by Grant Allen
If any man can seriously doubt that these changes are really due to a colour-sense in the little creatures which live upon the beautiful flowers, if he can imagine that the plant has produced its gorgeous petals for no other purpose than that of suicidal wastefulness; that the Mantis has grown into the perfect semblance of a leaf from pure wanton causeless mimicry; that the lurid red of fly-fertilised blossoms bears its likeness to the mangled flesh of animals by a simple freak of creative power; then the whole science and philosophy of the last hundred years have been thrown away upon him, and he may return at leisure to the blind and hopeless chance of the eighteenth-century atheists. Even if we could allow the strangely gratuitous hypothesis of a distinguished naturalist, that the colours of organic beings were originally developed by natural causes, with a sort of divine afterthought regarding the pleasure which man might derive from their contemplation, yet we cannot blind ourselves to the absolute necessity of their performing from the very first some special utilitarian function. Not even the watchmaker deity of Paley himself, one may suppose, would have invented flowers in the Secondary age for the sole gratification of man in the Post-tertiary. To put it briefly, if insects have not a colour-sense, then the whole universe must be nothing more than a singularly happy concourse of fortuitous atoms. The theist and the evolutionist are equally ready to disclaim with all their might this grotesque and monstrous supposition.
CHAPTER VI.
BIRDS OR MAMMALS AND FRUITS.
What insects are to bright-hued flowers, birds and mammals are to bright-hued fruits. And we might almost say, though with more reservation, what flowers are to the colour-sense in insects, fruits are to the colour-sense in birds and mammals.
Accordingly, we may fairly conclude that bright-coloured fruits belong to a much later geological age than entomophilous blossoms. We need not here transport ourselves in imagination to the green expanse of palæozoic jungles, unenlivened by scarlet flowers or gaudy insects; We have only to place ourselves amid the comparatively modern flora of the Tertiary age, surrounded by forest trees of familiar aspect, and tenanted by animals whose shape differs but little from those of our own historical epoch. Already the ground spreads a carpet of soft grass beneath our feet; already simple forms of insect-fertilised blossoms stand out in profusion as brilliant points of colour among the green foliage around. It is true we see no highly-differentiated daisies or thistles, with their clustered heads of tubular bells; no strangely-shaped orchids or snapdragons, with their forms nicely adjusted to those of the fertilising bees; but we find a fair abundance of unspecialised flowers, with a regular corolla of separate pieces, such as we know so well in the buttercup, the poppy, or the geranium. Moreover, we may see among them, not merely the little dingy creeping insects of the Carboniferous deltas, but flitting butterflies with coloured wings, and flower-haunting beetles of exquisite metallic sheen. These brighter forms of insect life have been developed in the vast cycles of that immeasurable interim by the selective action of sexual preference, working through a taste for brilliant hues which has been originally formed in the search for food; but this portion of our subject we must remove from its proper historical place, owing to the exigencies of logical treatment, and relegate it to a later chapter, where we may consider the question of sexual selection in its ensemble, as exhibited throughout all departments of the animal kingdom. For the present, we must content ourselves by taking for granted the existence among the Tertiary forests of gaily-tinted insects and gorgeous lizards, as well as that of crimson leaves and orange blossoms. Nay, more, we may probably allow that the higher vertebrate types which lived in those primitive modern wilds possessed some more or less distinct form of colour-perception, derived perhaps from their earlier marine ancestors, and kept alive by exercise upon these varied objects in their actual environment. Yet, in spite of such facts and probabilities, it will be well worth while briefly to trace the origin of bright-hued fruits, as we have already traced the origin of bright-hued flowers, both because the colour-sense of the highest vertebrates probably owes much to the reaction of these brilliant food-stuffs, and because the taste for colour in man himself may be plausibly referred to the arboreal habits of his ante-human progenitors. Furthermore, we shall see reason to conclude hereafter that the plumage of the most beautiful birds and the fur or skin of the most highly-coloured mammals are due to the love for bright hues originally developed in connection with the pulpy fruits; and this conclusion affords another reason why we should first inquire into the history of their evolution.
