Popular Science Monthly/Volume 28/November 1885/Concerning Clover
EVERY group of organisms, every genus and every species of plant or animal, has certain strong points which enable it to hold its own in the struggle for existence against its competitors of every kind. Most groups have also their weak points, which lay them open to attack or extinction at the hands of their various enemies. And these weak points are exactly the ones which give rise most of all to further modifications. A species may be regarded in its normal state as an equilibrium between structure and environing conditions. But the equilibrium is never quite complete; and the points of incompleteness are just those where natural selection has a fail* chance of establishing still higher equilibrations. These are somewhat abstract statements in their naked form: let us see how far definiteness and concreteness can be given to them by applying them in detail to the case of a familiar group of agricultural plants—the clovers.
To most people clover is the name of a single thing, or, at most, of two things, purple clover and Dutch clover; but to the botanist it is the name of a vast group of little flowering plants, all closely resembling one another in their main essentials, yet all differing infinitely from one another in two or three strongly marked peculiarities of minor importance, which nevertheless give them great distinctness of habit and appearance. In England alone we have no less than twenty-one recognized species of clover, of which at least seventeen are really distinguished among themselves by true and unmistakable differences, though the other four appear to me to be mere botanist's species, of no genuine structural value. If we were to take in the whole world, instead of England alone, the number of clovers must be increased to several hundreds. The question for our present consideration, then, is twofold: first, what gives the clovers, as a class, their great success in the struggle for existence, as evidenced by their numerous species and individuals; and, secondly, what has caused them to break up into so large a number of closely allied but divergent groups, each possessing some special peculiarity of its own, which has insured for it an advantage in certain situations over all its nearest congeners?
Clover is, of course, by family, a pea-flower, one of the great group of the Papilionaceæ, a tribe of the vast leguminous race. Now, everybody knows the general appearance of the pea-blossom, a form of flower which reappears throughout the whole group, in such different plants as gorse, laburnum, peas, beans, vetches, wistaria, lupine, and acacia; and it is clearly this form of flower which gave the original ancestor of the papilionaceous plants its main advantage in the struggle for existence over almost all its compeers. In other respects, the various members of the pea-flower tribe differ widely from one another. Some of them are tall, woody trees, like the laburnum; some are bushy shrubs, like the broom; some are low, creeping herbs, like the clover; and some are lithe, trailing climbers, like the pea and the scarlet-runner. So again with their foliage: some have hard, spiky leaves, like furze; some have regular trefoils, like medic; some have long sprays of many leaflets, like the sainfoin; and some have clinging tendrils, like the peas and vetches. Once more, in the pod and seed there are infinite varieties of shape, size, and arrangement, as one may see by comparing peas with horse-beans, or the short, hairy pod of gorse with the long, smooth capsule of the vetch, the inflated globe of the bladder senna, and the twisted, snail-like spiral of the medic. In fact, there is hardly a single particular in which the papilionaceous plants do not differ from one another immensely, except only their peculiar flower. Clearly, then, it is the flower almost alone which has given them their fair start in the struggle for life. I say almost—not quite—alone, because, as we shall see hereafter, they owe much also to their relatively large and richly stored seeds. In this one point they early reached a state of equilibrium; in other points, they went on varying and adapting themselves to an infinite variety of external circumstances.
Though it is not my intention to deal at any length here with any of the papilionaceous tribe except the clovers, a few words must first be premised about this peculiar and successful type of flower. It consists, like most other blossoms of the dicotyledonous race, of five petals, inclosing ten stamens, and with a single ovary, or embryo pod, in its very center. But anybody who has ever looked at a pea-blossom knows very well that it is not regular and radially symmetrical like a dog-rose; it has its parts bilaterally arranged, so that an insect lighting upon the flower in search of honey necessarily brushes his breast against the stamens and pistil, and therefore cross-fertilizes the embryo pods by carrying pollen from one blossom to the sensitive surface of the next. The five petals have undergone special modification so as to suit this special mode of impregnation. The upper petal, known as the standard, is usually broad and expanded, serving as an advertisement to attract insects; and in many advanced species it is variegated with convergent lines of different colors, which guide the bee toward the exact spot where the nectaries are engaged in elaborating honey for his benefit. The two next in order, called the wings, are generally shorter and smaller, and in most advanced types they possess two little indentations, one on each side, specially adapted to afford a foothold for the legs of the visiting bee, in the exact position that will enable him at once to reach the honey and to brush off the pollen against the sensitive surface. The two lowest petals of all are usually united by their under edge, so as to form a single organ, known as the keel, and closely inclosing the stamens and pistil. As a rule, too, all ten stamens are united into a single tube or sheath; or else the nine lower ones are so united, while the upper one is free. In spite of the general uniformity of floral type, however, many special modes of insect fertilization prevail among the various pea-flowers. Sometimes the blossom bursts open elastically when the bee lights upon it, dusting him all over with the ripe pollen; sometimes a small quantity is pumped out from the sharpened point of the keel by the weight of the insect's body; sometimes the pollen is deposited from his breast on the spirally curled summit of the pistil; sometimes it is swept off by a little brush of hairs, situated close beside the sensitive surface of the embryo pod. All that it is here necessary to bear in mind, however, is the general fact that the papilionaceous type of flower has gained its present high position as a dominant floral pattern by its beautiful and varied adaptation to insect fertilization.
