Popular Science Monthly/Volume 34/December 1888/The Origin of Forest-Groupings
|THE ORIGIN OF FOREST-GROUPINGS.|
THE organic relations or homologies of structure, showing connections of different beings with one another, which, in ignorance of their real bearing, were formerly made useful only for the classification of animals and plants into natural groups, have acquired a new significance since the doctrine of evolution has been brought to bear upon questions of origin and of the progress through time and space of living beings and their relations with those beings whose former existence is revealed by paleontology. The question whether there are evidences of affiliation of the former by the latter, of a direct relationship, has received attention from students; and, as one of the attempts to solve it, we may mention M. Gaudry's essay on "The Links in the Animal World," in which the development of the mammalia through geological times is investigated on the basis of the osseous frame. A difficulty that has never been surmounted besetting the study of the terrestrial mammalia with aërial respiration, arises out of their power of changing their place, which, while it is limited by geographical restrictions in the case of quadrupeds, is complete with birds. It is easy to conceive that overlappings and general irregularities have time and again been introduced into the combinations of groups of animals which any given country has successively contained. By that fact alone, the new-comers of each period, at points where we have not been able to observe their ancestors in the country of their origin, have an air of having risen suddenly and been preceded by nothing.
This is not so much the case in the vegetable kingdom, and is least so with its most eminent representative, the tree, particularly the forest tree or the tree that has become social. It is true that while we sometimes possess of fossil animals entire skeletons, and nearly always parts on which we can justly base a classification, we ordinarily get of the ancient trees only isolated specimens of leaves, or more rarely of fruits and seeds. We, nevertheless, generally succeed in determining these relics, and, by comparing them with analogous living ones, in forming conclusions, the probability of which carries conviction. Thus, with the aid of the data furnished by stratigraphy, we can not only reconstitute the forests of former days, but can also arrange them chronologically, grasp their mutual relations, establish their filiations, and finally explain how they have in the past been displaced and renewed.
It is necessary to take account of the peculiarity that trees are enrooted or fixed in the soil, so that only their seeds can leave them and be carried away, but never to a very great distance. This fixity is certainly one of the causes of the regularity and relative slowness of the modifications to which arborescent vegetation has been subjected in the periods anterior to ours. The new-comers of each region can never have rapidly traversed distances. It has been rather by slow steps, and by the aid of at first partial introductions, that the flora of all the epochs has been transformed. Instead of leaps, we meet with modifications aided by time, which were worked out through a long duration before becoming definitive. An attentive examination of the vegetable impressions collected over many successive levels and at points distributed along the course formerly followed by the vegetation, and marking its advance, should therefore enable us to recover the partial terms of the presumed filiation of the types whose origin we are investigating.
One phenomenon has been remarked in intimate relation with this gradual and successive substitution of plants; it is the cooling of the globe, operating insensibly, but subject to a general movement, the progress of which, although extremely slow, has never been arrested. Plants have pursued their migrations under the rule of this phenomenon, moving toward the south and gradually abandoning the north, beginning with the extreme north, or the immediate environs of the pole. The discovery of numerous vegetable fossils at different points in the arctic regions, in Spitzbergen, Greenland, Grinnell Land, etc., has been sufficient to give rise to terms of comparison and demonstrate what was the character of fossil vegetation when that of Europe more or less resembled the present vegetation of countries near the tropics. Hence it has been possible to establish with fair probability not only the general march but also the filiation of a number of plants; and it has been ascertained that the direct ancestors of part of our trees originally inhabited the interior of the polar circle, while many others, confined now to southern countries, once had European predecessors. "The forest" may be defined as an association of trees freely grouped over a space; or, as the vegetable kingdom delivered to its own forces and meeting conditions favorable to its becoming master of the soil and spreading its wealth over it. The "virgin" forest is the forest into which man has penetrated only in passing, or upon which he has never laid hand to attack or modify it. It is peculiarly the forest of hot countries, or of the intertropical zone, where everything concurs to stimulate luxuriance of the vegetable kingdom. Even in temperate climates, whose pretensions of this kind are modest, we have only to transport ourselves into some region where the native forest yet exists in all its primitive grandeur, to perceive at once the might and majesty of the vegetable kingdom thus abandoned to itself, and having uncontested possession of the territory. Forestal associations interpret the influence of the climate to which they are adapted. They change in aspect and composition according to the latitude, and present characteristic diversities combined in a determined and successive order as we advance from the neighborhood of the polar circle toward the south. In the review which we can make of them, they constantly present a double aspect, as they may be considered in themselves, or as with reference to their relations with the past, and their bonds of kindred with anterior vegetations. But, previous to placing ourselves at the latter point of view, we should glance at existing plants to determine the features of the order which now presides over their distribution.
