Popular Science Monthly/Volume 1/October 1872/On the Derivation of American Plants
|ON THE DERIVATION OF AMERICAN PLANTS.|
By Prof. ASA GRAY.
THE session being now happily inaugurated, your presiding officer of the last year has only one duty to perform before he surrenders the chair to his successors. If allowed to borrow a simile from the language of my own profession, I might liken the President of this Association to a biennial plant. He flourishes for the year in which he comes into existence, and performs his appropriate functions as presiding officer. When the second year comes round he is expected to blossom out in an address and disappear. Each president, as he retires, is naturally expected to contribute something from his own investigations, or his own line of study; usually to discuss some particular scientific topic. Now, although I have cultivated the field of North American botany with some assiduity for more than fifty years, have reviewed our vegetable hosts, and assigned to no small number of them their name and their place in the ranks, yet, so far as our own wide country is concerned, I have been, to a great extent, a close botanist. Until this summer I had not seen the Mississippi, nor set foot upon a prairie. To gratify a natural interest, and to gain some title for addressing a body of practical explorers, I have made a pilgrimage across the continent; I have sought and viewed in their native haunts many a plant and flower which, for me, had long bloomed unseen, or only in the Hortus siccus. I have been able to see for myself what species and what form constitute the main features of the vegetation of each successive region, and record—as the vegetation unerringly does—the permanent characteristics of its climate. Passing on from the eastern district, marked by its equally-distributed rainfall, and therefore naturally forest-clad, I have seen the trees diminish in numbers, give place to wide prairies, restrict their growth to the borders of streams, and then disappear from the boundless drier plains; have seen grassy plains change into brown and sere desert—desert in the common sense, but hardly anywhere botanically so—have seen a fair growth of coniferous trees adorning the more favored slopes of a mountain-range, high enough to compel summer showers; have traversed that broad and bare elevated region shut off on both sides by high mountains from the moisture supplied by either ocean, and longitudinally intersected by sierras which seemingly remain as naked as they were born; and have reached at length the westward slopes of the high mountain-barrier, which, refreshed by the Pacific, bear the noble forests of the Sierra Nevada and the Coast Range, and among them trees which are the wonder of the world. As I stood in their shade, in the groves of Mariposa and Calaveras, and again, under the canopy of the commoner redwood, raised on columns of such majestic height and ample girth, it occurred to me that I could not do better than to share with you, upon this occasion, some of the thoughts which possessed my mind. In their development they may perhaps lead us up to questions of considerable scientific interest.
I shall not detain you with my remarks (which would now be trite) upon the rise or longevity of these far-famed Sequoia trees, or of the sugar-pines, incense-cedar, and firs, associated with them, of which even the prodigious bulk of the dominating Sequoia does not sensibly diminish the grandeur. Although no account and no photographic representation of either species of the far-famed Sequoia trees can give an adequate idea of their singular majesty—still less of their beauty—yet my interest in them did not culminate merely nor mainly in consideration of their size and age. Other trees in other parts of the world may claim to be older. Certain Australian gum-trees (eucalypti) are said to be taller. Some, we are told, rise so high that they might even cast a flicker of shadow upon the summit of the Pyramid of Cheops. Yet the oldest of them doubtless grew from seed which was shed long after the names of the pyramid-builders had been forgotten. So far as we can judge from the actual counting of the layers of several trees, no Sequoia now alive can much overdate the Christian era. Nor was I much impressed with an attraction of man's adding. That the more remarkable of these trees should bear distinguishing appellations seems proper enough. But the tablets of personal names which are affixed to many of them in the most visited groves—as if the memory of more or less notable people of our day might be made more enduring by the juxtaposition—does suggest some incongruity. When we consider that a hand's-breadth at the circumference of any one of the venerable trunks so placarded has recorded in annual lines the lifetime of the individual thus associated with it, one may question whether the next hand's-breadth may not measure the fame of some of the names thus ticketed for adventitious immortality. Whether it be the man or the tree that is honored in the connection, probably either would live as long in fact and in memory without it.