Such being the general nature of the pea-flowers as a whole, we have next to inquire what are the special peculiarities which have enabled the clovers in particular to fill their peculiar niche in the existing economy of Nature. Clearly, the positions which clovers are adapted to adorn are not the high places in the hierarchy of vegetal life. They are not tall forest-trees or bushy shrubs; they are not long, creeping trailers or climbers; they are herbs of low and procumbent character, best fitted for filling up the interspaces of taller vegetation, and for vying with the grasses as elements of the close, tender, delicate greensward. The points which have enabled them to survive, therefore, are just those which allow a plant to thrive under such special conditions; and we must ask briefly what those points may be before we proceed to consider the specific characteristics of the various individual clovers.
In foliage the clovers are distinguished by their graceful trefoil leaves which are an adaptation of the typical papilionaceous pattern to the special necessities of their humble situation. For the common form of pea-leaf consists of a long leaf-stalk, with one terminal leaflet, and with several pairs of lateral leaflets, arranged opposite each other along a central line. In the clovers, however, and in most other small field forms of papilionaceous plants, only one pair of lateral leaflets is developed; and this arrangement allows the leaf-stalk to be elevated among the surrounding grasses in such a way as to get freely at the sun and air, which are necessary for the nutrition of the plant. But the chief peculiarity of the clovers is the arrangement of their flowers in dense heads. Instead of the blossoms growing separately or in pairs, as with most peas and vetches, or in long, loose bunches, as with laburnum and sainfoin, the flowers of the clovers, much reduced in size, are crowded into compact little bundles, for the most part at the end of a long stalk. What we ordinarily call the flower of a purple clover is, in fact, such a head of clustered flowers. This dense clustering of the flowers makes them, though individually small, very conspicuous in the mass to bees and other insects, and so largely increases their chance of cross-fertilization. For the same purpose they usually secrete abundant honey, and they possess in many cases the familiar fragrant clover perfume. Moreover, in most though not in all species the bases of the five petals have grown together into a narrow tube, inclosing the honey; and in the common purple clover this tube is so deep that no British insect except the humble-bee has a proboscis long enough to reach the nectaries. Such peculiarities are quite sufficient to give the clovers an immense advantage in the struggle for existence; and it is not surprising that they should have become exceptionally numerous in species and individuals, even among the richly endowed and dominant papilionaceous family.
Every race, however, has its weak as well as its strong points; and the weak point of the highly successful clovers lies in the unprotected position of their seeds and pods. Hence, in accordance with the general principles above laid down, it is in these particulars that we might expect to find the various species differ most from one another, since this is just the part on which natural selection of favorable varieties is most likely to be exerted. As in the papilionaceous family as a whole, the flower is the organ which remains almost identical throughout, because it is the organ which gives the family its true importance; so in the restricted clover group the trefoil leaflets and the clustered heads of flowers remain almost identical throughout, and for the like reason. But in any classification of the various species of clover, it will be seen by anybody who looks into the matter that all the distinctive characters are drawn from differences in the pod and calyx after flowering, because this is the weak point of the genus, and the one in which alone diversities of habit have been likely to arise and to be perpetuated by survival of the fittest. The other organs have long since reached their equilibrium; these organs alone remain in need of further equilibration.