The ultimate object of flowering is the production of seeds, that is to say, of embryo plants, destined to replace their parents, and continue the life of the species to future generations. The vessel which includes one or more such seeds, the produce of a single flower, is known in botanical parlance as a fruit. But as it often happens that the ripe pistils of more than one blossom become united together into a single mass, instances of which habit may be seen in the fig and the mulberry, it becomes convenient to describe these aggregate seed-vessels also as compound fruits. In the language of ordinary life, however, a fruit means something very different from such hard and dry seed-vessels as those of poppies, beans, or thistles. We understand by the word, in daily usage, some sweet, soft, pulpy object, more or less connected with the seeds, and usually possessing some bright-coloured portion. To these latter structures we shall generally give the designation of fruits-proper, to distinguish them from such among their like as are merely fruits by botanical courtesy. Fruits-proper, then, in this restricted sense, form the special object of our present investigation.
The botanical fruit consists of a covering or pericarp, often extremely thin and almost papery, enclosing one or more ripe seeds. But other connected portions of the plant, for example, the swollen flower-stalk and the receptacle, frequently coalesce so thoroughly with these essential organs that it becomes impossible to distinguish them, even for technical purposes. This is especially the case with fruits-proper, where the edible portion quite as often consists of some irregular adjunct as of the juicy pericarp itself. Accordingly, in the following account, I shall take the liberty of dealing a little broadly with the technical terms of botany, suppressing all unnecessary detail, and only dwelling upon the simplest and most salient points.
The ideal form of fruit would consist of a plain pericarp enclosing a single seed; and though such fruits are comparatively rare, one may reasonably suppose that the earliest flowering plants would in all probability produce seed-vessels of a very simple kind, in which the separate seeds were small and inconspicuous, though, relatively numerous. Indeed, the gymnosperms, or pine and cycad group, which appeared upon the earth before any other phanerogamous plants, cannot be said to possess any fruit at all in the proper sense of the term. Moreover, there is reason to believe that early plants produced large quantities of seeds which were relatively ill provided with coverings or nutriment, and which depended rather upon their number than upon any special adaptation for their chance of survival. But as time went on, slight adventitious variations in the nature of the seeds or their coverings might prove useful in protecting them from some one or other among the numerous dangers to which their fellows were exposed, and so might give them an extra advantage in the struggle for existence. The most noticeable among these variations is, that which consists in the extra supply of energetic material for the sprouting plantlet.
A young plant consists of an embryo whose growth depends upon the liberation of energy contained in its hydro-carbonaceous or albuminoid materials, by union with the free oxygen of the air. The energetic substances upon which it feeds were laid up for it by the parent plant, either in the growing seed-leaves (cotyledons), as in the case of the pea and bean, or in a separate albuminous mass, as in the case of the wheat tribe. But such energetic materials are exactly the portions of plants which form the best food-stuffs for animals; and accordingly, as birds and mammals multiplied upon the face of the earth, it must happen that those very
seeds which possessed the best chance of survival through their stores of nutriment would also be the ones which lay most exposed to the ravages of animal foes. Hence plants are compelled to adopt many devices whereby they may secure themselves against such depredations.
One common plan is that by which some underground structure, such as a bulb, tuber, corm, or root, is made to supplement, or in many cases almost to supersede, the natural mode of reproduction by seeds. This is seen in numerous plants, such as potatoes, onions, beets, and many grasses. But perhaps the most interesting case is that of the ground-nuts, whose “hypogean” fruit is buried deep in the earth, so as to escape the notice of all but burrowing animals. Yet even these species, which try to conceal their stores of food by hiding them under the soil, fall a prey at last to the snouts of rodents or swine. In fact, it must naturally happen that, as young plants and animals feed on exactly the same energetic substances, every device on the part of the plant will soon be met by a counter-device on the part of the hungry animal.