One notable thing about these Sequoia trees is their isolation. Most of the trees associated with them are of peculiar species, and some of them are nearly as local. Yet every pine, fir, and cypress in California is in some sort familiar, because it has near relations in other parts of the world. But the redwoods have none. The redwood—including in that name the two species of "big trees"—belongs to the general cypress family, but is sui generis. Thus isolated systematically, and extremely isolated geographically, and so wonderful in size and port, they, more than other trees, suggest questions. Were they created, thus local and lonely, denizens of California only; one in limited numbers in a few choice spots on the Sierra Nevada, the other only along the Coast Range from the bay of Monterey to the frontiers of Oregon? Are they veritable Melchisedeks, without pedigree or early relationship, and possibly fated to be without descent? Or are they now coming upon the stage (or rather were they coming but for man's interference) to play a part in the future? Or are they remnants, sole and scanty survivors of a race that has played a grander part in the past, but is now verging to extinction? Have they had a career, and can that career be ascertained or surmised, so that we may at least guess whence they came and how and when? Time was, and not long ago, when such questions as these were regarded as useless and vain—when students of natural history, unmindful of what the name denotes, were content with a knowledge of things as they now are, but gave little heed as to how they came to be so. Now such questions are held to be legitimate, and perhaps not wholly unanswerable. It cannot now be said that these trees inhabit their present restricted areas simply because they are there placed in the climate and soil of all the world most congenial to them. These must indeed be congenial, or they would not survive. But when we see how Australian eucalyptus trees thrive upon the California coast, and how these very redwoods flourish upon another continent; how the so-called wild-oat (Avena sterilis of the Old World) has taken full possession of California; how that cattle and horses, introduced by the Spaniard, have spread as widely and made themselves as much at home on the plains of the La Plata as on those of Tartary, and that the cardoon thistle-seeds, and others they brought with them, have multiplied there into numbers probably much exceeding those extant in their native land; indeed, when we contemplate our own race, and our own particular stock, taking such recent but dominating possession of this New World; when we consider how the indigenous flora of islands generally succumbs to the foreigners which come in the train of man; and that most weeds (i. e., the prepotent plants in open soil) of all temperate climates are not "to the manor born," but are self-invited intruders, we must needs abandon the notion of any primordial and absolute adaptation of plants and animals to their habitat which may stand in lieu of explanation, and so preclude our inquiring any further. The harmony of Nature and its admirable perfection need not be regarded as inflexible and changeless. Nor need Nature be likened to a statue, or a cast in rigid bronze, but rather to an organism, with play and adaptability of parts, and life and even soul informing the whole. Under the former view, Nature would be "the faultless monster which the world ne'er saw," but inscrutable as the Sphinx, whom it was vain, or worse, to question of the whence and whither. Under the other, the perfection of Nature, if relative, is multifarious and ever renewed; and much that is enigmatical now may find explanation in some record of the past.
The larger part of the genera of our own region which I have enumerated as wanting in California are present in Japan or Mantchooria, along with many other peculiar plants divided between the two. There are plants enough of the one region which have no representatives in the other. There are types which appear to have reached the Atlantic States from the South, and there is a larger infusion of subtropical Asiatic types into temperate China and Japan; among these there is no relationship between the two countries to speak of. There are also, as I have already said, no small number of genera and some species, which, being common all round or partially round the northern temperate zone, have no special significance because of their occurrence in these two antipodal floras, although they have testimony to bear upon the general question of geographical distribution. The point to be remarked is that a very large proportion of the genera and species which are peculiar to North America as compared with Europe, and largely peculiar to Atlantic North America as compared with the California region, are also represented in Japan and Mantchooria, either by identical or by closely-similar forms. The same rule holds on a more northward line, although not so strikingly. If we compare the plants, say of New England and Pennsylvania (latitude 45° 47'), with those of Oregon, and then with those of Northeast Asia, we shall find many of our own curiously represented in the latter, while only a small number of them can be traced along the route even so far as the western slope of the Rocky Mountains. And