between two vowels. Harebell or heather-bell or bluebell, the flower is one of our prettiest and most graceful native forms; and the exquisite depth of its color has always made it a prime favorite with our poets and our children alike. How it first got that beautiful color is the problem which I wish, if possible. to settle to-day.

I am not going to inquire at present why the harebell is colored at all. That question I suppose everybody has now heard answered a dozen times over at least. We all know nowadays that the colors of flowers are useful to them in attracting the insects which fertilize their embryo seeds; and that only those flowers possess bright hues which thus depend upon insects for the impregnation of their ovules. Wind - fertilized blossoms, in which the pollen of one head is carried by chance breezes to the stigma of another, are always small, green, and comparatively inconspicuous. It is only those plants which are indebted to bees or butterflies for the due setting of their seeds that ever advertise their store of honey by bright-hued petals. All this, as I say, we have each of us heard long ago. So the specific question which I wish to attack to-day is not why the harebell is colored, but why it is colored blue. And, in getting at the answer to this one test-question, I hope incidentally to answer the wider question why any given flower whatsoever should be blue, let us say, or red, or lilac, rather than orange, yellow, white, or any other possible color in nature except the one which it actually happens to be.

Briefly put, the general conclusion at which I have arrived is this: all flowers 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, 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.

In the first place, it is well to observe that when we speak of the colors of flowers we generally mean the color of the petals alone. For in most cases the stamens and other central organs, which form, botanically speaking, the really important part of the blossom, are yellow. or at least yellowish; while the petals may be blue, red, pink, orange, lilac, or even green. But as the central organs are comparatively small, whereas the petals are large and conspicuous, we naturally speak of flowers in everyday talk as having the color of their petals, which form by far the greater and most noticeable part of their whole surface. Our question, then, narrows itself down to this — Why are the petals in any particular blossom of one color rather than another?

Now petals, as I have more than once already explained to the readers of this magazine, are in all probability originally I enlarged and flattened stamens, which have been set apart for the special work of attracting insects. It seems likely that i all flowers at first consisted of the central organs alone — that is to say, the pistil, which contains the ovary with its embryo seeds; and the stamens, which produce the pollen, whose cooperation is necessary in order to fertilize these same embryo ovules and to make the pistil mature into the ripe fruit. But in those plants which took to fertilization by means of insects — or, one ought rather to say, in those plants which insects took to visiting for the sake of their honey or pollen, and so unconsciously fertilizing — the flowers soon began to produce an outer row of barren and specialized stamens, adapted by their size and color for attracting the fertilizing insects; and these barren and specialized stamens are what we commonly call petals. Any flowers which thus presented brilliant masses of color to allure the eyes of the beetles, the bees, and the butterflies would naturally receive the greatest number of visits from their insect friends, and would therefore stand the best chance of setting their seeds, as well as of producing healthy and vigorous offspring as the result of a proper cross. In this way, they would gain an advantage in the struggle for life over their less fortunate compeers, and would hand down their own peculiarities to their descendants after them.

* In a part of this article I shall have to go over ground already considered in a valuable paper read by Sir John Lubbock before the British Association at York last August, and I shall take part of my examples from his interesting collection of facts as reported in Nature. But, at the same time. I should like at the outset to point out that I venture to differ on two points from his great authority. In the first place, I do not think all flowers were originally green, because I believe petals were first derived from altered stamens, not from altered sepals or bracts, and that modern green flowers are degraded types, not survivals of early forms. And in the second place. I think yellow petals preceded white petals in the order of time, and not vice versa. I may also perhaps be excused for adding that I had already arrived at most of the substantive conclusions set forth in this article before the appearance of Sir John Lubbock's paper, and bad incidentally put forward the greater part of them, though dogmatically and without fully stating my reasons, in an article on the "Daisy's Pedigree," published in the Cornhill Magazine, and in another on the "Rose family," published in Belgaria, both for August, 1881. At the same time, must express my indebtness for many new details to Sir John Lubbock's admirable paper. Of course this note is only appended for the behoof of scientific readers. But as the stamens of almost all flowers, certainly of all the oldest and simplest flowers, are yellow, it would naturally follow that the earliest petals would be yellow too. When the stamens of the outer row were flattened and broadened into petals, there would be no particular reason why they should change their color; and, in the absence of any good reason, they doubtless retained it as before. Indeed, I shall try to show, a little later on, that the earliest and simplest types of existing flowers are almost always yellow, seldom white, and never blue; and this in itself would he a sufficient ground for believing that yellow was the original color of all petals.* But as I am personally somewhat heretical, in believing, contrary to the general run of existing scientific opinion, that petals are derived from flattened stamens, not front simplified and attenuated leaves, I shall venture to detail here the reasons for this belief; because it seems to me of capital importance in connection with our present subject. For if the petals were originally a row of stamens set apart for the function of attracting insects, it would be natural and obvious why they should begin by being yellow; but if they were originally a set of leaves, which became thinner and more brightly colored for the sante purpose, it would be difficult to see why they should first have assumed any one color rather than another.