The domain of the forest extends beyond the polar circle in Europe and Siberia, where it reaches and even passes a little above the seventieth degree. In America it retreats from that region about Labrador and Hudson Bay, the polar circle being hardly indented in the interval between Mackenzie River and Bering Strait. But in this domain, as in those that succeed it, an essential distinction should be made between the resinous forests, consisting almost exclusively of conifers, and those that are composed of "foliage-trees." In the north, the forests of resinous trees extend over great spaces. In central or more southerly regions, these forests prefer the mountainous masses. Besides the conifers, many foliage-trees—among them the birches, alders, aspens, willows, and mountain-ash—penetrate within the polar circle, and constitute a part of the arctic forests. South of the sixtieth degree in Europe, and of a lower latitude in America, there extends a richer assemblage of varieties, but insensibly connected with the preceding one. The birch, oak, elm, maples, ashes, and limes are its characteristic trees, while the foliage-trees and conifers of the preceding group are not excluded from it. The latter show a tendency to graduate themselves on the slopes as they ascend them, much as, in going from southern to northern countries, we pass through, regions occupied by like series of vegetation. Pursuing our course from north to south, we find the beech giving place in lower latitudes to varieties of oak; and it is one of the effects of this movement that other oaks appear at first in scattered colonies, as does also the chestnut, which, aside from its requirements as to the composition of the soil, seems to find, especially in southern Europe, the conditions normal to its forest development. The association of foliage-trees, whose outline is thus sketched, which would cover central Europe with a continuous forest if the continent had not been taken possession of by cultivation, is found on the southern slopes of the great mountain-ranges, only under specially favorable conditions of altitude and moisture. Its principal characteristics, besides the particular grouping of species, result from the winter caducity of the leaves, to the law of which the holly, the box, and the ivy—types also belonging to the next southern or Mediterranean group—are almost the only exceptions.
The Mediterranean group touches abruptly on the preceding one, and derives its name from the Mediterranean Sea, of which it occupies the whole periphery. In all the regions within this perimeter a similar forest flora covers with the same species a soil generally hilly, under a climate dry and warm, while subject to violent contrasts. The evergreen oaks, other oaks with semi-persistent foliage, the laurel, olive, pomegranate, terebinths, some of the maples, the oleander, and the carob; numerous shrubs with persistent leaves—laurestinuses, arbutuses, mock-privets, daphnes, heaths, cistuses, etc.—contribute to the constitution of this assemblage, which is all the more striking because an astonishing richness of characteristic details is concealed in it under an apparent uniformity. A more careful examination of the elements of which this flora is composed is demanded if we undertake to seek their origin. Besides the foliage-trees, the group includes conifers which are peculiar to it. The pines alone cover a large extent, one of them, the Aleppo pine, being very generally diffused, while a number of other species have each their determined station and place. The mountains in the interior of the region bear species that are special to them, and it is to such scattered islands of vegetation that we are indebted for such choice garden-plants as the spruces of Andalusia, Numidia, Mount Parnassus, Cephalonia, and Cilicia. In the same category are the cedars, which, with different names and varietal distinctions, people the ridges above a certain level of altitude of the Taurus, Lebanon, and Atlas. These are the mountaineer conifers, adapted to the Alpine stations of the Mediterranean region, where the altitude permits the beech, chestnut, maples, lindens, and birch to reappear and maintain themselves here and there in sporadic colonies.