these repositories of Eastern-American types in Japan and neighboring districts are in all degrees if likewise. Sometimes the one is undistinguishable from the other; sometimes there is a difference of as great but hardly of as tangible character; sometimes the two would be termed marked varieties if they grew naturally in the same forest, or in the same region; sometimes they are what the botanists call representative species, the one answering closely to the other, but with some differences regarded as specific; sometimes the two are nearly of the same genus or not quite that, but of a single or very few species in each country, when the point which interests us is that this peculiar limited type should occur in two antipodal places and nowhere else. It would be tedious, and, except to botanists, abstruse, to enumerate instances, yet the whole strength of the case depends upon the number of such instances. I propose, therefore, if the Association does me the honor to print this discourse, to append in a note a list of the more remarkable ones. But I would mention two or three cases as specimens. Our Rhus toxicodendron, or poison-ivy, is exactly repeated in Japan, but is found in no other part of the world, although a species like it abounds in California. Our other species of Rhus (R. venenata), commonly called poison-dogwood, is in no way represented in Western America, but has so close an alliance in Japan that the two were taken for the same by Thunberg and Linnæus, who called them both R, vernix. Our Northern fox-grape, Vitis labrosca, is wholly confined to the Atlantic States, except that it reappears in Japan and that region. Wistaria was named for a woody leguminous climber, with showy blossoms; native of the Middle Atlantic States. The other species which we prize so highly in cultivation, W. sinensis, is from China, as its name indicates, or perhaps only from Japan, where it is certainly indigenous. Our yellow-wood (cladrastis) inhabits a very limited district on the western slope of the Alleghanies. Its only and very near relative (maackia) is in Mantchooria. The hydrangeas have some species in our Alleghany region. All the rest belong to the Chino-Japanese region and its continuation westward. The same may be said of the Syringas (Philadelphus), except that there are one or two nearly the same in California and Oregon. Our blue choste (cantophyllum) is confined to the woods of the Atlantic States, but has lately been discovered in Japan. A peculiar relative of it, diphyllæ, confined to the higher Alleghanies, is also repeated in Japan, with a slight difference, so that it may largely be distinguished as another species. Another relative is our twin-leaf (Jeffersonia) of the Alleghany region alone. A second species has lately turned up in Mantchooria. A relative of this is podophyllum, our mandrake, a common inhabitant of the Atlantic United States, but found nowhere else. There is one other species of it, and that is in the Himalayas. Here are four most peculiar genera of one family, each of a single species in the Atlantic United States, which are duplicated on the other side of the world, either in identical or almost identical species, or in an analogous species, while nothing else of the kind is known in any other part of the world. I ought not to omit ginseng, the root so prized by the Chinese, and which they obtained from their northern provinces and Mantchooria. We have it also from Corea and Northern Japan. The Jesuit fathers identified the plant in Canada and the Atlantic States, brought it in the Chinese name by which we know it, and established the trade in it, which was for many years most profitable. The exportation of ginseng to China probably has not yet entirely ceased. Whether the Northeastern Asiatic and the Atlantic American ginsengs are exactly of the same species or not is somewhat uncertain, but they are hardly if at all distinguishable. There is a shrub—ellittia—which is so rare and local that it is known only at two stations on the Savannah River, in Georgia. It is of peculiar structure, and was without near relative until one was lately discovered in Japan (in Triwitalavia) so like it as hardly to be distinguishable, except by having the parts of the blossom in threes instead of fours. We suppose ellittia had happened to be collected only once, a good while ago, and all knowledge of the limited and secluded locality was lost; and meanwhile the Japanese form came to be known. Such a case would be paralleled with an actual one. A specimen of a peculiar plant was detected in the herbarium of the elder Michaux, who collected it (as his autograph ticket shows) somewhere in the high Alleghany Mountains more than eighty years ago. No one has seen the living plant since, or knows where to find it, if haply it still flourishes in some secluded spot. At length it is found in Japan; and I had the satisfaction of making the identification. One other relative is also shown in Japan; and another has just been detected in Thibet. Whether the Japanese and the Alleghanian plants are exactly the same or not, it needs complete specimens of the two to settle. So far as we know, they are just alike. And even if some difference came to be known between them, it would not appreciably alter the question as to how such a result came to pass.
