Popular Science Monthly/Volume 9/October 1876/The Local Distribution of Plants and the Theory of Adaptation
|THE LOCAL DISTRIBUTION OF PLANTS AND THE THEORY OF ADAPTATION.|
THERE is one class of facts in the geographical distribution of plants which has not received, at the hands of botanists, the degree of attention which its importance justifies.
I do not refer to those wide general phenomena which a comparison of the floras of different countries renders so striking, and by which the more humble and restricted class to which I would call attention is usually eclipsed. Such general considerations are, it is true, exceedingly interesting and important, and are in no danger of receiving too much attention. Nothing could be more absorbing than a close comparative analysis of the vegetation of different hemispheres, continents, islands, and zones, of the globe. The most casual survey of such fields reveals marvels, the mere acquaintance with which excites in the mind of the botanist the liveliest interest and pleasure. The strange and leafless euphorbias of South Africa, with their naked, green, parenchymous branches; the equally singular and grotesque cactuses of America answering to them; the anomalous vegetation of Australia, with its shadeless forests due to their vertical foliage; the absence of oaks east of the Ural Mountains, and of heaths on this side the Atlantic; the confinement of the genus Rosa to the northern and of the genus Calceolaria to the southern hemisphere—these and numberless other kindred facts connected with the general distribution of plants over the globe are justly calculated to excite the most intense interest, and have given rise to a variety of theories designed to account for them.
The phenomena, however, to which I would more particularly refer, come much nearer home, and may be presumed to have attracted the attention, more or less forcibly, of every one at all conversant with plants. They constitute a distinct class, and may be described in general terms as facts unfavorable to the received theory of adaptation.
It has long been regarded as a law of life, applicable alike to animal and vegetable forms, that each species is exactly adapted to the particular habitat where it occurs; and naturalists, assuming this law, have sought to solve the problem how this remarkable adaptation has been brought about, instead of pausing to question the alleged law of adaptation itself. And yet there have never been wanting numerous and obvious facts, especially in the vegetable kingdom, which, if interpreted at all, must be conceded to be incompatible with such a law, at least unless materially modified and greatly enlarged.
Mr. Thomas Meehan has remarked the fact that "almost all of our swamp-trees grow much better when they are transferred to drier places, provided the land is of fair quality. He referred, among others, to sweet-bay, red maple, weeping-willow, etc., as within his own repeated observations growing better out of swamps than in them." He further observes that "plants as a general rule, even those known as water-plants, prefer to grow out of water, except those that grow almost entirely beneath the surface."
A great many facts are at hand to prove that those plants which are found habitually growing in wet ground may be easily made to grow in dry ground. The Iris versicolor (blue flag), which, in a state of Nature, grows universally in marshes, and keeps perpetual company with Nuphar (pond-lily) and Sagittaria (arrow-head), is a common occupant of the driest gardens. The Lobelia cardinalis (cardinal flower), which I have found below tide-water mark, is also a common garden-flower, and not difficult to cultivate. Almost as much may be said for Lobelia syphilitica (great lobelia). The calla, the caladiums, and the anthuriums, belong to this class, and the list might be indefinitely extended.
But differences of moisture in the soil are not the only ones which are often overcome by natural or artificial changes in the conditions of growth. Most of our prettiest wild-flowers which are found growing in deep, shaded glens in pure leaf-mould, have been captured by florists, and made to thrive as well, and often better, under a cultivation which, with their most faithful efforts to imitate it, must be a complete alteration of their native condition of life. Of such might be mentioned at random the Trillium (wake-robin), the Cypripedium (lady's-slipper), the Dicentra (Dutchman's breeches), the Uvularia (bell wort), the Erythronium (dog's-tooth violet), etc.
So, too, plants growing under other conditions, as on hillsides, and in open woods or meadows, as the violets, hepaticas, anemones, and others, offer no difficulty to the florist.
These are cases in which the transfer is from apparently more favorable conditions to those less favorable. But similar results follow from a reversal of this order. Plants may be successfully transferred to ordinary garden-soil from localities which we would naturally suppose to be less favorable to growth, but to which these seem to be specially adapted. The columbine (Aguilegia), which grows on rocks, often with scarcely any soil in which to root, or emerges from narrow crevices between them, is planted in gardens where it thrives equally well. The same is true of the Cacti, which, taken from the arid plains where their indurated watery stalks and branches store up the water which the climate so long denies them, thrive under cultivation with undiminished vitality. The Agave, or American aloe, furnishes a similar illustration, and every few years a gorgeous century-plant blooms under cultivation, to the infinite delight of its owner.
