LECTURE X
THE ORIGIN OF FLOWERS
Introduction—Precursors of Darwin—Pollination by wind—Arrangements in flowers for securing cross-fertilization—Salvia, Pedicularis—Flowers visited by flies—Aristolochia—Pinguicula—Daphne—Orchids—Flowers are built up of adaptations—Mouth-parts of insects—Proboscis of butterflies—Mouth-parts of the cockroach—Of the bee—Pollen baskets of bees—Origin of flowers—Attraction of insects by colour—Limitation of the area visited—Nägeli's objection to the theory of selection—Other interpretations excluded—Viola calcarata—Only those changes which are useful to their possessors have persisted—Deceptive flowers—Cypripedium—Pollinia of Orchis—The case of the Yucca-moth—The relative imperfection of the adaptations tells in favour of their origin through natural selection—Honey thieves.
When one species is so intimately bound up with another that neither can live for any length of time except in partnership, that is certainly an example of far-reaching mutual adaptation, but there are innumerable cases of mutual adaptation, in which, although there is no common life in the same place, yet the first form of life is adjusted in relation to the peculiarities of the second, and the second to those of the first. One of the most beautiful, and, in regard to natural selection, the most instructive of these cases is illustrated by the relations between insects and the higher plants, relations which have grown out of the fact that many insects have formed the habit of visiting the flowers of the plants for the sake of the pollen. In this connexion the theory of selection has made the most unexpected and highly interesting disclosures, for it has informed us how the flowers have arisen.
In earlier times the beauty, the splendour of colour, and the fragrance of flowers were regarded as phenomena created for the delight of mankind, or as an outcome of the infinite creative power of Mother Nature, who loves to run riot in form and colour. Without allowing our pleasure in all this manifold beauty to be spoilt, we must nowadays form quite a different conception of the way in which the flowers have been called into being. Although here, as everywhere else in Nature, we cannot go back to ultimate causes, yet we can show, on very satisfactory evidence, that the flowers illustrate the reaction of the plants to the visits of insects, and that they have been in large measure evoked by these visits. There might, indeed, have been blossoms, but there would have been no flowers—that is to say, blossoms with large, coloured, outer parts, with fragrance, and with nectar inside, unless the blossoms had been sought out by insects during the long ages. Flowers are adaptations of the higher flowering plants to the visits of insects. There can be no doubt about that now, for—thanks to the numerous and very detailed studies of a small number of prominent workers—we need not only suppose it, we can prove it with all the certainty that can be desired. The mutual adaptations of insects and flowers afford one of the clearest examples of the mode of operation and the power of natural selection, and the case cannot therefore be omitted from lectures on the theory of descent.
That bees and many other insects visit flowers for the sake of the nectar and pollen has been known to men from very early times. But this fact by itself would only explain why adaptations to flower-visiting have taken place in these insects to enable them, for instance, to reach the nectar out of deep corolla-tubes, or to load themselves with a great quantity of pollen, and to carry it to their hives, as happens in the case of the bees. But what causes the plants to produce nectar, and offer it to the insects, since it is of no use to themselves? And further, what induces them to make the pillage easier to the insects, by making their blossoms visible from afar through their brilliant colours, or by sending forth a stream of fragrance that, even during the night, guides their visitors towards them?
As far back as the end of the eighteenth century a thoughtful and clear-sighted Berlin naturalist, Christian Konrad Sprengel, took a great step towards answering this question. In the year 1793 he published a paper entitled 'The Newly Discovered Secret of Nature in the Structure and Fertilization of Flowers[8],' in which he quite correctly recognized and interpreted a great many of the remarkable adaptations of flowers to the visits of insects. Unfortunately, the value of these discoveries was not appreciated in Sprengel's own time, and his work had to wait more than half a century for recognition.
[8] Das neu-entdeckte Geheimniss der Natur im Bau u. der Befruchtung der Blumen, Berlin, 1793.
Sprengel was completely dominated by the idea of an all-wise Creator, who 'has not created even a single hair without intention,' and, guided by this idea, he endeavoured to penetrate into the significance of many little details in the structure of flowers. Thus he recognized that the hairs which cover the lower surface of the petals of the wood-cranesbill (Geranium sylvaticum) protect the nectar of the flower from being diluted with rain, and he drew the conclusion, correct enough, though far removed from our modern ideas as regards the directly efficient cause, that the nectar was there for the insects.
He was also impressed by the fact that the sky-blue corolla of the forget-me-not (Myosotis palustris) has a beautiful yellow ring round the entrance to the corolla-tube, and he interpreted this as a means by which insects were shown the way to the nectar which is concealed in the depths of the tube.
Fig. 40. Potentilla verna, after Hermann Müller. A,
seen from above. Kbl, sepals. Bl, petals. Nt, nectaries
near the base of the stamens. B, section through the
flower. Gr, stigma. St, stamen. Nt, nectary.
We now know that such 'honey-guides' are present in most of the flowers visited by insects, in the form of spots, lines, or other marking, usually of conspicuous colour, that is, of a colour contrasting with the ground colour of the flower. Thus, in species of Iris, regular paths of short hairs lead the way to the place where the nectar lies. In the spring potentilla (Potentilla verna) (Fig. 40) the yellow petals (A, Bl) become bright orange-red towards their bases, and this shows the way to the nectaries, which lie at the bases of the stamens (st), and are protected by hairs, the so-called 'nectar-covers' (Saftdecke) of Sprengel, from being washed by rain.
The recognition of the honey-guides led Sprengel on to the idea that the general colouring of the flower effects on a large scale what the honey-guides do in a more detailed way—it attracts the attention of passing insects to where nectar is to be found; indeed, he went an important step further by recognizing that there are flowers which cannot fertilize themselves, in which the insect, in its search for honey, covers itself with pollen, which is then rubbed off on the stigma of the next flower visited, fertilization being thus effected. He demonstrated this not only for the Iris, but for many other flowers, and he drew the conclusion that 'Nature does not seem to have wished that any flower should be fertilized by its own pollen.' How near Sprengel was to reaching a complete solution of the problem is now plain to us, for he even discovered that many flowers, such as Hemerocallis fulva, remained infertile if they were dusted with their own pollen.
Even the numerous experiments of that admirable German botanist, C. F. Gärtner, although they advanced matters further, did not suffice to make the relations between insects and flowers thoroughly clear; for this the basis of the theory of Descent and Selection was necessary. Here, again, it was reserved for Charles Darwin to lead the way where both contemporaries and predecessors had been blindly groping. He recognized that, in general, self-fertilization is disadvantageous to plants; that they produce fewer seeds, and that these produce feebler plants, than when they are cross-fertilized; that, therefore, those flowers which are arranged to secure cross-fertilization have an advantage over those which are self-fertilized. In many species, as Sprengel had already pointed out, self-fertilization leads to actual infertility; only a few plants are as fertile with their own pollen as with that of another plant; and Darwin believed that, in all flowering plants, crossing with others of the same kind, at least from time to time, is necessary if they are not to degenerate.
Thus the advantage which the flowers derive from the visits of insects lies in the fact that insects are instrumental in the cross-fertilization of the flowers, and we can now understand how the plant was able to vary in a manner favourable to the insect-visits, and to exhibit adaptations which serve exclusively to make these visits easier; we understand how it was possible that there should develop among flowers an endless number of contrivances which served solely to attract insects, and even how, for the same end, the insignificant blossoms of the oldest Phanerogams must have been transformed into real flowers.
We must not imagine, however, that the obviously important crossing of plant-individuals, usually called 'cross-pollination,' can be effected only by means of insects. There were numerous plants in earlier times, and there is still a whole series in which cross-fertilization is effected through the air by the wind; these are the anemophilous or wind-pollinated Angiosperms.