Each and every one of the analogous cases I have been detailing—and of which I could adduce very many more—raises the same question, and would be satisfied with the same answer. These singular relations attracted my curiosity early in the course of my botanical studies, when comparatively few of them were known, and my serious attention in later years, when I had numerous and new Japanese plants to study in the collections made (by Morris, Williams, and Morrow) during Commodore Perry's visit in 1853, and especially, by Mr. Charles Wright, in Commodore Rodgers's expedition in 1855. I then discussed this subject somewhat fully, and translated the facts within my reach. This was before I ever had developed the rich fossil botany of the arctic zone, before the immense antiquity of existing species of plants was recognized, and before the publication of Darwin's now famous volume on the "Origin of Species" had introduced and familiarized the scientific world with those now current ideas respecting the history of species, with which I attempted to deal in a moderate and feeble way. My speculation was based upon the former glaciation of the northern temperate zone, and the inference of a warmer period preceding (and, perhaps, following). I considered that our own vegetation, or its proximate ancestry, must have occupied the arctic and sub-arctic regions in Pliocene times, and that it had been gradually pushed southward as the temperature lowered and the glaciation advanced even beyond its present habitation; that plants of the same stock and kindred, probably ranging round the arctic zone as the present arctic species do, made their forced migration southward upon widely-different longitudes, and receded more or less as the climate grew warmer; that the general difference of climate which marks the eastern and the western sides of the continents—the one extreme, the other mean—was doubtless even then established, so that the same species and the same sort of species would be likely to secure and retain foothold in the similar climates of Japan and the Atlantic United States, but not in intermediate regions of different distribution of heat and moisture; so that different species of the same genus as in torreya, or different genera of the same group, as Redwood, taxodium and glyptostribus, or different associations of forest-trees, might establish themselves each in the region best suited to their particular requirements, while they would fail to do so in any other. These views implied that the sources of our actual vegetation and the explanation of these peculiarities were to be sought in and presupposed an ancestry in Pliocene or still earlier times, occupying the high northern regions. And it was thought that the occurrence of peculiarly North American genera in Europe, in the Tertiary period (such as taxodium, carya, llquidamber, sassafras, negundo, etc.), might best be explained on the assumption of early interchange and diffusion through Northern Asia, rather than by that of the fabled Atlantis. The hypothesis supposed a gradual modification of species in different directions under altering conditions, at least to the extent of producing varieties, sub-species, and representative species, as they may be variously regarded; likewise the single and local origination of each type, which is now almost universally taken for granted.
The remarkable facts in regard to the Northeast American and Northeast Asiatic floras, which these speculations were to explain, have since increased in number, more especially through the admirable collections of Dr. Maximowits in Japan and adjacent countries, and the critical comparisons he has made and is still engaged upon. I am bound to state that in a recent general work by a distinguished botanist, Prof. Guisebach of Göttingen, these facts have been emptied of all special significance, and the relations between the Japanese and the Atlantic United States floras may be said to be more intimate than might be expected from the situation, climate, and present opportunity of interchange. This extraordinary conclusion is reached by regarding as distinct species all the plants common to both countries between which any differences have been discerned, although such differences would probably count for little if the two grew in the same country, thus transferring many of my list of identical to that of representative species, and by simply eliminating from consideration the whole array of representative species—i. e., all cases in which the Japanese and the American plant are not exactly alike. As if, by pronouncing the cabalistic word species the question was settled, or rather the greater part of it remanded out of the domain of science, as if, while complete identity of forms implied community of region, any thing short of it carried no presumption of the kind—so leaving all these singular duplicates to be wondered at, indeed, but wholly beyond the reach of inquiry. Now, the only known cause of such likeness is inheritance, and as all transmission of likeness is with some difference in individuals, and as changed conditions have resulted, as is well known, in very considerable differences, it seems to me that if the high antiquity of our actual vegetation could be rendered probable, not to say certain, and the former habitation of any of our species, or if very near relatives of them in high northern regions could be ascertained, my whole case would be made out.