Another distinct grouping of species in the bosom of this region is dependent on the nature of the soil. It includes the cork-oak, the chestnut, and the maritime pine, which are limited to the siliceous zone, and with them a whole train of plants and shrubs which are found with striking uniformity wherever the mineral composition of the soil is of similar character. So strict an adaptation, so absolute a selection, could not have been the work of a small number of centuries, but they may properly be attributed to causes that existed in a remote past.
To complete our review of the Mediterranean forest-grouping, we should consider, besides the dominant forms, some exceptionally sheltered parts of the regions, and other regions in which intermittent rigors of temperature have spared only the hardiest types. Three elements may be distinguished in the aggregate of about two hundred species included in the forestal vegetation of this region: First, the principal and characteristic element, in which plants with persistent leaves predominate, and which includes among its rarer types species in a declining condition, which are cantoned upon the best-sheltered or most southern points, and are transitional toward tropical types; next, the mountaineer element, to which altitude is favorable; and a third element, to which heat and moisture are agreeable. The last includes plants with deciduous leaves, which, while they can accommodate themselves to a cold climate, are better adapted to southern temperatures, and do not grow spontaneously in the central region; with which corresponds a group of trees—large, not very numerous, and usually monotypal—which deserves attention because of what it has been in the past, and which may be said to represent the southern prolongation of the central group, elbowing into the Mediterranean region. Its members, including the alder, the Eastern witch-elm, the hornbeam, the plane, the liquidambar, the fig, vine, some ashes, lindens, and walnut, are more in contrast with the mass of the Mediterranean plants than with the types equivalent in order that are domiciled farther north. It might be said of them that, while they are found associated with the former plants, they belong naturally to the category of the latter, as they would visibly had not the free extension of these to the southward been arrested. This view is confirmed by the examination of the forestal flora of a corresponding latitude in America. The absence on that continent of any vegetable domain equivalent to that of the Mediterranean disengages that element, and supplies, through the plane, the liquidambar, the persimmon, and American vines and walnuts direct from the ancient world, a parallelism or a repetition of forms which the study of paleontology helps to illustrate.
The principal elements of the Mediterranean group—hollies. laurels, olives, myrtles, etc.—with their narrow, elongated, coriaceous, entire or spiny foliage, only slightly divided, do not display the luxuriant fullness of tropical forms, but seem to lead toward them. They touch upon them on some sides, indicating the influence of a special medium, determined by conditions of intermittent heat and dryness. A peculiar feature of the Mediterranean group, and one which may help to determine its significance, is the capricious and uneven distribution in the interior region of plants of the most decided characteristics, evidently outside of the range of common species, manifesting affinities with hot country types. These species are narrowly cantoned in certain stations. The Pinus excelsa of the Himalayas is found only on a single mountain of Macedonia; the Algerine Thuya only in the Atlas; the false cork-oak in only a few specimens at a single spot; the carob at a few places on the littoral; the poplar of the Euphrates on the banks of the Jordan and at one point in the province of Constantine. Numerous instances of this kind betoken the existence of a former condition which has been more or less changed by subsequent events, that the group has suffered from revolutions which have displaced and partly eliminated elements that were formerly more widely distributed. The group has been impoverished, probably by the depreciation of some elements, certainly by the destruction of others.
This brief review of the forestal zones from north to south is sufficient for the study we have in view of the paleontological origin of the principal types of trees. This origin, which is at the best hard to determine, could not be sought with any probability, except as to those species concerning which we have data of a character to cast light on their history in the past, their former migrations, and their career through time as well as through space. Europe, North America, and the arctic zone furnish these data. They are not to be found in India, China, and Australia, for the guiding thread would be wanting there. Knowledge of many fossils is not enough of itself to conduct to the desired end. Vegetable impressions are useful indications, which, taken singly, have a relative value, but rarely lead to results of a material bearing.