To these belong most of the catkin-bearers, such as hazel and birch, and also the grasses and sedges, the hemp and the hop, and so forth. In these plants there is no real flower, but only an inconspicuous blossom, without brightly-coloured outer envelopes, without fragrance or nectar; all of them have smooth pollen grains, which easily separate into fine dust and are carried away by the wind until they fall, by chance, far from their place of origin, on the stigma of a female blossom.
Fig. 41. Flower of Meadow Sage (Salvia pratensis),
after H. Müller. st´, immature anthers concealed
in the 'helmet' of the flower. st´´, mature anther
lowered. gr´, immature stigma. gr´´, mature
stigma. U, the lower lip of the corolla, the
landing-stage for the bee.
By far the greater number of the phanerogams, however, especially all our indigenous 'flowers,' are, as a rule, fertilized by means of insects, and it is amazing to see in what diverse ways, often highly specialized, they have adapted themselves to the visits of insects. Thus there are flowers in which the nectar lies open to view, and these can be feasted on by all manner of insects; there are others in which the nectar is rather more concealed, but still easily found, and reached by insects with short mouth-parts, e.g. large flowers blooming by day and bearing much pollen, like the Magnolias. These have been called beetle-flowers, because they are visited especially by the honey-loving Longicorns.
Other flowers blooming by day are especially adapted to fertilization by means of bees; they are always beautifully coloured, often blue; they are fragrant, and contain nectar deep down in the flower, where it can only be reached by the comparatively long proboscis of the bee. Different arrangements in the different flowers secure that the bee cannot enjoy the nectar without at the same time effecting the cross-pollination. Thus the stamens of the meadow sage (Salvia pratensis) are at first hidden within the helmet-shaped upper lip of the flower (Fig. 41, st´), but bear lower down on their stalk a short handle-like process, which turns the pollen-bearing anther downwards (st´´) as soon as it is pressed back by an intruding insect. The pollen-sacs then strike downwards on the back of the bee, and cover it with pollen. When the bee visits another more mature flower, the long style, which was at first hidden within the helmet, has bent downwards (gr´´), and now stands just in front of the entrance to the flower, so that the bee must rub off a part of the pollen covering its back on to the stigma, and fertilization is thus effected.
There are other flowers which are specially disposed to suit the visits of the humble-bees, as, for instance, Pedicularis asplenifolia, the fern-leaved louse-wort, a plant of the high Alps (Fig. 42). The first thing that strikes us about this plant is the thickly tufted hair covering on the calyx (k), which serves to keep off little wingless insects from the flower; then there is the strange left-sided twisting of the individual flowers, whose under lip allows only a strong insect like the humble-bee to gain access, towards the left, to the corolla-tube (kr), in the depths of which the nectar is concealed. While the humble-bee is sucking up the nectar it becomes dusted over with pollen from the anthers, which falls to dust at a touch, and when it insinuates itself into a second flower its powdered back comes first into contact with the stigma of the pistil (gr) which projects from the elongated bill-shaped under lip, dusting it over with the pollen of the first visited flower. Butterflies and smaller bees cannot rob this flower; it is strictly a humble-bee's flower.
Fig. 42. Alpine Lousewort (Pedicularis asplenifolia). A, flower seen from the left side, enlarged three times; the arrows show the path by which the humble-bee enters. B, the same flower, seen from the left, after removal of the calyx, the lower lip and the left half of the upper lip. C, ovary (ov), nectary (n), and base of style. D, tip of style, bearing the stigma. E, two anthers turned towards one another. o, upper lip. u, lower lip. gr, style. st, anthers. kr, corolla-tube. k, calyx.
There are not a few of such flowers adapted to a very restricted circle of visitors, and in all of them we find contrivances which close the entrance to all except what we may call the welcome insects; sometimes there are cushions of bristles which prevent little insects from creeping up from below, or it is the oblique position of the flower which prevents their getting in from the stem; sometimes it is the length and narrowness of the corolla-tube, or the deep and hidden situation of the nectar, which only allows intelligent insects to find the treasure.
Very remarkable are those flowers which are adapted to the visits of flies, for they correspond in several respects to the peculiarities of these insects. In the first place, flies are fond of decaying substances and the odours given off by these, and so the flowers which depend for their cross-fertilization on flies have taken on the dull and ugly colours of decay, and give out a disagreeable smell. But flies are also shy and restless, turning now hither, now thither, and cannot be reckoned among the 'constant' insect visitors, that is to say, they do not persistently visit the same species; it is, therefore, evident that they might easily carry away the pollen without any useful result ensuing. Moreover, their intelligence is of a low order, and they do not seek nectar with the perseverance shown by bees and humble-bees. It is not surprising, therefore, to find that many of the flowers adapted for the visits of flies are so constructed that they detain their visitors until they have done their duty, that is to say, until they have effected, or at least begun, the process of cross-pollination.
Fig. 43. Flower of Birthwort (Aristolochia
clematitis) cut in half. A, before
pollination by small flies. b, the
bristles. B, after pollination. P, pollen
mass. N, stigma, b, the bristles.
b´, their remains. After H. Müller.
Fig. 44. Alpine Butterwort
(Pinguicula alpina).
A, section through the flower.
K, calyx. bh, bristly prominences.
sp, spur. st, stamen.
n, stigma. B, stigma and
stamen more magnified.
After H. Müller.
Our birthwort (Aristolochia clematitis) and the Cuckoo-pint (Arum maculatum) are pit-fall flowers, whose long corolla-tubes have an enlargement at the base, in which both pistil and stamens are contained. In the birthwort (Fig. 43) the narrow entrance-tube is thickly beset with stiff hairs (A, b), whose points are all directed towards the base. Little flies can creep down quite comfortably into the basal expansion, but once there they are kept imprisoned until the flower, in consequence of the pollination of the stigma, begins to wither, the first parts to go being these very bristles (B, b´), whose points, like a fish-weir, prevented the flies from creeping out. Other 'fly-flowers,' as for instance the Alpine butterwort (Pinguicula alpina) (Fig. 44), securely imprison the plump fly as soon as it has succeeded in forcing itself in far enough to reach, with its short proboscis, the nectar contained in the spur (sp) of the corolla. The backward-directed bristles hold it fast for some time, and it is only by hard pressing with the back against the anthers (st) lying above it, and against the stigma (n), that it ultimately succeeds in getting free, but it never does so without having either loaded itself with pollen, or rubbed off on the stigma the pollen it brought with it from another similar flower. The Alpine butterwort is protogynous, that is to say, the pistil ripens first, the pollen later, so that the possibility of self-fertilization is altogether excluded.
It would be impossible to give even an approximate idea of the diversity of the contrivances for securing fertilization in flowers without spending many hours over them, for they are different in almost every flower, often widely so, and even in species of the same genus they are by no means always alike; for not infrequently one species is adapted to one circle of visitors, and its near relative to another. Thus the flower of the common Daphne (Daphne mezereum) (Fig. 45, A and C) is adapted to the visits of butterflies, bees, and hover-flies, while its nearest relative (Daphne striata) (Fig. 45, B and D) has a somewhat narrower and longer corolla-tube, so that only butterflies can feast upon it. This example shows that there are exclusively 'butterfly flowers,' but specialization goes further, for there are flowers adapted to diurnal and others to nocturnal Lepidoptera. The former have usually bright, often red colours, and a pleasant aromatic fragrance, and in all of them the nectar lies at the bottom of a very narrow corolla-tube. To this class belong, for instance, the species of pink, many orchids, such as Orchis ustulata, and Nigritella angustifolia of the Alps, which smells strongly of vanilla; also the beautiful campion (Lychnis diurna) and the Alpine primrose (Primula farinosa). The flowers adapted to nocturnal Lepidoptera are characterized by pale, often white colour, and a strong and pleasant smell, which only begins to stream out after sunset, and indeed many of these flowers are quite closed by day. This is the case with the large, white, scentless bindweed (Convolvulus sepium), which is chiefly visited and fertilized by the largest of our hawk-moths (Sphinx convolvuli). The pale soapwort (Saponaria officinalis) exhales a delicate fragrance which attracts the Sphingidæ from afar, and the sweet smell of the honeysuckle (Lonicera periclymenum) is well known, and has the same effect; an arbour of honeysuckle often attracts whole companies of our most beautiful Sphingidæ and Noctuidæ on warm June nights, to the great delight of the moth-collecting youth.