The needful facts, of which I was ignorant when my essay was published, have now been for some years made known, thanks mainly to the researches of Heer upon ample collections of arctic fossil plants. These are confirmed and extended by new investigations, the results of which have been indicated to me by the latter. The taxodium, which everywhere abounds in the Miocene formations in Europe, has been specifically identified, first by Goeppert, then by Heer, with our common cypress of the Southern States. It has been found, fossil in Spitzbergen, Greenland, and Alaska, in the latter country along with the remains of another form, distinguishable, but very like the common species; and this has been identified by Lesquereux in the Miocene of the Rocky Mountains. So there is one species of tree which has come down essentially unchanged from the Tertiary period, which for a long while inhabited both Europe and North America, and also at some part of the period the region which geographically connects the two (once doubtless much more closely than now), but survives only in the Atlantic United States and Mexico. The same Sequoia which abound in the same Miocene formations in North Europe has been now abundantly found in those of Iceland, Spitzbergen, Greenland, Mackenzie River, and Alaska. It is named Sequoia langsdupii, but is pronounced to be very much like Sequoia sempervirens, our living redwood of the Californian coast—to be the ancient representative of it. Fossil specimens of a similar, if not the same, species have been recently detected in the Rocky Mountains by Hayden, and determined by our eminent paleontological botanist, Lesquereux, and he assures me that he has the common redwood itself from Oregon, in a deposit of Tertiary age. Another Sequoia (Sequoia stembergii), discovered in Miocene deposits in Greenland, is pronounced to be the representative of Sequoia gigantea, the big tree of Californian sierra. If the taxodium of the Tertiary time in Europe and throughout the arctic regions is the ancestor of our present bald cypress, which is assumed in regarding them as specifically identical, then I think we may with our present light fairly assume that the two redwoods of California are the probable descendants of the two ancient species which so closely resemble them. The forests of the arctic zone in Tertiary times contained at least three other species of Sequoia, as determined by their remains, one of which, from Spitzbergen, also much resembles the common redwood of California. Another, "which appears to have been the commonest coniferous tree on Disco," was common in England and some other parts of Europe. So the Sequoias, now remarkable for their restricted station and numbers, as well as for their extraordinary size, are of an ancient stock; their ancestors and kindred formed a large part of the forests which flourished throughout the polar regions, now desolated and ice-clad, and which extended into low latitudes in Europe. On this continent one species at least had reached to the vicinity of its present habitat before the glaciation of the region. Among the fossil specimens already found in California, and which our trustworthy paleontological botanist has not yet had time to examine, we may expect to find evidence of the early arrival of these two redwoods upon the ground which they now, after much vicissitude, scantily occupy. Differences of climate, or circumstances of migration, or both, must have determined the survival of Sequoia upon the Pacific; very similar would seem to have been the fate of a more familiar gymnospermous tree, the ginko or salistiria. It is now indigenous to Japan only. Its ancestor, as we may fairly call it, since, according to Heer, "it corresponds so entirely with the living species that it can scarcely be separated from it," once inhabited Northern Europe and the whole arctic region round to Alaska, and had even a representative farther south in our Rocky Mountain district. For some reason, this and glystrophobes survived only on the shores of Eastern Asia. Libocearus, on the other hand, appears to have cast in its lot with the Sequoias. Two species, according to Heer, were with the ancient ones in Spitzbergen. Of the two now living, one L. decurrens—the incense-cedar—is one of the noblest associates of both the present redwoods; the other is far south in the Andes of Chili. The genealogy of the torreyas is more obscure; yet it is not unlikely that the yew-like trees, named taxides, which flourished with the Sequoias in the Tertiary arctic forests, are the remote ancestors of the three species of torreya, now severally in Florida, in California, and in Japan. As to the pines and firs, these were more numerously associated with the ancient Sequoias of the polar forests than with their present representatives, but in different species, apparently more like those of Eastern than of Western North America. They must have encircled the whole polar zone then as they encircle the present temperate zone now.