And why is the pod a weak point? For this reason. The seeds of clover, though small, are very richly stored with starches and other food-stuffs for the growth of the young plant. Such richness is, of course, in itself an advantage to the race, because it allows the seedlings to start well equipped on the path of life, with some accumulated capital handed on to them by the mother-plant. But what will feed a seedling will feed an animal as well; and it is just these rich little beans in the clover-pod which give it all its dangerous value as a fodder for cattle. Hence, in the wild state those clovers which have their seeds least protected are most likely to be eaten off and killed down by birds or animals, while those which have them most protected are most likely to survive and become the parents of future generations. Here, then, we have the basis upon which natural selection can act in differentiating the primitive ancestral clover into various divergent species. Whatever accidental variation happens to give any particular clover protection for its seeds in any special habitat will certainly be preserved and increased, while all opposite variations will be cut off and demolished at once. So far as their foliage and their flowers are concerned, the clovers as a body are practically in a state of stable equilibrium; so far as their fruit and seeds are concerned, they are still undergoing modification by natural selection.
Clearly to illustrate this fundamental point, let us first look at some neighboring and closely allied plants, which are not exactly clovers, but which resemble them in almost all important particulars. These also show the same devices for specially protecting their seeds and pods from birds or animals. Take, for example, the genus of the medics. These are mostly small greensward plants, with trefoil leaflets like the clovers, but with the flowers in rather tall, one-sided spikes or loose bunches. Their pods are usually long and many-seeded, but they have this curious peculiarity, that instead of growing straight like that of a pea or bean, they coil up spirally like a snail-shell. When ripe they fall off the plant entire, and thus defeat the hopes of birds and other creatures which wait patiently for the opening of the pods. The simpler medics, such as the agricultural lucern, have smooth, spiral pods alone, and therefore they can be employed successfully as fodder for cattle. But this, which proves an advantage from the point of view of the farmer, is naturally a disadvantage from the point of view of the plant in a wild state, because it insures the seeds being eaten; and hence the more developed and weedy medics have acquired stout protective prickles, fringing their globular spirals, and making them very distasteful morsels to cows or horses. We have two such prickly medics in England, one closely coiled and rolled round like a ball, and thickly set with curved hooks; the other loose like a corkscrew, with two rows of sharp bristles at the adjacent edges; and both these, as I learn from farmers, are extremely objectionable weeds in meadows, rendering the bay almost uneatable. Indeed, I am assured that cattle will never touch even fresh meadow-grass containing them except when absolutely driven by hunger. It is noteworthy that our two doubtfully native smooth medics (lucern and nonesuch) both grow naturally in rough, dry places, and are only largely found as "artificial grasses"—that is to say, were introduced and maintained by human agency; while our two more truly wild species are meadow and pasture weeds, and are therefore amply protected by prickles against herbivorous animals. Again, bird's-foot trefoil, whose pretty yellow flowers form such ornaments to our sunny banks in summer, has a long, hard, dry pod, too stringy to be edible, and filled with pith between the beans; while lady's-fingers, a somewhat similar type, has an inflated hairy calyx completely inclosing the short pod in its protective and inedible capsule. Strangest of all, however, is the small, matted bird's-foot, whose pod never opens to shed the seeds, but divides between them into little joints or "articles," each inclosing a single bean, and so cheating the expectant birds of their promised food. These examples, which might be multiplied indefinitely, will sufficiently serve to show the importance of protection for the seeds as a basis of differentiation among the papilionaceous flowers.
With the restricted tribe of clovers the need for such protection has almost alone produced all the species into which the genus has long since split up. Originally, of course, we must suppose that there existed one united type of ancestral clover, differing from the other papilionaceous plants in the points which now distinguish the whole clover genus, but possessing none of the special distinctive marks which specifically divide one kind of clover from another. This hypothetical ancestor had probably rather large, purplish flowers, collected in compact heads on a common foot-stalk, with the five petals separate, and with a small three or four-seeded pod completely inclosed within the faded brown petals. From some such form the existing clovers have sprung by differentiations almost entirely affecting the pods and seeds, though they have also varied a little in color, according to the individual tastes of their particular insect visitors, as well as in the degree of union effected between their petals. Without going beyond the limits of our own native clovers, we will look first at those types in which the arrangement of the pod is simplest, and then pass on gradually to those in which it is more and more complex, till we arrive at last at that most marvelous English species which actually buries its own pods entire in the ground by a wonderful series of apparently purposive movements and gyrations.