Fig. 45. Daphne mezereum (A and C) and Daphne striata (B and D). The former visited by butterflies, bees, and flies, the latter by butterflies only. A and B, vertical sections of the flowers. St, stamens. Gr, style. n, nectary. C and D, flowers seen from above. After H. Müller.
I cannot conclude this account of flower-adaptations without considering the orchids somewhat more in detail, for it is among them that we find the most far-reaching adaptations to the visits of insects. Among them, too, great diversity prevails, as is evident from the fact that Darwin devoted a whole book to the arrangements for fertilization in orchids, but the main features are very much the same in the majority. Figure 46 gives a representation of one of our commonest species (Orchis mascula), A shows the flower in side view, B as it appears from in front. The flower seems as it were to float on the end of the stalk (st), stretching out horizontally the spur (sp) which contains the nectar. Between the large, broad under lip (U), marked with a honey-guide (sm), and offering a convenient alighting surface, and the broad, cushion-like stigma (n) lies the entrance to the spur. Fertilization occurs in the following way:—The fly or bee, when it is in the act of pushing its proboscis into the nectar-containing spur, knocks with its head against the so-called rostellum (r), a little beak-like process at the base of the stamens (p). The pollen masses are of very peculiar construction, not falling to dust, but forming little stalked clubs, with the pollen grains glued together, and so arranged that they spring off when the rostellum is touched and attach themselves to the head of the insect, as at D on the pencil (Fig. 46). When the bee has sucked up the nectar out of the spur, and then proceeds to penetrate into another flower of the same species, the pollinia have bent downwards on its forehead (E), and must unfailingly come in contact with the stigma of the second flower, to which they now remain attached, and effect its fertilization. What a long chain of purposeful arrangements in a single flower, and no interpretation of them is available except through natural selection!
Fig. 46. Common Orchis (Orchis mascula). A, flower in side view. st, stalk. sp, spur with the nectary (n). ei, entrance to the spur. U, lower lip. B, flower from in front. p, pollinia. Sm, honey-guide. ei, entrance to the nectar. na, stigma. r, rostellum. U, lower lip. C, vertical section through the rostellum (r), pollinium (p). ei, entrance. D, the pollinia removed and standing erect on the tip of a lead-pencil. E, the same, somewhat later, curved downwards.
And how diversely are these again modified in the different genera and species of orchids, of which one is adapted to the visits of butterflies exclusively, as Orchis ustulata, another to those of bees, as Orchis morio, and a third to those of flies, as Ophrys muscifera. These flowers are adapted to insect visits in the minutest details of the form of the petals, which are smooth, as if polished with wax, where insects are not intended to creep, but velvety or hairy where the path leads to the nectar, and at the same time to the pollen and the stigma. And then there is the diversity in the form and colour of the 'honey-guides' on the 'alighting surface,' that is, the under lip of the flower, upon which the insect sits and holds fast, while it pushes its head as far as possible into the spur, so that its proboscis may reach the nectar lying deep within it! Even though we cannot pretend to guess at the significance of every curve and colour-spot in one of the great tropical orchids, such as Stanhopea tigrina, yet we may believe, with Sprengel, that all this has its significance, or has had it for the ancestors of the plant in question, and in fact that the flower is made up of nothing but adaptations, either actual or inherited from its ancestors, although sometimes perhaps no longer of functional importance.
So far, then, we have illustrated the fact that there are hundreds and thousands of contrivances in flowers adapted solely to the visits of insects and to securing cross-fertilization, and these adaptations go so far that we might almost believe them to be the outcome of the most exact calculation and the most ingenious reflection. But they all admit of interpretation through natural selection, for all these details, which used to be looked upon as merely ornamental, are directly or indirectly of use to the species; directly, when, for instance, they concern the dusting of the insect with the pollen; indirectly, when they are a means of attracting visits.
Moreover, the evidence of the operation of the processes of selection becomes absolutely convincing when we consider that, as in symbiosis, there are always two sets of adaptations taking place independently of one another—those of the flowers to the visits of the insects, and those of the insects to the habit of visiting the flowers. To understand this clearly we must turn our attention to the insects, and try to see in what way they have been changed by adapting themselves to the diet which the flowers afford.
As is well known, several orders of insects possess mouth-parts which are suited for sucking up fluids, and these have evolved, through adaptation to a fluid diet, from the biting mouth-parts of the primitive insects which we see still surviving in several orders. Thus the Diptera may have gradually acquired the sucking proboscis which occurs in many of them by licking up decaying vegetable and animal matter, and by piercing into and sucking living animals. But even among the Diptera several families have more recently adapted themselves quite specially to a flower diet, to honey-sucking, like the hover-flies, the Syrphidæ,and the Bombyliidæ, whose long thin proboscis penetrates deep into narrow corolla-tubes, and is able to suck up the nectar from the very bottom. The transformation was not so important in this case, since the already existing sucking apparatus only required to be a little altered.
Again, in the order Hemiptera (Bugs) the suctorial proboscis does not owe its origin to a diet of flowers, for no member of the group is now adapted to that mode of obtaining food.
Fig. 47. Head of a Butterfly. A, seen from
in front. au, eyes. la, upper lip. md, rudiments
of the mandibles. pm, rudimentary
maxillary palps. mx´, the first maxillæ
modified into the suctorial proboscis. pl,
palps of labium or second maxillæ, cut off
at the root, remaining in B—which is a side
view. at, antennæ. Adapted from Savigny.
The proboscis of the Lepidoptera, on the other hand, depends entirely on adaptation to honey-sucking, and we may go the length of saying that the order of Lepidoptera would not exist if there were no flowers. This large and diverse insect-group is probably descended from the ancestors of the modern caddis-flies or Phryganidæ, whose weakly developed jaws were chiefly used for licking up the sugary juices of plants. But as flowering plants evolved the licking apparatus of the primitive butterflies developed more and more into a sucking organ, and was ultimately transformed into the long, spirally coiled suctorial proboscis as we see it in the modern butterflies (Fig. 47). It has taken some pains to trace this organ back to the biting mouth-parts of the primitive insects, for nearly everything about it has degenerated and become stunted except the maxillæ (mx´). Even the palps (pm) of these have become so small and inconspicuous in most of the Lepidoptera that it is only quite recently that remains of them have been recognized in a minute protuberance among the hairs. The mandibles (md) have quite degenerated, and even the under lip has disappeared, and only its palps are well developed (B, pl). But the first maxillæ (mx´), although very strong and long, are so extraordinarily altered in shape and structure that they diverge from the maxillæ of all other insects. They have become hollow, probe-like half-tubes, which fit together exactly, and thus form a closed sucking-tube of most complex construction, composed of many very small joints, after the fashion of a chain-saw, which are all moved by little muscles, and are subject to the will through nerves, and are also furnished with tactile and taste papillæ. Except this remarkable sucking proboscis there are no peculiarities in the body of the butterfly which might be regarded as adaptations to flower-visiting, with a few isolated exceptions, of which one will be mentioned later. This is intelligible enough, for the butterfly has nothing more to seek from the flower beyond food for itself; it does not carry stores for offspring.