Equally striking results take place under the influence of man without his design or selection. There are a great many indigenous plants which are rarely found outside of the influence of human cultivation. They emerge from their obscure natural retreats at the approach of civilization, spread rapidly over fields and pastures, and often become formidable enemies of the farmer and the gardener. Under the general name of weeds they are proscribed and pursued, and no effort is spared for their extermination. They also invade towns and cities, overrun vacant lots, disfigure parks and plats, and force themselves into pavements and "crannied walls." Ambrosia trifida (the great rag-weed) forms forests in waste grounds and neglected gardens. A. artemisiæfolia (Roman wormwood) is one of the farmer's most persistent pests, and resists all efforts at extermination. The cocklebur and thorny clotbur (Xanthium strumarium and X. spinosum) warn us of theirpresence wherever we go. Polygonum aviculare (knot-grass) and other species invade our door-yards and threaten to cross our thresholds. Euphorbia maculata (spotted spurge) spreads its prostrate and symmetrical mats over the dry and gravelly walks. Spergularia rubra (sand spurrey) unfolds its rosy petals to the hottest July sun upon the parching bricks beneath our feet. Erigeron Canadense (horse-weed), Epilobium angustifolium (great willow-herb), Gnaphalium polycephalum (common everlasting), and a host of other indigenous weeds, overrun the cultivated fields and commons wherever man has impressed his influence upon primitive Nature.
This phenomenon, however, becomes still more obtrusive when we turn to introduced species. And, if it be claimed that the transfer from waste places in the Old World to similar waste places in the New is not a change of conditions, we have only to remove our point of observation to Europe or Asia to render all that has been said of indigenous plants applicable also to adventive ones. For, unless we are willing to go further in admitting the transmutation of species than the founders of that doctrine, we must assume that each of these species has had a native habitat somewhere, and its preference for proximity to human habitations is unexplainable on any theory of original adaptation.
Illustrations on this point would be quite superfluous, as these plants constitute the bulk of all our weeds, and present themselves at every turn. I might mention the ubiquitous ox-eye daisy (Leucanthemum vulgare), the iniquitous Canada thistle (Cirsium arvense), and the obnoxious burdock (Lappa officinalis), as examples of species which share with man, not only his cosmopolitan character, but also some of his vices. But these foreign immigrants often furnish us with one of the most striking exemplifications of the anomaly, if such it may still be called, which I am endeavoring to illustrate.
It frequently happens that a plant, taken from one country into another having an entirely different flora, thrives more vigorously than it did at home, and even threatens to drive out indigenous species. Some of the species last mentioned belong to this class, particularly the Canada thistle, which, notwithstanding its popular name, has been introduced into this country from Europe, and has spread not only over Canada and New England, but far south and west. Cnicus lanceolatus (common thistle) is only less prominent because less troublesome. The same is true of many plants of the mint family, particularly Nepeta glechoma (ground-ivy). On the other hand, some American species, like Erigeron Canadense, have migrated by the aid of man into almost every country on the globe, always thriving best where civilization is highest. But some of these do not confine themselves to the circle of man's protective influence. Sometimes they strike out into the forest or spread over the plains, carrying dismay to the native vegetation. Mr. Darwin, speaking of the introduction into South America of the cardoon (Cynara cardunculus), a congener of the artichoke, says: "It occurs in these latitudes on both sides of the Cordillera, across the continent. I saw it in unfrequented spots in Chili, Entre Bios, and the Banda Oriental. In the latter country alone, very many (probably several hundred) square miles are covered by one mass of these prickly plants, and are impenetrable by man or beast. Over the undulating plains, where these great beds occur, nothing else can now live. Before their introduction, however, the surface must have supported, as in other parts, a rank herbage. I doubt whether any case is on record of an invasion on so grand a scale of one plant over the aborigines." He also mentions other analogous cases, though of a character less marked.
This love of change, if I may so characterize it, seems to inhere in the entire vegetable kingdom. Not even climate avails to overcome it, as is evidenced by the rapid invasion from the tropics of many plants whenever the presence of man in any manner creates the conditions favorable to their migration. Conspicuous among these are Chenopodium (pigweed), Amarantus (amaranth), Ipœma (morning glory), and others.
If we take a wider view of this class of phenomena, we may perceive that it is only by an extension of the same principle that all the beneficial changes made by man in the vegetable kingdom have become possible. Every plant he has improved and rendered subservient to his purposes has become what it is in obedience to an inherent tendency to exchange its original condition for a better one. And it is by taking advantage of this tendency and creating such better conditions that man has drawn it into his service.