Our common English purple clover (for convenience' sake I adopt throughout Mr. Bentham's vernacular names) may be taken as a good specimen of the simpler and less-protected kinds. The mere fact that it is grown extensively for fodder shows that it has no deterrent prickles or bristles to ward off the attacks of herbivorous animals; and indeed, throughout the clover group, it may be noted that birds and insects, rather than large mammals, seem to be the enemies especially guarded against by the majority of plants. Purple clover is a perennial, with long, hairy stems, the hairs serving to prevent ants from creeping up to the blossoms and uselessly rifling the honey intended to attract the fertilizing bees. The young flower-heads are also inclosed in two papery wings or stipules, which effectually protect them from injury before they open. The petals are united into a very long tube, accessible only (as before noted) to the humble-bee; and in New Zealand, where our European humble-bee is unknown, it has been found necessary to import several nestfuls, in order to make the acclimatized clover set its seed for agricultural purposes. But the devices for the protection of the pod are here comparatively slight. Each pod contains, as a rule, only a single seed, and it is externally guarded simply by the wire-like calyx-teeth, which are long, thin, and awl-shaped, and fringed on either side by a row of thick-set hairs. The two lowest are longer than the others, apparently as a protection against crawling insects. After flowering, the petals remain upon the heads, turn brown, and inclose the ripening pod. These brown heads of overblown flowers have such a dead, withered appearance that they seem sufficiently to deceive all intending depredators. As a whole, the species seems to survive mainly because of its protected young flower-heads, its special attractions for fertilization, and its habit of inclosing the pods in the dry petal-tube. It should be noticed, however, that, though artificially propagated in meadows and pastures, it would not probably be a very successful plant if left entirely to its own devices. Man has intervened to give it his powerful aid by sowing its seed, and by fencing it off from cattle, so that it has now become, in spite of itself, one of our most abundant and ubiquitous clovers.
Next in order we may take a series of small, wild, purplish clovers, closely allied to this cultivated type, but more specially adapted for protection against animal foes. Of these the little knotted clover, which grows in our dry pastures and banks, is an excellent simple example. It is a small, tufted annual, often growing in very closely cropped, sheep-eaten crofts, and therefore with an acquired habit of creeping close to the ground, and spreading its foliage flat against the earth. Its calyx-teeth are short and almost prickly, and its little knotted heads grow so close in the angles of the leaves that even a sheep has hard work to bite them off with his nipping front teeth. The rough clover is another of these dwarf creepers, much like knotted clover in general appearance, but even more prostrate, and with its flower-heads still more closely wrapped up in the angles of the leaves, whose wings or stipules almost completely inclose them. The greatest difference, however, resides in the calyx, whose teeth here, after flowering, become broader and stiffer, curve backward, and give the whole plant a stringy, dry, innutritions look. This species or variety also grows mostly on sheep-bitten banks, and manages wonderfully to set its seed in spite of the manifold dangers to which it is exposed. Boccone's clover, confined in Britain to the Lizard Promontory in Cornwall, is a larger southern form of the same central type, closely allied to the knotted clover. It grows much taller, but has an equally forbidding type of pods; and I notice in Southern France, where it is very abundant, that the dry stalks and oblong heads of fruit are always left uncropped on bare banks and road-sides where goats and sheep have been browsing—a fact which clearly shows that even those omnivorous grazers consider it an unpalatable morsel.
To the same group, I think, but in a more developed degree, belong three or four other British species, whose protections are somewhat less easy to understand. Of these, clustered clover appears like a still higher type of rough clover. It is a slender, creeping annual, with very small, globular flower-heads, almost buried in the angles of the stem and leaves; and it has short, broad calyx-teeth, rigidly curved backward after flowering, and with hard, sharp points. This, I take it, is a protection against browsing animals. The sea clover, on the other hand, seems rather to guard against birds or insects. In the flowering state, it looks almost exactly like a small purple clover; but as the seeds ripen it assumes a very different aspect. First of all, the calyx-teeth grow out into rather broad green leaves, so that the whole head looks more like a mass of foliage than a bunch of ripening fruit. The lower tooth, especially, becomes very long and leaf-like; and it may be remarked that, as a rule, the two lower teeth in clovers differ more or less conspicuously from the upper ones, pointing apparently to some special danger of attack from below. As the pod slowly ripens, two lips grow out on either side of the calyx, and finally meet on the top of the pod, so as to hermetically seal it, leaving only a tightly closed aperture in the very middle. Thus the calyx has, as it were, a false bottom, appearing to be empty when it is not really so, and by this means deceiving would-be intruders. It must be noticed, however, that such a deceptive device would be useless against a herbivorous animal, which could crop off the entire head; it would only serve against birds or insects, which might pick out the seeds one by one. That it does effectually protect the tiny beans is certain, for in no case will you find a calyx without a pod inside it. At the same time, so thoroughly has the calyx with its outgrowth of lips usurped the place of the primitive pod-covering that the real pod is reduced to a mere papery envelope, and can only be detected as inclosing the seed by a somewhat careful dissection. In this sea clover, too, the entire head, when ripe and dry, has a very forbidding aspect, the mass looking decidedly prickly and stringy, like a teazle; and I observe that it generally remains uncropped until the calyx and seeds fall of themselves, especially in Southern Europe, where it grows very tall. Why it should be confined to the neighborhood of the sea and of a few tidal rivers, more especially to salt-marshes, it would be hard to say; probably the special danger against which it defends itself is one found only under these circumstances, in which case it would there alone have any advantage over its competitors. Indeed, it must not be supposed that all these questions arc yet by any means finally solved. The sole object of the present paper is to point out the common principle running through the variations of the clover pattern, and to suggest such partial explanations of their causes as have yet occurred to a single observer.