The accepted doctrine as to the nature of petals is that discovered by Wolff and afterwards rediscovered by Goethe, after whose name it is usually called; for of course, as in all such cases, the greater man's fame has swallowed up the fame of the lesser. Goethe held that all the parts of the flower were really modified leaves, and that a gradual transition could be traced between them, from the ordinary leaf through the stemleaf and the bract to the sepal (or division of the calyx), the petal, the stamen, and the ovary or carpel. Now, if we look at most modern flowers, such a transition can undoubtedly be observed; and sometimes it is very delicately graduated. so that you can hardly say where each sort of leaf merges into the next. But, unfortunately for the truth of the theory as ordinarily understood, we now know that in the earliest flowers there were no petals or sepals. but that primitive flowering plants had simply leaves on the one hand, and stamens and ovules on the other. The oldest types of flowers at present surviving, those of the pine tribe and of the tropical cycads (such as the wellknown zamias of our conservatories), have still only these simple elements. But if petals and sepals are later in origin (as we know them to be) than stamens and carpels, we cannot say, it seems to me, that they mark the transition from one form to the other, any more than we can say that Gothic architecture i marks the transition from the Egyptian style to the classical Greek. I do not mean to deny that the stamen and the ovary are themselves by origin modified leaves — that part of the Wolffian theory is absolutely irrefutable — but what I do mean to say is this, that, with the light shed upon the subject by the modern doctrine of evolution, we can no longer regard petals and sepals as intermediate stages between the two. The earliest flowering plants had true leaves on the one hand, and specialized pollen-bearing or ovule-bearing leaves on the other hand, which latter are what we call stamens and carpels; but they had no petals at all. and the petals of modern flowers have been produced at some later period. I believe, also, they have been produced by a modification of certain external stamens, not by a modification of true leaves. Instead of being leaves arrested on their way towards becoming stamens, they are stamens which have partially reverted towards the condition of leaves. They differ from true leaves, however, in their thin, spongy texture, and in the bright pigments with which they are adorned.

All stamens show a great tendency easily to become petaloid, as the technical botanists call it; that is to say, to flatten out their filament or stalk, and finally to lose their pollen-bearing sacs or anthers. In the waterlilies — which are one of the oldest and simplest types of flowers we now possess, still preserving many antique points of structure unchanged — we can trace a regular gradation from the perfect stamen to the perfect petal. In the centre of the flower, we find stamens of the ordinary sort. w ith rounded stalks or filaments, and long, yellow anthers full of pollen at the end of each; then, as we move outward, we find the filaments growing flatter and broader, and the pollensacs less and less perfect; next we find a few stamens which look exactly like petals, only that they have two abortive anthers stuck awkwardly on to their summits; and, finally, we find true petals, broad and flat, yellow or white as the case may be, and without any trace of the anthers at all. Here in this very ancient flower we have stereotyped for us as it were, the mode in which stamens first developed into petals, under stress of insect selection.

* I must add that i do not in the least doubt the tee, of Wohlff's great generalisation in the way in which he meant it — the existence of a homology between the leaf and all the floral organs; I only mean that the conception requires to be modified a little by the light later evolutionary discoveries. "But how do you know," some one may ask, "that the transition was not in the opposite direction? How do you know that the waterlily had not petals alone to start with, and that these did not afterwards develop, as the Wolffian hypothesis would have us believe, into stamens "Well, for a very simple reason. The theory of Wolff and Goethe is quite in compatible with the doctrine of development, at least if accepted as a historical explanation (which Wolff and Goethe of course never meant it to be). Flowers can and do exist without petals, which are no essential part of the organism, but a mere set of attractive colored advertise. ments for alluring insects; but no flower can possibly exist without stamens, which are one of the two essential reproductive organs in the plant. Without pollen. no flower can set its seeds. A parallel from the animal world will make this immediately obvious. Hive-bees consist of three kinds the queens or fertile females, the drones or males, and the workers or neuters. Now it would be absurd to ask whether the queens were developed from an original class of neuters, or the neuters from an original class of fertile females. Neuters left to themselves would die out in a single generation: they are really sterilized females, set apart for a special function on behalf of the hive. It is just the same with petals: they are sterilized stamens, set apart for the special function of attracting insects on be. half of the entire flower. But to ask which came first, the petals or the stamens, is as absurd as to ask which came first, the male and female bees or the neuters.*