I must refrain from all enumeration of the angiospermous or ordinary deciduous trees and shrubs, which are now known by their fossil remains to have flourished throughout the polar regions when Greenland better deserved its name, and enjoyed the present climate of New England and New Jersey. Then Greenland and the rest of the north abounded with oaks, representing the several groups of species which now inhabit both our eastern and western forest districts; several poplars are very like our balsam-poplar or balm-of-Gilead-tree; more beeches than there are now, a hornbeam, and a hop-hornbeam, some birches, a persimmon, and a plane-tree, near representatives of those of the Old World, at least of Asia, as well as of Atlantic North America, but all wanting in California; one juglans, like the walnut of the Old World; two or three grape-vines are near our Southern fox-grape or muscadine, the other near our Northern frost-grape; a tilia, very like our basswood of the Atlantic States, only a liquidamber; a magnolia, which recalls our Magnolia grandiflora; a liriodendron, sole representative of our tulip-tree; and a sassafras very like the living tree. Most of these, it will be noticed, have their nearest or their only living representatives in the Atlantic States, and, when elsewhere, mainly in Eastern Asia. Several of them, or of species like them, have been detected in our Tertiary deposits west of the Mississippi by Newberry and Lesquereux. Herbaceous plants, as it happens, are rarely preserved in a fossil state, else they would probably supply additional testimony to the antiquity of our existing vegetation, its wide diffusion over the northern and more frigid zone, and its enforced migrations under changes of climate. Supposing, then, that our existing vegetation, as a whole, is a continuation of that of the Tertiary period, may we conclude that it absolutely originated then? Evidently not. The preceding Cretaceous period has furnished to Caruthers in Europe a fossil print like that of the Sequoia gigantea of the famous groves, associated with pines of the same character as those that accompany the present tree; has furnished to Heer, from Greenland, two more Sequoias, one of them identical with a Tertiary species, and one nearly allied to Sequoia languidrupii, which in turn is a probable ancestor of the Californian redwood; has furnished to Lesquereux, in North America, the remains of another ancient Sequoia, a glyptotrobus; a liquidamber, which well represents our sweet-gum-tree; oaks, analogous to living ones, leaves of a plane-tree, which are also in the Tertiary, and are scarcely distinguishable from our own Platanus Occidentalis; of a magnolia and tulip-tree; and "of a sassafras undistinguishable from our living species."
I need not continue the enumeration. The facts will justify the conclusion which Lesquereux—a very scrupulous investigator—has already announced, that "the essential types of our actual flora are marked in the Cretaceous period, and have come to us after passing, without notable changes, through the Tertiary formations of our continent." According to these views, as regards the plants, at least, the adaptation to successive times and changed conditions has been maintained, not by absolute reversals, but by gradual modifications. I, for one, cannot doubt that the present existing species are the lineal successors of those that garnished the earth in the old time before them, and that they were as well adapted to their surroundings then as those which flourish and bloom around us are to their conditions now. Order and exquisite adaptation did not wait for man's coming, nor were they ever stereotyped. Organic Nature, by which I mean the system and vitality of living things, their adaptation to each other and to the world, with all its apparent and indeed real stability, should be likened, not to the ocean, which varies only by tidal oscillations from a fixed level to which it is always returning, but rather to a river so vast that we can neither discern its shores nor reach its sources, whose onward flow is no less actual because too slow to be observed by the ephemera which hover near its surface or are borne upon its bosom. Such ideas as these, though still repugnant to some, and not long since to many, have so possessed the minds of the naturalists of the present day that hardly a discourse can be pronounced or an investigation prosecuted without reference to them. I suppose that the views here taken are little, if at all, in advance of the average scientific mind of the day. I cannot regard them as less noble than those which they are succeeding. An able philosophical writer, Miss Frances Power Cobbe, has recently and truthfully said:
I agree with the writer that this first conclusion is premature and unworthy; I will add deplorable. Through what faults or infirmities of dogmatism on the one hand, and skepticism on the other, it came to be so thought, we need not here consider. Let us hope, and confidently expect, that it is not to last; that the religious faith which survived, without a shock, the notion of the fixity of the earth itself, may equally outlast the notion of the absolute fixity of the species which inhabit it; that, in the future, even more than in the past, faith in an order which is the basis of science will not (as it cannot reasonably) be dissevered from faith in an Ordainer, which is the basis of religion.
- Retiring Address of Prof. Gray before the American Association for the Advancement of Science, upon resigning the presidency, at the late meeting in Dubuque, Iowa, August 21, 1872.