Each species of plant must, of course, solve for itself the problem, during the course of its development, whether its energies will be best employed by hoarding nutriment for its own future use in bulbs and tubers, or by producing richly-endowed seeds which will give its offspring a better chance of rooting themselves comfortably, and so surviving in safety amid the ceaseless competition of rival species. The various cereals, such as wheat, barley, rye, and oats, have found it most convenient to grow afresh with each season, and to supply their embryos with an abundant store of food for their sustenance during the infant stage of plant life. Their example has been followed by peas and other pulses, by the wide class of nuts, and by the majority of garden fruits. On the other hand, the onion and the tiger-lily store nutriment for themselves in the underground stem, surrounded by a mass of overlapping or closely-wound leaves, which we call a bulb; the iris and the crocus lay by their stock of food in a woody or fleshy stalk; the potato makes a rich deposit of starch in its subterraneous branches or tubers; the turnip, carrot, radish, and beet use their root as the storehouse for their hoarded food-stuffs; while the orchis produces each, year a new tubercle by the side of its existing root, and this second tubercle becomes in turn the parent of its next year’s flowering stem. Perhaps, however, the common colchicum or meadow-saffron affords the most instructive instance of all; for during the summer it sends up green leaves alone, which devote their entire time to the accumulation of food-stuffs in a corm at their side; and when the autumn comes round, this corm produces, not leaves, but a naked flower-stalk, which pushes its way through the moist earth, and stands solitary before the October winds, depending wholly upon the stock of nutriment laid up for it in the corm.
Now, if we look at the materials used as food by man or other frugivorous creatures, we shall see that they consist almost universally of these reservoirs of energetic material, laid up by the plant for itself or its descendants. It is true that the graminivorous animals, like deer, sheep, cows, and horses, live mainly off the green leaves of grasses and creeping plants. But we know how small an amount of food they manage to extract from these fibrous masses, and how constantly their whole existence is devoted to the monotonous and imperative task of grazing for very life. Those animals, however, who have learnt to live at the least cost to themselves always choose the portions of a plant which it has stored with nourishment for itself or its offspring. Men and monkeys feed naturally off fruits, seeds, and bulbs. Wheat, maize, rye, barley, oats, rice, millet, pease, vetches, and other grains or pulses, form the staple sustenance of half mankind. Other fruits largely employed for food are plantains, bananas, bread-fruit, dates, cocoa-nuts, chestnuts, mangoes, mangostines, and papaws. Among roots, tubers, and bulbs stored with edible materials may be mentioned beet, carrot, radishes, turnips, swedes, ginger, potatoes, yam, cassava, onions, and Jerusalem artichokes. But if we look at the other vegetables used as food, we shall observe at once that they are few in number and unimportant in economical value. In cabbage, Brussels sprouts, lettuce, succory, spinach, and water-cress, we eat the green leaves; yet nobody would ever dream of making a meal off any of these poor food-stuffs. The stalk or young sprout forms the culinary portion of asparagus, celery, sea-kale, rhubarb, and angelica, none of which vegetables are remarkable for their nutritious properties. In all the remaining food-plants some part of the flowering apparatus supplies the table, as in true artichokes, where we eat the receptacle, richly stocked with nutriment for the opening florets; or in cauliflower, where we choose the aborted flower-buds themselves. In short, we find that men and the higher animals generally support themselves upon those parts of plants in which energy has been accumulated either for the future growth and unfolding of the plant itself, or for the sustenance of its tender offspring.
Doubtless the earliest seeds differed but little from the spores of cryptogams in the amount of nourishment with which they were provided, and the mode in which they were dropped upon the nursing soil beneath. But during the great secondary and tertiary ages of geology, throughout whose long course first the conifers and then the true flowering plants slowly superseded the gigantic horse-tails and tree-ferns of the coal-measures, many new devices for the dispersion and nutrition of seeds were gradually developed by the pressure of natural selection. Those plants which merely cast their naked embryos adrift upon the world to shift for themselves in the fierce struggle of stout and hardy competitors must necessarily waste their energies in the production of an immense number of seeds. In fact, calculations have been made which show that a single scarlet corn-poppy produces in one year no less than 50,000 embryos; and some other species actually exceed this enormous figure. If, then, any plant happens, by a favourable combination of circumstances, to modify the shape of its seed in such a manner that it can be more readily conveyed to open or unoccupied spots, it will be able in future to economise its strength, and thus to give both itself and its offspring a better chance in the struggle for life. There are many ways in which natural selection has effected this desirable consummation.