In many other cases besides the waterlily, we know that stamens often turn into petals. Thus the numerous colored rays of the mesembryantheinums or ice-plant family are acknowledged to be flattened stamens. In double roses and almost all other double flowers the extra petals are produced from the stamens of the interior. In short, stamens generally can be readily converted into petals, especially in rich and fertile soils or under cultivation. Even where stamens always retain their pollensacs, they have often broad, flattened, petaloid filaments, as in the star of Bethlehem and many other flowers. Looking at the question as a whole, we can sec how petals might easily have taken their origin from stamens, while it is difficult to understand how they could have taken their origin from ordinary leaves — a process of which if it ever took place, no hint now remains to us. We shall see hereafter that the manner in which certain outer florets in the compound flower-heads of the daisy or the aster have been sterilized and specialized for the work of attraction, affords an exact analogy to the manner in which it is here suggested that certain stamens may at an earlier date have been sterilized and specialized for the same purpose, thus giving rise to what we know as petals.

We may take it for granted, then (to return from this long but needful digression), that the earliest petals were derived from flattened stamens, and were therefore probably yellow in color, like the stamens from which they took their origin. The question next arises — How did some of them afterwards come to be orange, red, purple, or blue?

A few years aco, when the problem of the connection Between flowers and insects still remained much in the state where Sprengel left it at the end of the last century, it would have seemed quite impossible to answer this question. But nowadays, after the full researches of Darwin. Wallace, Lubbock, and Hermann Müller into the subject, we can give a very satisfactory solution indeed. We now know, not only that the colors of flowers as a whole are intended to attract insects in general, but that certain colors are definitely intended to attract certain special kinds of insects. Thus, to take a few examples only out of hundreds that might be cited, the flowers which lay themselves out for fertilization by miscellaneous small flies are almost always white; those which depend upon the beetles are generally yellow; while those which bid for the favor of bees and butterflies are usually red, purple, lilac, or blue. Certain insects always visit one species of flower alone; and others pass from blossom to blossom of one kind only on a single day, though they may vary a little from kind to kind as the season advances, and one species replaces another. Muller, the most statistical of naturalists, has noticed that while bees form seventy-five per cent. of the insects visiting the very developed composites, theyform only fourteen per cent. of those visiting umbelliferous plants, which have, as a rule, open but by no means showy white flowers. Certain blossoms which lay themselves out to attract wasps are, as he quaintly puts it, "obviously adapted to a less zesthetically cultivated circle of visitors." And some livid red flowers actually resemble in their color and odor decaying raw meat, thus inducing bluebottle flies to visit them and so carry their pollen from head to head.

Down to the minutest distinctions between species, this correlation of flowers to the tastes of their particular guests seems to hold good. Hermann Muller notes that the common galium of our heaths and hedges is white, and therefore visited by small flies; while the lady's bedstraw. its near relative, is yellow, and owes its fertilization to little beetles. Mr. H. O. Forbes counted on one occasion the visits he saw paid to the flowers on a single bank; and he found that a particular bumblebee sucked the honey of thirty purple deadnettles in succession, passing over without notice all the other plants in the neighborhood; two other species of bumblebee and a cabbage butterfly also patronized the same deadnettles exclusively. Fritz Muller noticed a lantana in South America which changes color as its flowering advances; and he observed that each kind of butterfly which visited it stuck rigidly to its own favorite color, waiting to pay its addresses until that color appeared. Mr. Darwin cut off the petals of a lobelia and found that the hive-bees never went near it, though they were very busy with the surrounding flowers. But perhaps Sir John Lubbock's latest experiments on bees are the most conclusive of all. lie had long ago convinced himself, by trials with honey placed on slips of glass above yellow, pink, or blue paper, that bees could discriminate the different colors; and he has now shown in the same way that they display a marked preference for blue over all others. The fact is, blue flowers arc, as a rule, specialized for fertilization by bees, and bees therefore prefer this color; while conversely the flowers have at the same time become blue because that was the color which the bees prefer. As in most other cases, the adaptation must have gone on pari passu on both sides. As the beeflowers grew bluer, the bees must have grown fonder and fonder of blue; and as they grew fonder of blue, they must have more and more constantly preferred the bluest flowers.