Suffocated clover is another of the tiny creeping types, apparently protected for the most part against browsing quadrupeds. It is a wee tufted form, with minute flowers stuck close in small dense heads, as if gummed to the short stems, and very crowded along their course. We may regard it as the last effort of a very degraded race to keep up its existence in the most closely gnawed pastures, on sand or gravel, where only very dwarfed and scrubby plants can escape destruction. The reader will notice that under such circumstances two types of clover succeed, each in its own way. If the heads become very small, close, and inconspicuous, or tightly pressed against the wiry trailing stems, they escape the observation of browsing animals. If, on the other hand, though tall and noticeable, they develop prickly or stiffened teeth, they are rejected as unfit for food by the creatures which devour the surrounding herbage.
Reversed clover takes its name from a peculiarity which seems to , be connected with its mode of fertilization, for it has its standard petal turned outward, instead of inward as in all other clovers. The meaning and object of this change I do not know; but its most marked feature is still one bearing upon preservation of the seed, for, after flowering, the upper part of the calyx becomes much inflated, and is traversed by large membranous veins. At the same time it arches over the lower half, leaving three small teeth below, and two swollen ones at the top, so as to form a sort of bladder-like capsule over the concealed pod. In this case, again, the protection is obviously designed against birds or insects. In the curious strawberry clover, common among dry meadows and road-sides in Southern Britain, the same device has been carried a step further. Each flower in the head is here surrounded by a long involucre of lobed bracts, and, after flowering, the calyx swells immensely, so as to transform the entire head into a compact globular ball of little bladders, each inclosing a single pod. This arrangement has been popularly compared to a strawberry, but it is much more like a raspberry, being a delicate pink in hue, and composed of twenty or thirty small round capsules. The upper half of the bladder is likewise thickly covered with fine down, doubtless very objectionable to the skin of the tongue, and the whole is netted and veined in the most delicate and beautiful fashion. Hardly any other clover possesses so advanced a plan for protecting its little pod.
Another type is presented to us by the large crimson clover, not truly indigenous in Britain, but commonly cultivated for fodder in the south of England. It is a soft, hairy plant, and, like other fodder clovers, it does not possess any very advanced protective device. Still, even here, the calyx has extremely long, narrow teeth, thickly covered with smooth hairs, which serve to keep its beans safe. The analogy of a prickly pear or a rose-hip will show how very unpleasant such hairs feel in the mouth. The beautiful, small barefoot clover derives its expressive name from a further development of the same principle. The long teeth of the calyx project beyond the flowers, and are enveloped in soft, downy hair, which gives the whole head a very dainty, feathery appearance. As soon as the flowers are faded, the head looks like a mere mass of soft fluff, unenticing to herbivorous animals, and effectually concealing the seeds from birds or insects. The starry clover of Southern Europe, naturalized in England at Shoreham and a few other spots, starts from much the same point, but has specialized itself both against large and small depredators. On the one hand, its smooth, woolly calyx, much like that of crimson clover during the flowering stage, spreads out after blossoming into a star-shaped pattern, and forms with its neighbors a dry, bristly, interlacing head, thickly studded with sharp hairs; and this suffices to protect it from cattle and goats. On the other hand, the mouth of the calyx, being thus exposed by the spreading of the teeth, is closed by a perfect chevalde-frise of convergent tufted hairs, all meeting in the center of the throat; and this barrier answers the same purpose as that of the sea clover, though in a different manner, by forming a false bottom to ex^ elude insects. I notice on the dry Mediterranean hills that these bristly heads are rejected by the goats and sheep, like those of Boccone's clover, and even donkeys refuse to eat them.