This willingness and often eagerness in plants to change their habitat, sometimes without the least acclimation, enlarges, therefore, from the mere lusus naturæ which it at first appeared to be, into a law which is coextensive with plant-life. In view of the facts adduced, and others which will occur to the reader, we may conclude that the law of adaptation as popularly held requires extensive qualification if allowed to stand at all; that it is rather apparent than real; that large classes of facts are marshaled against it, and that some wider law is perpetually overruling it. The adaptations of Nature of which we hear so much are not perfect. Nature does not provide each species with a habitat best suited to its fullest development. But every plant is at all times ready to change its habitat for a better one, and this is actually going on whenever occasion permits.
Let us now inquire whether the facts enumerated admit of any general explanation. Mr. Meehan proposes to account for the better growth of swamp-trees in drier soil by maintaining that their seeds cannot germinate in dry ground. If this be true, it is a worse commentary on the theory of adaptation than I am willing myself to make without further proof. Certainly no intelligent adapting power could originate so gross and apparently gratuitous an inadaptation as an organism doomed to live out its existence under conditions unfavorable to its healthy development, because, forsooth, it could begin its career only under such conditions! But, as both the theory and the commentary rest on a teleological basis, they are both worthless from a scientific point of view.
But, however this may apply to the trees enumerated by him, it certainly does not apply to many plants of the same class which I have named, for florists propagate them from the seed when they choose. Still less can this explanation be admitted to account for any of the other classes from which illustrations have been drawn. And, indeed, I am not aware that any attempt has ever been made to bring forward a rational explanation of a general character for the facts under consideration. Botanists, generally, seem to have been either too much dazed by the light of those more universal and striking features to which attention was called at the outset, or too intent on the special study of the facts themselves, independent of the lessons they inculcate, to have worked out a solution for the problem I have been seeking to present. But the chief obstacle, after all, to such a solution, is to be found in the satisfaction which every one seems to feel with the old explanation, viz., that plants grow in particular places because they are adapted to them and to no other, which, as we have seen, is opposed by a strong array of facts.
In this theory of perfect natural adaptation, whether it be left to stand upon its old teleological basis, or be placed, as some modern investigators would place it, upon a genetic one, a very important factor has been left out, viz., the influence which plants exert upon one another. Adaptation, as the term is employed, is applied to a supposed correlation between the plant and its inorganic environment; and to this alone is attributed their entire local distribution. But facts of the class above considered prove that this is not only an inadequate explanation of such distribution, but that it is in many cases no explanation at all, since they so generally disregard inorganic conditions, and thrive equally well or better under entirely different ones from those which Nature furnished. Their distribution must, therefore, be almost entirely attributed to some other conditions; and to what other conditions are they subjected but to organic ones, to those which they reciprocally impose upon each other? It is to these organic conditions, then, to the mutual influence of different kinds of vegetation, growing, as it always does in a state of Nature, in close local proximity and contact, that we must look for the chief laws that control the local distribution of plants.
The modification, therefore, of the adaptation theory, or rather the substitute for it, which, in the light of these facts, I would propose might be called the law of mutual repulsion, by which every individual, to the extent of its influence, repels the approach of every other and seeks the sole possession and enjoyment of the inorganic conditions surrounding it—this mutual repulsion results at length in a statical condition which is always brought about through the action of the vital forces themselves, and which, as soon as reached, determines absolutely the exact place and degree of development of each species and each individual.
It is this statical condition which is apt to be lost sight of in the modern philosophy of evolution. The modification of species, the survival and advancement of some and the depauperating and extinction of others, all forms of variation and transmutation—these are dynamical phenomena, and only take place under the influence of disturbing agencies. Changes of this kind are slow and secular, and lie beyond the reach of direct observation, perceptible only to the eye of reason on the closest comparison of large masses of dependent facts. They, therefore, long escaped observation, and Nature remained until recent times a sealed book with respect to them. What wonder, then, that this still deeper and more occult law of biological statics should have remained still longer undetected, or only dimly seen? For, underlying this dynamical movement in organized beings, there must exist a universal statical condition throughout organic as throughout inorganic Nature.
The changes of which science has at length caught a glimpse can be nothing more than the regular and cyclical or fitful and spasmodic disturbances of a deeper and universal state of forced equilibrium, which pervades the vital as it is known to pervade the mechanical world. And just as astronomers and physicists, confining their investigations to the more obvious and perceptible motions of celestial and terrestrial bodies, long remained ignorant of the law of gravitation which constantly forces all things into a state of equilibrium, so in biology the statical condition has been lost sight of in the effort to obtain better views of that moving panorama which a broader knowledge of the phenomena of life so unmistakably unfolds. Yet, without a clear recognition of this statical law, it is impossible to account for the facts presented by the distribution of plants, and it will doubtless be found equally essential to the full comprehension of many other phenomena of Nature. But, when we recognize this law, the whole aspect of our question is changed. Plants appear to be no longer in a state of perfect adaptation to their surroundings.