We thus see how the special tastes of insects may have become the selective agency for developing white, pink, red, purple, and blue petals from the original yellow ones. But before they could exercise such a selective action, the petals must themselves have shown some tendency to vary in certain fixed directions. How could such an original tendency arise? For, of course, if the insects never saw any pink, purple, or blue petals, they could not specially favor and select them; so that we are as yet hardly nearer the solution of the problem than ever.

Here Mr. Sorby, who has chemically studied the coloring matter of leaves and flowers far more deeply than any other investigator, supplies us with a useful hint. lie tells us that the various pigments of bright petals are already contained in the ordinary tissues of the plant, whose juices only need to be slightly modified in chemical constitution in order to make them into the blues, pinks, and purples with which we are so familiar. "The colored substances in the petals," he says, "are in many cases exactly the same as those in the foliage from which chlorophyll has disappeared; so that the petals are often exactly like leaves which have turned yellow and red in autumn, or the very yellow or red leaves of early spring." " The color of many crimson, pink, and red flowers is due to the development of substances belonging to the erythrophyll group, and not unfrequently to exactly the same kind as that so often found in leaves. The facts seem to indicate that these various substances may be due to an alteration of the normal constituents of leaves. So far as I have been able to ascertain, their development seems as if related to extra oxidization, modified by light and other varying conditions not yet understood."

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 yellow through red to blue, rather than vice versa? I believe there is; and we get hints of it in the following fashion.

One of our common little English forget-me-nots, by name Myosotis versicolor (may I be pardoned for using a few scientific names just this once?) 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 colorchange is by no means uncommon; and in all the cases that I know of 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-flower tribe, Cheiranthus chamerleo, has at first a whitish flower, then a citronyellow, and finally emerges into red or violet. The petals of Stylidium fruticosum are pale yellow to begin with, and afterwards become light rose-colored. Au evening primrose, Œnothera tetraptera, has white flowers in its first stage and red ones at a later period of development. Cobæa scandensgoes from white to violet; Hibiscus mutabilis from white through flesh-colored to red. Fritz Muller's lantana is yellow on its first day, orange on the second, and purple on the third. The whole tribe of borages 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 oxidation of 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 wingplumes; the metallic hues of sunbirds and 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 fruitpigeons, 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 eyespots 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, like that of this harebell, probably marks the highest level of all.

On the other hand, Mr. Wallace's principle also explains why the bees and but terflies should prefer these specialized colors to all others, and should therefore select the 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 flowerfeeding. 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 honeyglands 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 riffraff, 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 and butterflies, the aristocrats of the arthropodous world, would grow for the most part to he 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, which can be visited by any insects whatsoever; and these are in large part yellow or white. A little higher are the flowers 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 onesided flowers, which have assumed special shapes to accommodate bees or other specific
honeyseekers; and these are often purple and not infrequently blue. Highly specialized in another way are the flowers 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 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. I shall proceed in the sequel to give examples.

One may say that the most profoundly modified of all existing flowers are the families of the composites, the labiates, the snapdragons, and the orchids. Now these are exactly the families in which blue and purple flowers are commonest; while in all of them, except the composites. white flowers are rare, and unmixed yellow flowers almost unknown. But perhaps the best way to test the principle will be to look at one or two families in detail, remembering of course that we can only expect approximate results, owing to the natural complexity of the conditions. Not to overburden the subject with unfamiliar names I shall seldom I go beyond the limits of our own native English flora.