Turning to a somewhat different class, there are some clovers which protect their seeds in a quite distinct manner, by merely turning them out of sight. Common Dutch clover does this in a simple yet very noticeable fashion. It bears its pretty white flowers in tall globular heads on a lengthened footstalk, which renders them extremely conspicuous objects to the fertilizing bees. But each flower is stalked within the head, and, as soon as it has been fertilized, it turns downward, and fades brown against the common footstalk. Every head of Dutch clover thus habitually consists of two parts—an upper part, containing erect open flowers or flower-buds, not yet fertilized; and a lower part, containing overblown flowers, already fertilized, and now engaged in setting their seed. This plan combines two distinct advantages at once. In the first place, the bees lose no time in discriminating between the mature honey-bearing blossoms and those already rifled, which insures more frequent visits and a larger general average of seed-setting. In the second place, the fruiting pedicels and pods, being turned down and concealed, are less likely to be visited by small animal foes, such as flying insects, which might lay their eggs within, and let the grub feed (as often happens) on the growing seed. Dutch clover is a fodder-plant, and therefore, probably, in its native state does not grow much in places exposed to the ravages of large herbivores. At the same time, the pod is many-seeded, and the plant spreads largely as well by creeping and rooting at the joints.
That the object of the turning down after flowering is distinctly to protect the pod, as well as to save time for the bees, may be seen, I think, from the analogous instance of the pretty little yellow hop clover. This common and graceful English plant has primrose-colored flowers, and (as usual with yellow blossoms) depends mainly for fertilization upon a smaller class of insects than Dutch or purple clover. But after the blossoms are fertilized, they turn down in the same manner as in Dutch clover, only far more markedly, giving the head a considerable resemblance to the hop-cones from which the species takes its name. After being thus reflexed, the faded but persistent petals close over the pod, and the standard becomes furrowed with deep marks, which seem to me intended to give a crumpled, withered appearance to the head. Simple as is this device, it nevertheless effectually conceals the pod within a closely imbricated set of scales or shields, each one folding over the next like tiles on a house, and entirely preventing the access of birds or insects to the seeds. The lesser clover and slender clover seem to me to be successively dwarfed and degraded states of the same plant, due apparently in part to bad soil, and in part to diminished need for special protection.
Last of all we come to the most advanced and developed type of any, the subterranean clover. In general appearance this plant closely resembles Dutch clover, from which, in all probability, it is a remote descendant. But, growing, as a rule, on dry, sandy, or gravelly pastures closely nipped by sheep or other herbivores, it has acquired a very remarkable and ingenious mode of escaping their depredations. Like the other species similarly circumstanced, it grows close to the ground, in small tufts; and it bears a few rather large white flowers, two or three together in a starved-looking head. Looked at closely in this stage, a number of small central knobs may be distinguished at the end of the common flower-stalk. These knobs are really the calyxes of undeveloped blossoms, completing the head. After flowering, the stalks lengthen and bend down to the ground, carrying the fertilized pods with them. Then the minor pod-stalks bend back, and the undeveloped central flowers grow out into short, thick awls or gimlets, with five finger-like lobes at their extremity, representing the five spreading teeth of the original calyx. These awls next begin digging their way into the earth by a slow, gyrating motion, and at last wear out a hole in which they bury the pod and bean entire. Thus the plant actually sows and manures its own seed, and so escapes all danger from the grazing animals. This extraordinary action may be considered as the high-water mark of ingenuity and foresight in the unconscious outcomes of natural selection among the clover kind.
In conclusion, it may be added that many of these clovers are very difficult to discriminate from one another in the flowering stage; it is only when the fruit begins to ripen and the calyx to assume its characteristic shape, that they can be readily identified by safe specific marks. Throughout, in short, all the clover traits remain almost the same, except in the matter of the fruiting pods. This is the one weak point of the genus, and this is therefore the place where natural selection has been able to produce fresh differentiating effects. Such a brief consideration of one small group of plants may serve to bring the general principle with which we started into the definite relief of concrete application; and it may also serve to show the vast variety of detail with which Nature effects the self-same object, even within the narrow limits of a single family or genus.—Gentleman's Magazine.