The bees, however, do this, and accordingly we find that in them the adaptations to flower-visiting are not confined to the mouth-parts.
As far as we can judge now, the flower-visiting bees are descended from insects which resembled the modern burrowing-wasps. Among these the females themselves live on nectar and pollen, and build cells in holes in the ground, and feed their brood. They do not feed them on honey, however, but on animals—on caterpillars, grasshoppers, and other insects, which they kill by a sting in the abdomen, or often only paralyse, so that the victim is brought into the cells of the nest alive but defenceless, and remains alive until the young larva of the wasp, which emerges from the egg, sets to work to devour it.
Fig. 48. Mouth-parts of the
Cockroach (Periplaneta orientalis),
after R. Hertwig. la, upper lip
or labrum. md, mandibles. mx1,
first maxillæ, with c, cardo, st,
stipes, li, internal lobe or lacinia,
le, external lobe or galea, and pm,
the maxillary palp. mx2, the
labium or second maxillæ, with
similar detailed parts.
Before I go on to explain the origin of the sucking proboscis of the bee from the biting mouth-parts of the primitive insects I must first briefly consider the latter.
The biting mouth-parts of beetles, Neuroptera, and Orthoptera (Fig. 48), consist of three pairs of jaws, of which the first, the mandibles (md), are simply powerful pincers for seizing and tearing or chewing the food. They have no part in the development of the suctorial apparatus either in bees or in butterflies, so they may be left out of account. The two other pairs of jaws, the first and second maxillæ (mx1 and mx2), are constructed exactly on the same type, having a jointed basal portion (st) bearing two lobes, an external (le) and an internal (li), and a feeler or palp, usually with several joints, directed outwards from the lobes (pm and pl). The second pair of maxillæ (mx2) differs from the first chiefly in this, that the components of the pair meet in the median line of the body, and fuse more or less to form the so-called 'under lip' or labium. In the example given, the cockroach (Periplaneta orientalis), this fusion is only partial, the lobes having remained separate (le and li); and the same is true of the bee, but in this case the inner lobes have grown into a long worm-like process which is thrust into the nectar in the act of sucking.
Fig. 49. Head of the Bee. Au, compound
eyes. au, ocelli. at, antennæ. la, upper lip.
md, mandibles. mx1, first maxillæ, with pm,
the rudimentary maxillary palp. mx2, second
maxillæ with the internal lobes (li) fused to
form the 'tongue.' le, the external lobes of
the second maxillæ, known as 'paraglossæ.'
pl, labial palp.
Even the burrowing-wasps exhibit the beginnings of variation in this direction, for the under lip is somewhat lengthened and modified into a licking organ. The adaptation has not gone much further than this, even in one of the true flower-bees, Prosopis, which feeds its larvæ with pollen and honey, and it is only in the true honey-bee that the adaptation is complete (Fig. 49). Here the so-called 'inner lobe' of the under lip (li) has elongated into the worm-shaped process already mentioned; it is thickly covered with short bristles, and is called the 'tongue' of the bee (li). The outer lobes of the under lip have degenerated into little leaf-like organs, the so-called accessory tongue or paraglossa (le), while the palps of the under lip (pl) have elongated to correspond with the tongue, and serve as a sensitive and probably also as a smelling organ, in contrast to the palps of the first maxillæ, which have shrunk to minute stumps (pm). The whole of the under lip, which has elongated even in its basal portions, forms, with the equally long first maxillæ, the proboscis of the bee. The first maxillæ are sheath-like half-tubes, closely apposed around the tongue, and form along with it the suctorial tube, through which the nectar is sucked up. Thus, of the three pairs of jaws in insects, only the first pair, the mandibles, have remained unaltered, obviously because the bee requires a biting-organ for eating pollen, for kneading wax, and for building cells.
But bees do not only feast on nectar and pollen themselves, they carry these home as food for their larvæ. The form already mentioned, Prosopis, takes up pollen and nectar in its mouth, and afterwards disgorges the pulp as food for its larvæ, but the rest of the true bees have special and much more effective collecting-organs, either a thick covering of hair on the abdomen, or along the whole length of the posterior legs, or finally, a highly developed collecting apparatus, such as that possessed by the honey-bee—the basket and brush on the hind leg. The former is a hollow on the outer surface of the tibia, the latter a considerable enlargement of the basal tarsal joint, which, at the same time, is covered on the inner surface with short bristles, arranged in transverse rows like a brush. The bee kneads the pollen into the basket, and one can often see bees flying back to the hive with a thick yellow ball of pollen on the hind leg. In those bees which collect on the abdomen, like Osmia and Megachile, the pollen mass forms a thick clump on the belly, and in the case of Andrena Sprengel observed long ago that it sometimes flew with a packet of pollen bigger than its own body on the hind leg.
All these are contrivances which have gradually originated through the habit of carrying home pollen for the helpless larvæ shut up in the cells. They have developed differently in the various groups of bees, probably because the primary variations with which the process of selection began were different in the various ancestral forms.
In the ancestors of those which carry pollen on the abdomen there was probably a thick covering of hair on the ventral surface of the body, which served as a starting-point for the selection, and, in consequence, the further course of the adaptation would be concerned solely with this hair-covered surface, while variations in other less hairy spots would remain un-utilized.
After all this it will no longer seem a paradoxical statement that the existence of gaily coloured, diversely formed, and fragrant flowers is due to the visits of insects, and that, on the other hand, many insects have undergone essential transformations in their mouth-parts and otherwise as an adaptation to a flower diet, and that an entire order of insects with thousands of species—the Lepidoptera—would not be in existence at all if there had been no flowers. We must now attempt to show, in a more detailed way, how, by what steps, and under what conditions, our modern flowers have arisen from the earlier flowering plants. In this I follow closely the classic exposition which we owe to Hermann Müller.
The ancestral forms of the modern higher plants, the so-called 'primitive seed plants' or 'Archisperms,' were all anemophilous, as the Conifers and Cycads are still. Their smooth pollen-grains, produced in enormous quantities, fell like clouds of dust into the air, were carried by the wind hither and thither, and some occasionally alighted on the stigma of a female flower. In these plants the sexes often occur separately on different trees or individuals, and there must be a certain advantage in this when the pollination is effected by the wind.
The male flowers of the Archisperms would be visited by insects in remote ages, just as they are now; but the visitors came to feed upon the pollen, and did not render any service to the plant in return; they rather did it harm by reducing its store of pollen. If it was possible to cause the insect to benefit the plant at the same time as it was pillaging the pollen, by carrying some of it to female blossoms and thereby securing cross-fertilization, it would be of great advantage, for the plant would no longer require to produce such enormous quantities of pollen, and the fertilization would be much more certain than when it depended on the wind. It is obvious that the successful pollination of anemophilous plants implies good weather and a favourable wind.
Fig. 50. Flowers of the Willow (Salix cinerea); after H. Müller. A, the male. B, the female catkin. C, individual male flower; n, nectary. D, individual female flower; n, nectary. E, Poplar, an exceptional hermaphrodite flower.