Multiplied observations and discoveries, and fossil beds of unusual richness, to be explored at many points from north to south, have been required to give a view of the floral past of a part of the globe. It was also necessary that these beds, instead of belonging to a single period, should be frequently separated by long intervals distributed through successive ages, so as to present the complete picture of the series of past times. Thus a comprehensive grasp has become possible of the vicissitudes through which the vegetable kingdom has gradually undergone transformation. Changes insensible at each single step have in the long run resulted in modifications of the aspect of the landscape and replacements of the types and species composing the floral carpet at a given movement by forms different from them, and also from those before which they were themselves destined to retire at a later period.
The impression which one feels in the midst of a deep forest is one of perennial duration. Except man, what is there to uproot those giants that have lived through centuries? What action can be conceived of that will exclude them from the ground which they possess so completely? The first impression would almost make these forest masses coeval with the globe, its natural product and spontaneous dress from the days of its youth. Such an impression would be a mistaken one. The forests have not been perpetuated in the same order from the beginning, but have changed much in the course of ages. Those which we now see have taken the place of other more ancient ones, and these substitutions have occurred many times, sometimes through partial modifications and sometimes also under such conditions that the old order has only indirect relations with the present one, or is even wholly foreign to it.
Since a serious mind can not suppose that at every revolution of plant-life there has been a total destruction of the anterior elements, followed by a creation conceived anew in all its details, we are forced to seek in the order which precedes the reason for the existence of that which has replaced it. This view implies an endless chain of causes and effects, of ancestral and derived forms, stretching along, now spreading, now continuing themselves, to spread out again, and—in what more particularly concerns the types of the vegetable kingdom—emigrating in a determined direction. This direction is found to have consisted, for plants, in a march from north to south in search of more favorable regions and stations better fitted to the exigencies of acquired adaptation, as rapidly as the terrestrial temperature declined from its pristine conditions, as latitudes took on their individual characteristics, and as the arctic zone, which had been temperate, grew cooler and became more and more differentiated. The polar circle was thus constituted a barrier that became more pronounced, less accessible, and was finally closed to arboreal vegetation, while under the operation of the same movement the present temperate zone became cooled in an equivalent measure, was impoverished, and gradually stripped of a considerable part of its floral wealth. The remnants that escaped this elimination in those successive and numerous retreats that filled the second half of the Tertiary period still occur, scattered and dwarfed, in the southern part of that zone, and upon points where the less sensible depression of temperature has permitted them to maintain themselves sporadically.
By regarding these considerations and this presumed march, we succeed in determining the connection between recent and fossil species, and evidences of affiliation between them. There also exist relations not to be neglected between some recent types and other old ones which we can not believe to be wholly lost, and others between present forestal groupings taken separately and those which have succeeded one another through the ages; but the further we go back, the more we address ourselves to a distant order of things, the less tangible are these relations found to be. Of all the Carboniferous vegetation there remain only isolated or dwarfed types, as of Equisetæ, ferns, and club-mosses. The singular Japanese gingko, and perhaps the dammara of the Indian Archipelago, can trace their ancestry back to that period. A greater number of estrays have survived from the Secondary ages; but they are still rare—auracarias, cedars, pines, thuyas, and a few colonies of cycads south of the equator. There has been something vague and undetermined about the "foliage" trees since they first appeared in the Cretaceous period; but their evolution and characteristic physiognomy have been tending to fix themselves. The magnolia, tulip-tree, plane-tree, ivy, etc., have hardly varied since then; but the subsequent modifications of other European flora have been so frequent and profound that no collection of existing species corresponds except by partial traits that have been questioned with the Cretaceous vegetation. The correspondence is somewhat closer with the vegetation of the Eocene, especially of the later Eocene. The examination of certain floras which are referred to this horizon has shown that vegetation has not varied much since then, except from the impoverishment which it has suffered by the subsequent elimination of some types, and the addition of a number of deciduous types of later introduction. Thus, a considerable proportion of forms, the ancestors of which appeared in the Eocene, have persisted in place from that epoch, while those which have since migrated are found in more southerly regions.