The thistle, the dandelion, and the cotton-bush provide their seeds with long tufts of light hair, thin and airy as gossamer, by which they are carried on the wings of the wind to bare spaces, away from the shadow of their mother-plant, where they may root themselves successfully in the vacant soil. The maple, the ash, and the pine supply their embryos with flattened wings, which serve them in like manner not less effectually. Both these we may classify as wind-dispersed seeds. A second set of plants have seed-vessels which burst open explosively when ripe, and scatter their contents to a considerable distance. The balsam forms the commonest example in our European gardens; but a well-known tropical tree, the sandbox, displays the same peculiarity in a form which is almost alarming, as its large, hard, dry capsules fly apart with the report of a small pistol, and drive out the disc-shaped nuts within so forcibly as to make a blow on the cheek decidedly unpleasant. These we may designate as self-dispersed seeds. Yet a third class may be conveniently described as animal-dispersed, divisible once more into two sub-classes, the involuntarily and the voluntarily aided. Of the former kind we have examples in those seeds which, like burrs and cleavers, are covered with little hooks, by whose assistance they attach themselves to the fur or wool of passers-by. The latter or voluntarily aided sort are exemplified in fruits-proper, the subject of our present investigation, such as apples, plums, peaches, cherries, haws, and brambleberries. Every one of these plants is provided with hard and indigestible seeds, coated or surrounded by a soft, sweet, pulpy, perfumed, bright-coloured, and nutritious covering known as fruit. By all these means the plant allures birds or mammals to swallow and disperse its undigested seed, giving in, as it were, the pulpy covering as a reward for the services thus conferred.
But before we go on to inquire into the mode of their development we must glance briefly at a second important difference in the constitution of seeds.
If we plant a grain of mustard-seed in moist earth, and
allow it to germinate, we shall see that its young leaves begin from the very first to grow green and assimilate energetic matter from the air around them. They are, indeed, compelled to do so, because they have no large store of nutriment laid up in the seed-leaves for their future use by the mother-plant. But if we treat a pea in the same manner, we shall find that it long continues to derive nourishment from the abundant stock of food treasured up in its big round seed-leaves. Now of course any plant which thus learns to lay by in time for the wants of its offspring gives its embryo a far better chance of surviving and leaving descendants in its turn than one which abandons its infant plants to their own unaided resources in a stern battle with the unkindly world. Exactly the same difference exists between the two cases as that which exists between the wealthy merchant’s son, launched on life with abundant capital accumulated by his father, and the street Arab, turned adrift as soon as he can walk alone, to shift or starve for himself in the lanes and alleys of a great city.
So then as plants went on varying and improving under the stress of over-population, it would naturally result that many species must hit independently upon this device of laying by granaries of nutriment for the use of their descendants. But side by side with the advancing development of vegetable life, animal life was also developing in complexity and perfect adaptation to its circumstances. And herein lay a difficult dilemma for the plant. On the one hand, in order to compete with its neighbours, it must lay up stores of starch and oil and albumen for the good of its embryos; while, on the other hand, the more industriously it accumulated these expensive substances, the more temptingly did it lay itself open to the depredations of the squirrels, mice, bats, monkeys, and other clever thieves, whose number was daily increasing in the forests round about. The plant becomes, in short, like a merchant in a land exposed to the inroads of powerful robbers. If he does not keep up his shop with its tempting display of wares, he must die for want of custom; if he shows them too readily and unguardedly, he will lay himself open to be plundered of his whole stock-in-trade. In such a case the plant and the merchant have recourse to the self-same devices. Sometimes they surround themselves with means of defence against the depredators; sometimes they buy themselves off by sacrificing a portion of their wealth to secure the safety of the remainder. Those seeds which adopt the former plan we call nuts, while to those which depend upon the latter means of security we give the name of fruits.