It is plain that the utilization of the insect-visitors in fertilization might be secured in either of two ways; the female blossoms might also offer something attractive to the insects, or hermaphrodite flowers might be formed. As a matter of fact, both ways have been followed by Nature. An example of the former is the willow, the cross-fertilization of which was forced upon the insects by the development in both female and male blossoms of a nectary (Fig. 50, C and D), a little pit or basin in which nectar was secreted. The insects flew now to male and now to female willow-catkins, and in doing so they carried to the stigma of the female blossom the pollen, which in this case was not dusty but sticky, so that it readily adhered to their bodies.
The securing of cross-fertilization by the development of hermaphrodite flowers has, however, occurred much more frequently, and we can understand that this method secured the advantageous crossing much more perfectly, for the pollen had necessarily to be carried from blossom to blossom, while, in cases like that of the willow, countless male blossoms might be visited for nectar one after the other before the insect made up its mind to fly to a female blossom of the same species. The beginnings of the modification of the unisexual flowers in this direction may be seen in variations which occur even now, for we not infrequently find, in a male catkin, individual blossoms, which, in addition to the stamens, possess also a pistil with a stigma. (Fig. 50 E shows such an abnormal hermaphrodite flower from a poplar.)
As soon as hermaphrodite flowers came into existence the struggle to attract insects began in a more intense degree. Every little improvement in this direction would form the starting-point of a process of selection, and would be carried on and increased to the highest possible pitch of perfection.
It was probably the outer envelopes of the blossoms which first changed their original green into other colours, usually those which contrasted strongly with the green, and thus directed the attention of the insects to the flowers. Variations in the colour of ordinary leaves are always cropping up from time to time, whether it be that the green is transformed into yellow or that the chlorophyll disappears more or less completely and red or blue coloured juices take its place. Many insects can undoubtedly see colour, and are attracted by the size of coloured flowers, as Hermann Müller found by counting the visits of insects to two nearly related species of mallow, one of which, Malva silvestris, has very large bright rose-red flowers visible from afar, while the other, Malva rotundifolia, has very inconspicuous small pale-red flowers. To the former there were thirty-one different visitors, to the latter he could only make sure of four. The second species, as is to be expected, depends chiefly on self-fertilization.
It has recently been disputed from various quarters that insects are attracted by the colours of the flowers, and these objections are based chiefly on experiments with artificial flowers. But when, for instance, Plateau, in the course of such experiments saw bees and butterflies first fly towards the artificial flowers, and then turn away and concern themselves no more about them, that only proves that their sight is sharper than we have given them credit for; for though they may be deceived at a distance, they are not so when they are near; it is possible, too, that the sense of smell turns the scale[9]. I have myself made similar experiments with diurnal butterflies, before which I placed a single artificial chrysanthemum midst a mass of natural flowers. It rarely happened indeed that a butterfly settled on the artificial flower; they usually flew first above it, but did not alight. Twice, however, I saw them alight on the artificial flower, and eagerly grope about with the proboscis for a few moments, then fly quickly away. They had visited the real chrysanthemums or horse-daisies with evident delight, and eagerly sucked up the honey from the many individual florets of every flower, and they now endeavoured to do the same in the artificial flower, and only desisted when the attempt proved unsuccessful. In this experiment the colours were of course only white and yellow; with red and blue it is probably more difficult to give the exact impression of the natural flower-colours; and in addition there is the absence of the delicate fragrance exhaled by the flower.
[9] The experiments of Plateau have since been criticized by Kienitz-Gerloff, who altogether denies their value (1903).
It must be allowed that the colour is certainly not the sole attraction to the flower; the fragrance helps in most cases, and even this is not the object of the insect's visits. The real object is the nectar, to which colour and fragrance only show the way. The development of fragrance and nectar must, like that of the colour, have been carried on and increased by processes of selection, which had their basis in the necessity for securing insect-visits, and as soon as these main qualities of the flower were established greater refinements would begin, and flower-forms would be evolved, which would diverge farther and farther, especially in shape, from the originally simple and regular form of the blossom.
The reason for this must have lain chiefly in the fact that, after insect-visits in general were secured by a flower, it would be advantageous to exclude all insects which would pillage the nectar without rendering in return the service of cross-fertilization—all those, therefore, which were unsuited either because of their minute size or because of the inconstancy of their visits. Before the butterflies and the bees existed, the regularly formed flat flower with unconcealed nectar would be visited by a mixed company of caddis-flies, saw-flies, and ichneumon-flies. But as the nectar changed its place to the deeper recesses of the flower it was withdrawn from all but the more intelligent insects, and thus the circle of visitors was already narrowed to some extent. But when in a particular species the petals fused into a short tube, all visitors were excluded whose mouth-parts were too short to reach the nectar; while among those which could reach it the process of proboscis-formation began; the under lip, or the first maxillæ, or both parts together, lengthened step for step with the corolla-tube of the flower, and thus from the caddis-flies came the butterflies, and from the ichneumon-flies the burrowing-wasps (Sphegidæ) and the bees.
At first sight one might perhaps imagine that it would have been more advantageous to the flowers to attract a great many visitors, but this is obviously not the case. On the contrary, specialized flowers, accessible only to a few visitors, have a much greater certainty of being pollinated by them, because insects which only fly to a few species are more certain to visit these, and above all to visit many flowers of the same species one after another. Hermann Müller observed that, in four minutes, one of the humming-bird hawk-moths (Macroglossa stellatarum) visited 108 different flowers of the same species, the beautiful Alpine violet (Viola calcarata), one after the other, and it may have effected an equal number of pollinations in that short time.
It was, therefore, a real advantage to the flowers to narrow their circle of visitors more and more by varying so that only the useful visitors could gain access to their nectar, and that the rest should be excluded. Thus there arose 'bee-flowers,' 'butterfly-flowers,' 'hawk-moth flowers,' and, indeed, in many cases, a species of flower has become so highly specialized that its fertilization can only be brought about by a single species of insect. This explains the remarkable adaptations of the orchids and the enormous length of the proboscis in certain butterflies. Even our own hawk-moths Macroglossa stellatarum and Sphinx convolvuli show an astonishing length of proboscis, which measures 8 cm. in the latter species. In Macrosilia cluentius, in Brazil, the proboscis is 20 cm. in length; and in Madagascar there grows an orchid with nectaries 30 cm. in length, filled with nectar to a depth of 2 cm., but the fertilizing hawk-moth is not yet known.
Thus we may say that the flowers, by varying in one direction or another, have selected a definite circle of visitors, and, conversely, that particular insect-groups have selected particular flowers for themselves, for those transformations of the flowers were always most advantageous which secured to them the exclusive visits of their best crossing agents, and these transformations were, on the one hand, such as kept off unwelcome visitors, and, on the other hand, such as attracted the most suitable ones.
From the botanical point of view the assumption that flowers and flower-visiting insects have been adapted to each other by means of processes of selection has been regarded as untenable, because every variation in the flower presupposes a corresponding one in the insect. I should not have mentioned this objection had it not come from such a famous naturalist as Nägeli, and if it were not both interesting and useful in our present discussion. Nägeli maintained that selection could not, for instance, have effected a lengthening of the corolla-tube of a flower, because the proboscis of the insects must have lengthened simultaneously with it. If the corolla-tube had lengthened alone, without the proboscis of the butterfly being at the same time elongated, the flower would no longer be fertilized at all, and if the lengthening of the proboscis preceded that of the corolla-tube it would have no value for the butterfly, and could not therefore have been the object of a process of selection.
