Part 23

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.

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_).

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.