The roses form a most instructive family to begin with. As a whole they are not very highly developed, since all of them have simple, open, symmetrical flowers, generally with five distinct petals. Bat of all the rose tribe, as I have endeavored to show elsewhere, the potentilla group, including our common English cinquefoils and silver-weed, seem to make up the most central, simple, and primitive members. They are chiefly low, creeping weeds, and their flowers are of the earliest pattern, without any specialization of form, or any peculiar adaptation to insect visitors. Now among the potentilla group, nearly all the blossoms are yellow, as are also those of the other early allied forms, such as agrimony and herb-bennet. Almost the only white potentillas in England are the barren strawberry and the true strawberry, which have diverged more than any other species from the norms of the race. Water-avens, how-ever, a close relative of herb-bennet. has a dusky purplish tinge and Sir John Lubbuc notes that it secretes honey, and is far oftener visited by insects than its kinsman. The bramble tribe, including the blackberry, raspberry, and dew-berry. have much larger flowers than the potentillas, and are very greatly frequented by winged visitors. Their petals are pure white, often with a pinky tinge, especially on big, well-grown blossoms. But there is one low, little-developed member of the blackberry group, the stone-bramble, with narrow, inconspicuous petals of a greenish yellow, merging into dirty white; and this humble form seems to preserve for us the transitional stage from the yellow potentilla to the true white brambles. One step higher. the cherries, apples, and pears have very large and expanded petals, white toward the centre, but blushing at the edges into rosy pink or bright red. Finally, the true roses, whose flowers are the most developed of all, have usually extremely broad pink petals (like those of our own dog-rose), which in some still bigger exotic species become crimson or damask of the deepest dye. They are more sought after by insects than any others of their family. At the same time, the roses as a whole, being a relatively simple family, with regular symmetrical flowers of the separate type, have never risen to the stage of producing blue petals. That is why our florists cannot turn out a blue rose. It is easy enough to make roses or any other blossoms vary within their own natural limits, revert to any earlier form or color through which they have previously passed; but it is difficult or impossible to make them take a step which they have never yet naturally taken. Hence florists generally find the most developed flowers are also the most variable and plastic in color; and hence, too, we can get red, pink, white, straw-colored, or yellow roses, but not blue ones. This, I believe, is the historical truth underlying De Candolle's division of flowers into a xanthic and a cyanic series.

Still more interesting, because covering a wider range of color, are the buttercup family, whose petals vary from yellow to every shade of crimson, purple, and blue. Here, the simplest and least differentiated members of the group are the common meadow buttercups, which, as everybody knows, have five open petals of a brilliant golden hue. Nowhere else is the exact accordance in color between stamens and petals more noticeable than in these flowers. There are two kinds of buttercup in England, however, which show us the transition from yellow to white actually taking place under our very eyes. These are the water crowfoot and its close ally the ivy-leaved crowfoot, whose petals are still faintly yellow toward the centre, but fade away into primrose and white as they approach the edge. The clematis and anemone, which are more highly developed, have white sepals (for the petals here are suppressed), even in our English species; and exotic kinds varying from pink to purple are cultivated in our flower-gardens. Columbines are very specialized forms of the buttercup type, both sepals and petals being brightly colored, while the former organs are produced above into long, bow-shaped spurs, each of uhich secretes a drop of honey; and various columbines accordingly range from red to purple and dark blue. Even the columbine, however, though so highly specialized, is not bilaterally but circularly symmetrical. This last and highest mode of adaptation to insect visits is found in larkspur, and still more developed in the curious monkshood. Now larkspur is usually blue, though white or red blos-soms sometimes occur by reversion; while monkshood is one of the deepest blue flowers we possess. Sir John Lubbock has shown that a particular bumble-bee (Bontbus hortorum), is the only north European insect capable of fertilizing the larkspur.

The violets are a whole family of bilateral flowers, highly adapted to fertilization by insects, and as a rule they are blue. I sere, too, however, white varieties easily arise by reversion; while one member of the group, the common pansy, is perhaps, the most variable flower in all nature.

Pinks do not display so wide a range in either direction. They begin as high up as white, and never get any higher than red or carnation. The small, undeveloped field species, such as the chick-weeds, stitchworts, and corn-spurries, have open flowers of very primitive character, and almost all of them are white. They are fertilized by miscellaneous small flies. Rut the campions and true pinks have a tubular calyx, and the petals are raised on long claws, while most of them also display special adaptations for a better class of insect fertilization in the way of fringes - or crowns on the petals. These higher kinds are generally pink or red. Our own beautiful purple English corn-cockle is a highly developed campion, so specialized that only butterflies can reach its honey with their long tongues, as the nectaries are situated at the bottom of the tube. Two other species of campion, however, show us interestingly the way in which variations of color may occur in a retrograde direction even among highly evolved forms. One of them, the day lychnis, has red, scentless flowers, opening; in the morning, and it is chiefly fertilized by diurnal butterflies. But its descendant, the night lychnis, has taken to fertilization by means of moths; and as moths can only see white flowers, it has become white, and has acquired a faint perfume as an extra attraction. Still, the change has not yet become fully organized in the species, for one may often find a night lychnis at the present time which is only pale pink, instead of being pure white.