This objection overlooks the facts that a species of plant and of butterfly consists not of one individual but of thousands or millions, and that these are not absolutely uniform, but in fact heterogeneous. It is precisely in this that the struggle for existence consists—that the individuals of every species differ from one another, and that some are better, others less well constituted. The elimination of the latter and the preferring of the former constitutes the process of selection, which always secures the fitter by continually rejecting the less fit. In the case we are considering, then, there would be, among the individuals of the plant-species concerned, flowers with a longer and flowers with a shorter corolla-tube, and among the butterflies some with a longer and some with a shorter proboscis. If among the flowers the longer ones were more certain to be cross-fertilized than the shorter ones, because hurtful visitors were better excluded, the longer ones would produce more and better seeds, and would transmit their character to more descendants; and if, among the butterflies, those with the longer proboscis had an advantage, because the nectar in the longer tubes would, so to speak, be reserved for them, and they would thus be better nourished than those with the shorter proboscis, the number of individuals with long proboscis must have increased from generation to generation. Thus the length of the corolla-tube and the length of the proboscis would go on increasing as long as there was any advantage in it for the flower, and both parties must of necessity have varied pari passu, since every lengthening of the corolla was accompanied by a preferring of the longest proboscis variation. The augmentation of the characters depended on, and could only have depended on, a guiding of the variations in the direction of utility. But this is exactly what we call, after Darwin and Wallace, Natural Selection.
We have, however, in the history of flowers, a means of demonstrating the reality of the processes of selection in two other ways. In the first place, it is obvious that no other interpretation can be given of such simultaneous mutual adaptations of two different kinds of organisms. If we were to postulate, as Nägeli, for instance, did, an intrinsic Power of Development in organisms, which produces and guides their variations, we should, as I have already said, be compelled also to take for granted a kind of pre-established harmony, such as Leibnitz assumed to account for the correlation of body and mind: plant and insect must always have been correspondingly altered so that they bore the same relation to each other as two clocks which were so exactly fashioned that they always kept time, though they did not influence each other. But the case would be more complicated than that of the clocks, because the changes which must have taken place on both sides were quite different, and yet at the same time such that they corresponded as exactly as Will and Action. The whole history of the earth and of the forms of life must, therefore, have been foreseen down to the smallest details, and embodied in the postulated Power of Development.
But such an assumption could hardly lay claim to the rank of a scientific hypothesis. Although every grain of sand blown about by the wind on this earth could certainly only have fallen where it actually did fall, yet it is in the power of any of us to throw a handful of sand wherever it pleases us, and although even this act of throwing must have had its sufficient reason in us, yet no one could maintain that its direction and the places where the grains fell were predestined in the history of the earth. In other words: That which we call chance plays a part also in the evolution of organisms, and the assumption of a Power of Development, predestinating even in detail, is contradicted by the fact that species are transformed in accordance with the chance conditions of their life.
This can be clearly demonstrated in the case of flowers. That the wild pansy (Viola tricolor), which lives in the plains and on mountains of moderate elevation, is fertilized by bees, and the nearly allied Viola calcarata of the High Alps by Lepidoptera, is readily intelligible, since bees are very abundant in the lower region, and make the fertilization of the species a certainty, while this is not so in the High Alps. There the Lepidoptera are greatly in the majority, as every one knows who has traversed the flower-decked meads of the High Alps in July, and has seen the hundreds and thousands of butterflies and moths which fly from flower to flower. Thus the viola of the High Alps has become a 'butterfly-flower' by the development of its nectaries into a long spur, accessible only to the proboscis of a moth or butterfly. The chance which led certain individuals of the ancestral species to climb the Alps must also have supplied the incentive to the production of the changes adapted to the visits of the prevalent insect. The hypothesis of a predestinating Power of Development suffers utter shipwreck in face of facts like these.
We have, furthermore, an excellent touchstone for the reality of the processes of selection in the quality of the variations in flowers and insects. Natural selection can only bring about those changes which are of use to the possessors themselves; we should therefore expect to find among flowers only such arrangements as are, directly or indirectly, of use to them, and, conversely, among insects only such as are useful to the insect.
And this is what we actually do find. All the arrangements of the flowers—their colour, their form, their honey-guides, their hairy honey-paths (Iris), their fragrance, and their honey itself—are all indirectly useful to the plant itself, because they all co-operate in compelling the honey-seeking insect to effect the fertilization of the flower. This is most clearly seen in the case of the so-called 'Deceptive' flowers, which attract insects by their size and beauty, their fragrance, and their resemblance to other flowers, and force their visitors to be the means of their cross-fertilization, although they contain no nectar at all. This is the case, according to Hermann Müller, with the most beautiful of our indigenous orchids, the lady's slipper (Cypripedium calceolaris). This flower is visited by bees of the genus Andrena, which creep into the large wooden-shoe-shaped under lip in the search for honey, only to find themselves prisoners, for they cannot get out, at least by the way they came in, because of the steep and smoothly polished walls of the flower. There is only one way for the bee; it must force itself under the stigma, which it can only do with great exertion, and not without being smeared with pollen, which it carries to the next flower into which it creeps. It can only leave this one in the same way, and thus the pollen is transferred to the stigma by a mechanical necessity.
Such remarkable cases remind us in some ways of those cases of mimicry in which the deceptions have to be used with caution or they lose their effect. One might be disposed to imagine that such an intelligent insect as a bee would not be deceived by the lady's slipper more than once, and would not creep into a second flower after discovering that there was no nectar in the first. But this conclusion is not correct, for the bees are well accustomed in many flowers to find that the nectar has already been taken by other bees; they could therefore not conclude from one unsuccessful visit that the Cypripedium did not produce nectar at all, but would try again in a second, a third, and a fourth flower. If these orchids had abundantly covered flower-spikes like many species of Orchis, and if the species were common, the bees would probably soon learn not to visit them, but the reverse is the case. There is usually only one or, at most, two open flowers on the lady's slipper, and the plant is rare, and probably occurs nowhere in large numbers.
If we could find a flower in which the nectar lay open and accessible to all insects, and which did not require any service from them in return, the case could not be interpreted in terms of natural selection; but we do not know of any such case.
Fig. 51. The Yucca-moth (Pronuba
yuccasella). M, laying eggs in
the ovary of the Yucca flower.
n, the stigma. After Riley.
Conversely, too, there are no adaptations in the insects which are useful only to the flowers, and which are not of some use, directly or indirectly, to the insect itself. Bees and butterflies certainly carry the pollen from one flower to the stigma of another, but they are not impelled to do this by a special instinct; they are forced to do it by the structure of the flower, which has its stamens so placed and arranged that they must shake their pollen over the visitor, or it may be that the anthers are modified into stalked, viscid pollinia which spring off at a touch, and fix themselves, so to speak, on the insect's head. And even this is not all in the case of the orchis, for the insect would never of its own accord transfer these pollinia on to the stigma of the next flower; this is effected by the physical peculiarity which causes the pollinia, after a short time, to bend forwards on the insect's head.
All this fits in as well as possible with the hypothesis: how could an instinct to carry pollen from one flower to the stigma of another have been developed in an insect through natural selection, since the insect itself has nothing to gain from this proceeding? Accordingly, we never find in the insect any pincers or any kind of grasping organ adapted for seizing and transmitting the pollen.
There is, however, one very remarkable case in which this appears to be so, indeed really is so, and nevertheless it is not contradictory to, but is corroborative of, the theory of selection. The excellent American entomologist, Riley, established by means of careful observations that the large white flowers of the Yucca are fertilized by a little moth which behaves in a manner otherwise unheard of among insects. Only the females visit the flowers, and they at once busy themselves collecting a large ball of pollen. To this end they have on the maxillary palps (Fig. 52, C, mxp) a long process (si), curved in the form of a sickle, and covered with hairs, which probably no other Lepidopteron possesses, with the help of which the moth very quickly sweeps together a ball of pollen, it may be three times the size of her own head. With this ball the insect flies to the next flower, and there she lays her egg, by means of an ovipositor otherwise unknown among Lepidoptera (Fig. 52, A, op), in the pods of the flower. Finally, she pushes the ball of pollen deep into the funnel-shaped stigmatic opening on the pistil (Fig. 51, n), and so effects the cross-fertilization. The ovules develop, and when the caterpillars emerge from the egg four to five days later they feed on these until they are ready to enter on the pupa stage. Each little caterpillar requires about eighteen or twenty seeds for its nourishment (Fig. 52, B, r).