It is in the Miocene, particularly in the later Miocene, that the relations become manifest of the deciduous trees of southern habit which we have spoken of as being dispersed over various points of the Mediterranean domain—the plane-tree, the liquidambar, the planera, the linden, the vine, some hornbeams and ashes, the datepalm, the pomegranate, etc.; and the lauriferous grouping of the Canary Islands, preserved intact by means of its insular situation and of the persistency of local climatological conditions, reproduces unchanged the picture of a mountain-forest of central Europe, as recent discoveries have shown it to have been in the later Miocene and earlier Pliocene. There are the same species, the same mixture of laurels, hollies, olives, to which are added recent Japanese or Caucasian forms of nuts, maples, elms, and toward the high summits pines and firs very like those of the higher mountains of Teneriffe, Morocco, and Asia Minor. Collections made by M. Marion in the marly sediments of Durfort, where bones of the southern elephant were also found, and in the tufas of Valentine, near Marseilles, show that a number of plants now found only further south, still in the second half of the Pliocene inhabited the hills and shore-lines of southern France. These plants were then, therefore, at home further north than they are now found; and their occurrence in the Pliocene points to a later flowing back of species, amounting to a definite retreat in the Quaternary age.
This retreat, perfectly logical and almost regular in its operation, is connected with changes of climate, which were themselves in relation with a progressive depression of the temperature of the earth. It is connected also, in a parallel order of phenomena, with the exhaustion of some races, and with the development, by a concomitant origination, of other young and new races, favored by the same circumstances that caused the elimination of the races that gave way to them. Considering all the elements of the question, we find that it is by the extension, at a given moment, of vegetable races previously localized and realizing a certain amount of variation, that species are constituted at the start. Once characterized—that is, after the acquisition of a total of characteristics, at first fleeting, then hereditarily fixed—the species is permanent nevertheless only in a relative fashion so long as there exist in it parts susceptible of differentiation anew. The amplitude of the limits between which it may range through the course of time depends on the proportion of the elements that remain variable to those that will not change. The morphological oscillations of which it offers an example are thus determined by its own tendencies to submit more or less readily to excitations from without. Hence there are evident inequalities in the specific type, sometimes running to obscure shades, sometimes clearly cut; the last especially after the exclusion of intermediate forms.
There exist, in fact, fleeting species, which can not be circumscribed by any precise limit; and others, fixed in their minutest traits, that are susceptible only of insignificant variations. The forms of the latter category, such, for example, as the sequoias of America and the cedars of the Atlas, persist in the places of which they have once taken possession, where some of them have been driven back and cantoned, so that quite contrary conditions or the intrusion of more vigorous forms have not sufficed wholly to exclude them. We see, from this manner of looking at things, that the relations of the present arborescent species with those of ancient ages are only the last consequence of a march or an antagonism of long continuance, which must of necessity have left traces. Regarding this closely, we find the indices of filiation of the living by fossil species not wanting, but fully confirming our interpretation; and the genetic bonds disclose themselves to the investigator from the moment when he consents to regard the species as having acquired by degrees the characteristics which it possesses, and also as susceptible of displacement by extension or by crowding back.
The morphological connection has certainly the signification of a relationship; but it can and must vary according as the relationship is more intimate or remote, direct and immediate, or indirect and collateral. We can, by the aid of a direct method, rather led by a kind of intuition than subjected to precise rules, found a judgment concerning the bearing of these analogical shades. While the opportunities of observing forms nearly allied to those that are familiar to us diminish as we go back in the past, yet, at whatever age we place ourselves, a near resemblance always induces the notion of a direct descent of the recent form from the one which predicted its traits in the midst of an order of things remote from that which has since prevailed. The observed resemblance may also be conceived to be the more decisive as the species in which it appears belongs to a more ancient period. The indices of genetic connection may thus go far back into the past, and as to certain types of trees little subject to variance, they are discerned in a quite remote past. In order logically to reach precise conclusions, he who engages in such researches should take account not only of the type, but also of the species considered in itself, that is, of the state of race, with its particular history, of which it is sometimes possible to follow the incidents. So far as is possible, the type, or union of allied forms, derived originally from the same stock, should not be confounded with the species, or the particularized race, which, its characteristics having been once acquired and its aptitudes determined, necessarily assumes a march in harmony with the tendencies that distinguish it, within an area suited to it. The plane-trees, poplars, tulip-trees, beeches, and chestnuts appear toward the middle of the Cretaceous period, but it does not follow that existing forms are the immediate descendants of these primitive forms, or of any one of them taken separately. The traces of our plane-trees, the visible ancestor of the tulip-tree, and the evident predecessors of existing poplars, make their first distinct appearance during the Tertiary. So do those of the beech and chestnut, the introduction of which into Europe took place through isolated individuals, distinguished by shades which were gradually effaced on approaching modern times.