The only other family of flowers with separate petals which I shall consider here is that of the pea-blossoms. These are all bilateral in shape, as everybody knows; but the lower and smaller species, such as the medick, lotus, and lady's fingers, are usually yellow. So also are broom and gorse. Among the mare specialized clovers, some of which are fertilized by bees alone, white, red, and purple predominate. Even with the smaller and earlier types, the most developed species, like lucerne, are likewise purple. But in the largest and most advanced types, the peas, beans, vetches, and scarlet runners, we get much brighter and deeper colors, often with more or less tinge of blue. In the sweet peas and many others, the standard frequently differs in hue from the keel or the wings — a still further advance in heterogeneity of coloration. Lupines, sain loin, everlasting pea, and wistaria are highly evolved meintiers of the same family, in which purple, lilac, mauve, or blue tints become distinctly pronounced.

When we pass on, however, to the flowers in which (as in this harebell) the petals have all coalesced into a tubular or campanulate corolla, we get even more striking results. Here, where the very shape at once betokens high modification, yellow is a comparatively rare color (especially as a ground-tone, though it often comes out in spots or patches), while purple and blue, so rare elsewhere, become almost the rule. For example, in the great family of the heaths, which is highly adapted to insect fertilization, more particularly by bees, purple and blue are the prevailing tints, so much so that, as we all have noticed a hundred times over, they often color whole tracts of hillside together. So far as I know, there are no really yellow heaths at all. The bell shaped blos-soms mark at once the position of the heaths with reference to insects; and the order, according to Mr. Bentham, supplies us with more ornamental plants than any other in the whole world.

It is the same with the families allied to my harebell here. They are, in fact, for the most part larger and handsomer blossoms of the same type as the heaths; and the greater number of them, like the hare-bell itself and the Canterbury bell, are deep blue. Rampion and sheep's bit, also blue, are clustered heads of similar blossoms. The little blue lobelia of our borders, which is bilateral as well as tubular, belongs to a closely related tribe. Not far from them are the lilac scabious, the blue devil's bit, and the mauve teasel. Amongst all these very highly evolved groups blue distinctly forms the prevalent color.

The composites, to which belong the daisies anti dandelions, also give us some extremely striking evidence. Each flower-head here consists of a number of small florets, crowded together so as to resemble a single blossom. So far as our present purpose is concerned, they fall naturally into three groups. The first is that of the dandelions and hawkweeds, with open florets, fertilized, as a rule, by very small insects; and these are generally yellow, with only a very few divergent species. The second is that of the thistleheads, visited by an immense number of insects, including the bees; and these are almost all purple, while some highly evolved species, like the cornflower or bluebottle and the true artichoke, are bright blue. The third is that of the daisies and asters, with tubular central florets and long, flattened outer rays; and these demand a closer examination here.

The central florets of the daisy tribe, as a rule, are brightE;olden; a fact which shows pretty certain),y that they are descended from a common ancestor who was also yellow. Moreover, these yellow florets are bell-shaped, and each contain a pistil and five stamens, like any other perfect flower. But the outer florets are generally sterile; and instead of being bell-shaped they are split down one side and unrolled, so as to form a long ray; while their corolla is at the same time much larger than that of the central blossoms. In short, they are sterilized members of the compound flower-head, specially set apart for the work of display; and thus they stand to the entire flower-head in the same relation as petals do to the simple original flower. The analogy between the two is complete. Just as the petal is a specialized and sterilized stamen told off to do duty as an allurer of insects for the benefit of the whole flower, sit the ray-floret is a specialized and sterilized blossom told off to do the selfsame duty for the benefit of the group of tiny flowers which make up the composite flower-head.