There is no longer a necessary correspondence and correlation between organism and habitat, no longer necessary that rhythmical (almost preëstablished) harmony between species and environment. This need only exist so far as is necessary to render the life of the species possible. Beyond this the greatest inharmony and inadaptation may be conceived to reign in Nature. Each plant may be regarded as a reservoir of vital force, as containing within it a potential energy far beyond and wholly out of consonance with the contracted conditions imposed upon it by its environment, and by which it is compelled to possess the comparatively imperfect organization with which we find it endowed. Each individual is where it is, and what it is, by reason of the combined forces which hedge it in and determine its very form. Each species is the perpetual and inexorable antagonist of every other. The "struggle" is not alone "for existence," it is also for place. In the plant races, as in the human, there is a recognized hierarchy, the laws of which are as yet to a great extent involved in mystery. But the first principle, as in the rest of Nature, is force. Each one encroaches with all the power of vegetal growth upon its neighbors. This pressure is enormous. Who shall calculate thisof molecular forces? Yet there is no displacement, no motion. So thoroughly has every nook and chink been filled that there is no room for motion. Like the all-pervading circumambient air, its power is not felt so long as no vacuum is produced. Each organism has long since reached the limit of its power to extend its dominion. The plant grows up from the germ to maturity under a constant surveillance, and every attempt to overstep its fixed limits is instantly checked. It stands in its fixed position, locked in the embrace of forces which permit it neither to advance nor retreat.
Such is the state of equilibrium which is always and necessarily reached in a state of Nature, and in which man first finds each newly-discovered flora. But let these statical conditions be once changed, whether by the advent of man or from whatever cause, and this equilibrium is immediately disturbed. The chained forces are set free; a general swarming begins; some individuals are destroyed, others are liberated; each pushes its advantage to the utmost, and all move forward in the direction of least resistance, till at length they again mutually neutralize each other, and again come, under new conditions and modified forms, into the former state of quiescence.
The most frequent and prominent cause of these disturbances of the natural fixity of vegetation is the influence of man. The results of this influence may be said to be the products of agriculture, horticulture, and floriculture, on the one hand, and, on the other, weeds. But there may be many other causes of disturbance besides that produced by man, such as the appearance of new animals, geological revolutions, or climatal and meteorological vicissitudes. Anything which destroys the stability which the perpetually-operating vegetal forces impose upon the plants of any region is certain to reveal a latent vitality, which, when liberated, proves itself capable of profiting by conditions far different from, and superior to, those under which it is originally found. The willow, the alder, the elm, and the sycamore, hug the banks of streams because baffled and beaten back at every attempt to invade the drier ground. The wild-columbine and the saxifrage are driven into their rocky fastnesses by more powerful rivals for the rich forest loams. The thistle and the chamomile flourish in lawns and commons because their human foes are less formidable than the enemies of the plain. The fruit-trees, the cereals, and the roses, reach those wonderful heights of development under man's care, because he not only proves their friend, but wards off all their enemies. And just here it should be remarked that the alleged tendency of cultivated plants to relapse, when neglected, into their original state, upon which Prof. Agassiz laid so much stress as an unanswerable argument against transmutation, becomes, under the law of mutual repulsion, the necessary result of remanding them to their old conditions. As man's care and protection were necessary to enable them to advance, so, when these are withdrawn, they must be expected to again yield to hostile forces, and fall back to the level of their original state. It is not the special adaptation of a plant for the spot on which it grows, so much as the hostile attitude of other plants around it, which restricts and determines its range. The elements which decide where plants shall grow, are to be found in vegetation itself, and not in inorganic conditions. The power of self-adaptation which they possess is sufficient to habituate almost any species to almost any inorganic conditions. Each species, therefore, keeps within its own restricted limits, not because it cannot live in other soils, but because prior occupants forbid it to come.
The law of adaptation may therefore be reduced to this: that every plant possesses the power of self-adaptation to such a degree that, no matter under what conditions it may be compelled, by the higher law of mutual repulsion, to live, it will mould its own organism into harmony with those conditions, and thus continue its existence; and this, whether it is required to adopt a more perfect or a less perfect form.
But what it actually is, is no criterion of what it is capable of becoming, and the locality in which it is found is no evidence that it is best adapted to such a locality. These data only prove that in the final balance of forces to which it is subjected it was assigned such a degree of development and such a habitat.
- See Popular Science Monthly for May, 1874, p. 126.
- "Naturalist's Voyage round the World," p. 119.