The principal requisite for tracing the presumed origins, whether of the type at its birth, of supposed ancestors, or of the direct antecedents of existing species, is to have in mind the exact succession of the periods and stages, giving the relative date of each of the determined first appearances and the order of the constitutive elements of the march which the forms of which we meet the traces have followed through time and space. The succession of ages, represented by beds deposited in a constant order, or stratigraphic geology, makes this known to us. It is also necessary to take account of the changes that have been impressed upon the whole vegetable kingdom during this long series of periods. The changes have been too profound, and attended by too complete renovations, for us to be able to find the cradles of existing plants in the primitive periods.
Three great plant-periods may be distinguished, starting from the moment when the surface of the globe first began to be covered with aerial vegetation: the primary or paleophytic period, or cryptogamic era, which derives its name from the domination of cryptogams; the secondary period, or mesophytic, during which gymnosperms—conifers and cycads—obtained the predominance, while foliage-trees were still absent; and the tertiary or neophytic period, also called angiospermic, from the presence of the higher plants and particularly of the foliage-trees.
The last of these periods, which began with the chalk and is still in continuance, is characterized by the appearance and extension of the higher plants, and was also coincident with the first signs of polar refrigeration and with the more and more marked decrease of terrestrial temperature with increase of latitude. This fact, at first hardly sensible, then gradually accented, exercised an influence within the polar circle before extending its action beyond, into the temperate zone, which was for a long time hot, and afterward warm, while the regions around the pole were already frozen. A remarkable relation certainly seems to exist between the beginning and the course of the climatological depression of the northern regions and the progress of vegetation, which perfected itself in a parallel line. Indeed, it closed the entire cycle of its definitive evolution by the adjunction of the angiosperms, the most perfect plants; and these acquired preponderance as rapidly as the cooling of the arctic regions went on. These regions appear to have been exempt till this time from the rigors of a cold season, and, by that fact, subtracted from the effects of the winter rest.
It is certain that the vegetable kingdom, as soon as it had acquired all the elements of which it is still composed, began to distribute itself in groups ordered according to the latitude; and this movement once started, the differences between the groups became continually more accentuated by the increasing exclusion in each of them of a part of the types which they originally contained. There resulted from this a constant impoverishment of northern countries as compared with southern ones, which gained, at least by contrast, what the former never ceased to lose. The movement has thus tended to a differentiation by zones, and has resulted in despoiling them, but in unequal proportions, increasingly as they are removed from the tropical zone, the only one which has been exempt from the spoliation.
Let us not forget that, parallel with this movement, working, moreover, with an extreme slowness and in harmony with it, another movement, purely organic and evolutionary, although incited, if not directed, by the former, has not ceased to push to development and to morphological differentiation various groups of plants; particularly of those which, relatively young and plastic, were susceptible, by this fact, of giving birth to new forms, and, by successive splittings, to new types. These are the angiosperms, which, having once gained preponderance, have offered the spectacle of an increasing multiplicity of races and forms. That multiplicity could only increase. The displacements resulting from the climatic depressions have aided in it by inducing changes of stations and opening new cantonments to races not yet fully established. The revolutions of the surface, continental contiguities, and the more or less accentuated orographic relief, have constituted other factors not less active in the general push of species, incessantly solicited to vary as they adapted themselves to the soil of the regions into which they penetrated, as they scattered, and as they struggled victoriously against rival species.
Such is the spectacle which the vegetation of the globe has not ceased to present; and the existing forests appear as the final resultant and ultimate consequence of that long series of alternatives which is summarized in the expression, "the struggle for existence."—Translated for the Popular Science Monthly from the Revue des Deux Mondes.