Now, the earliest ray-florets would naturally be bright yellow, like the tubular blossoms of the central disk from which they sprang. And to this day the ray florets of the simplest daisy types, such as the cornmarigold, the sunflower, and the ragwort, are yellow like the central flowers. In the camomile, however, the ox-eye daisy, and the mayweed, the rays have become white; and this, I think, fairly estailishes the fact that white is a higher development of color than yellow; for the change must have been made in order to attract special insects. Certainly, such a differentiation of the flowers in a single head cannot be without a good purpose. In the true daisy, again, the white rays become tipped with pink, which sometimes rises almost to rose-color and this stage is exactly analogous to rose-color; of apple-blossom, which similarly halts on the way from white petals to red. In the asters and Michaelmas daisies we get a further advance to purple, lilac, and mauve. while both in these and in the chrysanthemums true shades of blue not infrequently appear. The cinerarias of our gardeners are similar forms of highly developed groundsels from the Canary Islands.

* Our English archangels and few others are yellow. Such cases of reversion are wit uncommon, and are doubtless due to special insect selection in a retrograde direction.I must pass over the blue tubular gentians and periwinkles, with many other like cases, for I can only find room for two more families. One of these, the borage kind, has highly modified flowers, with a tube below and spreading lobes above; in addition to which most of the species possess remarkable and strongly developed appendages to the corolla, in the way of teeth, crowns, hairs, scales, parapets, or valves. Of the common British species alone, the forget-me-nots are clear sky-blue with a yellow eye; the viper's bugloss is at first reddish purple, and afterwards a deep blue; the lungwort is also dark blue; and so are the two alkanets, the true bugloss, the madwort, and the familiar borage of our claretcup, though all of them by reversion occasion' ally produce purple or white flowers. Iloundstongue is purple red, and most of the other species vary between purple and blue; indeed throughout the family most flowers are red at first and blue as they mature. Of these, borage at least is habitually fertilized by bees, and I believe the same to be partially true of many of the other species. The second highly evolved family to which I wish to draw attention is that of the labiates — perhaps the most specialized of any so far as regards insect fertilization. Not only are they tubular, but they are very bilateral and irregular indeed, displaying more modification of form than any other flowers except the orchids. Almost all of them are purple or blue. Among the bestknown English species are thyme, mint, marjoram, sage, and basil, which I need hardly say are great favorites with bees. Groundivy is bright blue; catmint, pale blue; prunella, violet purple; and common bugle, blue or fleshcolor. Many of the others are purple or purplish.* It must be added thaat in both these families the flowers are very liable to vary within the limit of the same species; and red, white, or purple specimens are common in all the normally blue kinds.

Sometimes, indeed, we may say that the new color has not yet begun to fix itself in the species, but that the hue still varies under our very eyes. Of this the little milkwort (a plant of the type with separate petals) affords an excellent example, for it is occasionally white, usually pink, and not infrequently blue; so that in all probability it is now actually in course of acquiring a new color. Much the same thing happens with the common pimpernel. Its ancestral form is probably the woodland loosestrife, which is yellow; but pimpernel itself is usually or ange red, while a blue variety is frequent on the Continent, and sometimes appears in England as well. Every botanist can add half-adozen equally good instances from his own memory.

So far I have spoken only of what the ladies would call selfcolor, as though every flower were of one unvaried hue throughout. I must now add a few words on the subject of the spots and lines which so often variegate the petals in certain species. On this subject, again, Mr. Wallace's hint is full of meaning. Everywhere in nature, he points out, spots and eyes of color appear on the most highly modified parts, and this rule applies most noticeably to the case of petals. Simple regular flowers, like the buttercups and roses, hardly ever have any spots or lines; but in very modified forms like the labiates and the orchids they are extremely common. The scrophularineous family, to which the snapdragon belongs, is one most specially adapted to insects, and even more irregular than that of the labi; ates; and here we find the most singular effects produced by dappling and mixture of colors. The simple yellow mullein, it is true, has no such spots or lines, nor have even many of the much higher blue veronicas; but in the snapdragons, the foxglove, the toadflax, the ivy-linaria, the eyebright, and the calceolarias, the intl. mate mixture of colors is very noticeable. In the allied tropical bignomas and gloxinias we see much the same distribution of hues. Many of the family are cultivated in gardens on account of their bizarre and fantastic shapes and colors. As to the orchids, I need hardly say anything about their wonderfully spotted and variegated flowers. Even in our small English kinds the dappling is extremely marked, especially upon the expanded and profoundly modified lower lip: but in the larger tropical varieties the patterns are often quaint and even startling in their extraordinary richness of fancy and apparent capriciousness of design. Mr. Darwin has shown that their adaptations to insects are more intimate and more marvellous than those of any other flowers whatsoever.