Fig. 52. The fertilization of the Yucca. A, ovipositor of the Yucca-moth. op, its sheath. sp, its apex. op1, the protruded oviduct. B, two ovaries of the Yucca, showing the holes by which the young moths escape, and (r) a caterpillar in the interior. C, head of the female moth, with the sickle-shaped process (si) on the maxillary palps for sweeping off the pollen and rolling it into a ball. mx1, the proboscis. au, eye. p1 base of first leg. D, longitudinal section through an ovary of the Yucca, soon after the laying of two eggs (ei). stk, the canal made by the ovipositor.
Here, then, we find an adaptation of certain parts of the moth's body in relation to the fertilization of the flower, but in this case it is as much in the interest of the moth as of the plant. By carrying the pollen to the stigma the moths secure the development of the ovules, which serve their offspring as food, so that we have here to do with a peculiar form of care for offspring, which is not more remarkable than many other kinds of brood-care in insects, such as ants, bees, Sphex-wasps, ichneumon-flies, and gall-flies.
It might be objected that this case of the Yucca is not so much one of effecting fertilization as of parasitism; but the eggs, which are laid in the seed-pods, are very few, and the caterpillars which emerge from them only devour a very small proportion of the seeds, of which there may be about 200 (Fig. 52, B). Thus the plants also derive an advantage from the moth's procedure, for quite enough seeds are left. The form and position of the stamens and of the stigma seem to be as exactly adapted to the visits of the moth as the moth is to the transference of the pollen, for the Yucca can only be fertilized by this one moth, and sets no seed if the moth be absent. For this reason the species of Yucca cultivated in Europe remain sterile.
Thus the apparent contradiction is explained, and the facts everywhere support the hypothesis that the adaptations between flowers and insects depend upon processes of selection.
This origin is incontrovertibly proved, it seems to me, in another way, namely, by the merely relative perfection of the adaptations, or rather, by their relative imperfection.
I have already pointed out that all adaptations which depend upon natural selection can only be relatively perfect, as follows from the nature of their efficient causes, for natural selection only operates as long as a further increase of the character concerned would be of advantage to the existence of the species. It cannot be operative beyond this point, because the existence of the species cannot be more perfectly secured in this direction, or, to speak more precisely, because further variations in the direction hitherto followed would no longer be improvements, even though they might appear so to us.
Thus the corolla of many flowers is suited to the thick, hairy head and thorax of the bee, for to these only does the pollen adhere in sufficient quantity to fertilize the next flower; yet the same flowers are frequently visited by butterflies, and in many of them there has been no adaptation to prevent these useless visits. Obviously this is because preventive arrangements could only begin, according to our theory, when they were necessary to the preservation of the species; in this case, therefore, only when the pillaging visits of the butterflies withdrew so many flowers from the influence of the effective pollinating visitor, the bee, that too few seeds were formed, and the survival of the species was threatened by the continual dwindling of the normal number. As long as the bees visit the flowers frequently enough to ensure the formation of the necessary number of seeds a process of selection could not set in; but should the bees find, for instance, that nearly all the flowers had been robbed of their nectar, and should therefore visit them less diligently, then every variation of the flower which made honey less accessible to the butterflies would become the objective of a process of selection.
Everywhere we find similar imperfections of adaptation which indicate that they must depend on processes of selection. Thus numerous flowers are visited by insects other than those which pollinate them, and these bring them no advantage, but merely rob them of nectar and pollen; the most beautiful contrivances of many flowers, such as Glycinia, which are directed towards cross-fertilization by bees, are rendered of no effect because wood-bees and humble-bees bite holes into the nectaries from the outside, and so reach the nectar by the shortest way. I do not know whether bees in the native land of the Glycinia do the same thing, but in any case they can do no sensible injury to the species, since otherwise processes of selection would have set in which would have prevented the damage in some way or other, whether by the production of stinging-hairs, or hairs with a burning secretion, or in some other way. If the actual constitution of the plant made this impossible, the species would become less abundant and would gradually die out.
Thus the relative imperfection of the flower-adaptations, which in general are so worthy of admiration, affords a further indication that their origin is due to processes of selection.
ADDITIONAL NOTE TO CHAPTER X.
It has been remarked that the chapter on the Origin of Flowers in the German Edition contains no discussion and refutation of the objections which have up till recently been urged against the theory of flowers propounded by Darwin and Hermann Müller. I admit that this chapter seemed to be so harmonious and so well rounded, and at the same time so convincing as to the reality of the processes of selection, that the feeble objections to it, and the attempts of opponents to find another explanation of the phenomena, might well be disregarded in this book.
However, the most important of these objections and counter-theories may here be briefly mentioned.
Plateau in Ghent was the first to collect facts which appeared to contradict the Darwinian theory of flowers; he observed that insects avoided artificial flowers, even when they were indistinguishable in colour from natural ones as far as our eyes could perceive, and he concluded from this that it is not the colour which guides the insects to the flowers, that they find the blossoms less by their sense of sight than by their sense of smell. But great caution is required in drawing conclusions from experiments of this kind. I once placed artificial marguerites (Chrysanthemum leucanthemum) among natural ones in a roomy frame in the open air, and for a considerable time I was unable to see any of the numerous butterflies (Vanessa urticæ), which were flying about the real chrysanthemums, settle on one of the artificial flowers. The insects often flew quite close to them without paying them the least attention, and I was inclined to conclude that they either perceived the difference at sight, or that they missed the odour of the natural flowers in the artificial ones. But in the course of a few days it happened twice in my presence that a butterfly settled on one of the artificial blooms and persistently groped about with fully outstretched tube to find the entrance to the honey. It was only after prolonged futile attempts that it desisted and flew away. That bees are guided by the eye in their visits to flowers has been shown by A. Forel, who cut off the whole proboscis, together with the antennæ, from humble-bees which were swarming eagerly about the flowers. He thus robbed them of the whole apparatus of smell, and nevertheless they flew down from a considerable height direct to the same flowers. An English observer, Mr. G. N. Bulman, has been led to believe, with Plateau, that it is a matter of entire indifference to the bees whether the flowers are blue, or red, or simply green in colour, if only they contain honey, and that therefore the bees could have played no part in the development of blue flowers, as Hermann Müller assumed they had, and that they could have no preference for blue or any other colour, as Sir John Lubbock and others had concluded from their experiments. This is correct in so far that bees feed as eagerly on the greenish blossoms of the lime-tree as they do on the deep-blue gentian of the Alpine meadows or the red blossoms of the Weigelia, the dog-roses of our gardens or the yellow buttercups (Ranunculus) of our meadows; they despise nothing that yields them honey. But it certainly does not follow from this that the bees may not, under certain circumstances, have exercised a selecting influence upon the fixation and intensification of a new colour-variety of a flower. This is less a question of a colour-preference, in the human sense, on the part of the bees than of the greater visibility of the colour in question in the environment peculiar to the flower, and of the amount of rivalry the bees meet with from other insects in regard to the same flower. In individual cases this would be difficult to demonstrate, especially since we can form only an approximate idea of the insect's power of seeing colour, and cannot judge what the colours of the individual blossoms count for in the mosaic picture of a flowery meadow. Yet this is the important point, for, as soon as the bees perceive one colour more readily than another, the preponderance of this colour-variety over other variations is assured, since it will be more frequently visited. In the same way we cannot guess in individual cases why one species of flower should exhale perfume while a nearly related species does not. But when we remember that many flowers adapted for the visits of dipterous insects possess a nauseous carrion-like smell, by means of which they not only attract flies but scare off other insects, we can readily imagine cases in which it was of importance to a flower to be able to be easily found by bees without betraying itself by its pleasant fragrance to other less desirable visitors.