Structurally speaking, the spots and lines on petals seem to be the direct result of high modification; but functionally, as Sprengel long ago pointed out, they act as honeyguides, and for this purpose they have no doubt undergone special selection by the proper insects. Lines are comparatively rare on regular flowers, but they tend to appear as soon as the flower becomes even slightly bilateral. and they point directly towards the nectaries. The geranium family affords an excellent illustration of this law. The regular forms are mostly uniform in hue; but many of the South African pelargoniums, cultivated in gardens and hothouses, are slightly bilateral, the two upper petals standing off from the three lower ones; and these two become at once marked with dark lines, which are in sonic cases scarcely visible, and in others fairly pronounced. From this simple beginning one can trace a gradual progress in heterogeneity of coloring, till at last the most developed bilateral forms have the two upper petals of quite a different hue front the three lower ones, besides being deeply marked with belts and spots of dappled color. In the allied tropzeolurn or Indian cress (the so-called nasturtium of old-fashioned gardens, thought the plant is really no more related to the watercress and other true nasturtiums than we ourselves are to the great kangaroo) this tendency is carried still further. Here, the calyx is prolonged into a deep spur, containing the honey, inaccesssible to any but a few large insects; and towards this spur all the lines on the petals converge. Sir John Lubbock observes that without such conventional marks to guide them, bees would waste a great deal of time in bungling about the mouths of flowers; for they are helpless, blundering things at an emergency, and never know their way twice to the same place if any change has been made in the disposition of the familiar surroundings.

Finally, there remains the question why have some flowers green petals? This is a difficult problem to attack at the end of a long paper; and indeed it is one of little interest for ninety-nine people out of a hundred; since the flowers with green petals are mostly so small and inconspicuous that nobody but a professional botanist ever troubles his head about them. The larger part of the world is somewhat surprised to learn that there are such things as green flowers at all; though really they are far commoner than the showy-colored ones. Nevertheless, lest I should seem to be shirking a difficulty altogether, I shall add that I believe green petals to be in almost every case degraded representatives of earlier yellow or white ones. This belief is clean contrary to the accepted view, which represents the green, wind-fertilized blossoms as older in order of time than their colored insect-fertilized allies. Nevertheless, I think all botanists will allow that such green orgreenish flowers as the hellebores, the plantains, the lady's mantle, the saladburner, the moschatel, the twayblade, and the parsley-piert are certainly descended from bright-hued ancestors, and have lost their colors on their petals though acquiring the habit of wind-fertilization or self-fertilization. Starting from these, I can draw no line as I go downward in the scale through such flowers as knawel, goosefoot, dog's mercury, nettle, and arrowgrass, till I get to absolutely degraded blossoms like glasswort, callitriche, and pondweed, whose real nature nobody but a botanist would ever suspect. Whether the catkins, the grasses, and the sedges were ever provided with petals I do not venture to guess; but certainly wherever we find the merest rudiment of a perianth I am compelled to believe that the plant has descended from brightcolored ancestors, however remotely. And when we look at the very de I graded blossoms of the spurges, which we know by the existence of intermediate links to be derived from perianthbearing forefathers, the possibility at least of this being also true of catkins and grasses cannot be denied. So far as I can see, the conifers and cycads are the only flowering plants which we can be quite sure never possessed colored and attractive petals. But this digression is once more only intended for the scientificallyminded reader.

If the general principle here put for ward is true, the special colors of different flowers are clue to no mere spontaneous accident, nay, even to no meaningless caprice of the fertilizing insects. They are due in their inception to a regular law of progressive modification : and they have been fixed and stereotyped in each species by the selective action of the proper beetles, bees, moths, or butterflies. Not only can we say why such a color, once happening to appear, has been favored in the struggle for existence, but also why that color should ever make its appearance in the first place, which is a condition precedent to its being favored or selected at all. For example, blue pigments are often found in the most highly developed flowers, because blue pigments are a natural product of high modification — a simple chemical outcome of certain extremely complex biological changes. On the other hand, bees show a marked taste for bit*. because blue is the color of the most advanced flowers and by always selecting such where possible, they both keep up and sharpen their own taste, and at the same time give additional opportunities to the blue flowers, which thus ensure proper fertilization. I believe it ought always to be the object of naturalists in this manner to show not only why such and such a "spontaneous" variation should have been favored whenever it occurred, but also to show why and how it could ever have occurred at all.

- Grant Allen.

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