Thus, therefore, we can understand the odourless but intensely blue species of gentian, if we may assume that its blue colour is more visible to bees than to other insects. If I were to elaborate in detail all the principles which here suggest themselves to me I should require to write a complete section, and I am unwilling to do this until I can bring forward a much larger number of new observations than I am at present in a position to do. All I wish to do here is to exhort doubters to modesty, and to remind them that these matters are exceedingly complex, and that we should be glad and grateful that expert observers like Darwin and Hermann Müller have given us some insight into the principles interconnecting the facts, instead of imagining whenever we meet with some little apparently contradictory fact, which may indeed be quite correct in itself, that the whole theory of the development of flowers through insects has been overthrown. Let us rather endeavour to understand such facts, and to arrange them in their places as stones of the new building.
Often the contradiction is merely the result of the imperfect theoretical conceptions of its discoverer, as we have already shown in regard to Nägeli. Bulman, too, fancies he has proved that bees do not distinguish between the different varieties of a flower, but visit them indiscriminately with the same eagerness, thus causing intercrossing of all the varieties, and preventing any one from becoming dominant. But are the varieties which we plant side by side in our gardens of the kind that are evolved by bees? That is to say, are their differences such as will turn the scale for or against the visits of the bees? If one were less, another more easily seen by the bees; or if one were more fragrant, or had a fragrance more agreeable to bees than the other, the result of the experiment would probably have been very different.
One more objection has been made. It is said that the bees, although exclusively restricted, both themselves and their descendants, to a diet of flowers, are not so constant to a particular flower as the theory requires. They do indeed exhibit a 'considerable amount of constancy,' and often visit a large number of flowers of the same species in succession, but the theory requires that they should not only confine themselves to this one species, but to a single variety of this species. These views show that their authors have not penetrated far towards an understanding of the nature of selection. Nature does not operate with individual flowers, but with millions and myriads of them, and not with the flowers of a single spring, but with those of hundreds and thousands of years. How often a particular bee may carry pollen uselessly to a strange flower without thereby lowering the aggregate of seeds so far that the existence of the species seems imperilled, or how often she may fertilize the pistil of a useful variation with the pollen of the parent species, without interrupting or hindering the process of the evolution of the variety, no mortal can calculate, and what the theory requires can only be formulated in this way: The constancy of the bees in their visits to the flowers must be so great that, on an average, the quantity of seeds will be formed which suffices for the preservation of the species. And in regard to the transformation of a species, the attraction which the useful variety has for the bees must, on an average, be somewhat stronger than that of the parent species. As soon as this is the case the seeds of the variety will be formed in preponderant numbers, although they may not all be quite pure from the first, and by degrees, in the course of generations, the plants of the new variety will preponderate more and more over those of the parent form, and finally will alone remain. In the first case we have before our eyes the proof that, in spite of the imperfect constancy of the bees, a sufficient number of seeds is produced to secure the existence of the species. Or does Mr. Bulman conclude from the fact that the bees are not absolutely constant that flowers are not fertilized by bees at all?
I cannot conclude this note without touching briefly upon what the opponents of the flower theory have contributed, and what explanation of the facts they are prepared to offer.
In his important work, Mechanische-physiologische Theorie der Abstammungslehre, published in 1884, Nägeli, as a convinced opponent of the theory of selection, attempted an explanation. He was quite aware that his assumption of an inward 'perfecting principle' would not suffice to explain the mutual adaptations of flowers and insects, and he refers the transformation of the first inconspicuous blossoms into flowers to the mechanical stimulus which the visiting insects exerted upon the parts of the blossom. By the pressure of their footsteps, the pushing and probing with their proboscis, they have, he says, transformed gradually, for instance, the little covering leaves at the base of a pollen vessel into large flower petals, caused the conversion of short flower-tubes into long ones, and of the pollen, once dry and dusty, into the firmly adhesive mass formed in the anther lobes of our modern flowers. The colour of the flowers depends, according to him, upon the influence of light, which certainly no more explains the yellow ring on a blue ground in the forget-me-not than it does the many other nectar-guides which show the insect the way to the honey. Nägeli works with the Lamarckian principle in the most daring way, and with the same naïveté as Lamarck himself in his time, that is, without offering any sort of explanation as to how the minute impression made, say by the foot or by the proboscis of an insect, upon a flower, is to be handed on to the flowers of succeeding generations. He treats the unending chain of generations as if it were a single individual, and operates with his 'secular' stimulus, and with 'weak stimuli, lasting through countless generations,' as though they were a proved fact. But I have not even touched upon the question as to whether these 'stimuli' could produce the changes he ascribes to them, even if they were continually affecting the flower. How the scale-like covering leaves of the pollen vessels could become larger and petal-like through the treading of an insect's foot is as difficult to see as why a honey-tube should become longer because of the butterfly's honey-sucking: might it not just as well become wider, narrower, or even shorter? I see no convincing reason why it should become longer! And even if it did so, it would necessarily continue to lengthen as time went on, and this is not the case, for we find corolla-tubes of all possible lengths, but, it is to be noted, always in harmony with the length of the proboscis of the visiting insect. In a similar way Henslow has recently attempted to refer the origin of flowers to the mechanical stimulus exercised upon it by the visiting insects. 'An insect hanging to the lower petal of a flower elongates the same by its weight, and the lengthened petal is transmitted by heredity.'...'The irritation caused by its feet in walking along the flower causes the appearance of colouring matter, and the colour is likewise transmitted.'...'As it probes for honey it causes a flow of sweet sap to that part, and this also becomes hereditary!'
In this case, also, it is simply taken for granted that every little passing irritation not only produces a perceptible effect, but that this effect is transmissible. In a later lecture we shall have to discuss in detail the question of the inheritance of functional modifications. It is enough to say here that, if this kind of transmission really took place even in the case of such minute and transitory changes, there could be no dispute as to the correctness of the 'Lamarckian principle,' since every fairly strong and lasting irritation could be demonstrated with certainty to produce an effect. When a butterfly, floating freely in the air, sucks honey from a tube, the irritation must be almost analogous to that caused by a comb lightly drawn by some one through our hair, and this is supposed to effect the gradual lengthening of the corolla-tube of the flower!
The secretion of honey, too, depends upon the persistent irritation of the proboscis! Then 'deceptive flowers,' like the Cypripedium we have mentioned, could not exist at all, for they contain no honey, although the proboscis of the bee must cause the same irritation in them as in other orchids which do contain honey. This whole 'theory' of direct effect is, moreover, only a crude and apparent interpretation, which explains the conditions only in so far as they can be seen from a distance; it fails as soon as they are more exactly examined; all the great differences in the position of the honey, its concealment from intelligent insects, its protection from rain by means of hairs, and against unwelcome guests by a sticky secretion, the development of a corolla-tube which corresponds in length to the length of the visiting insect's proboscis, the development of spurs on the flower, in short, all the numerous contrivances which have reference to cross-fertilization by insects remain quite unintelligible in the light of this theory—it is a mere pis aller explanation for those who continue to struggle against accepting the theory of selection.