Popular Science Monthly/Volume 16/February 1880/Saporta's World of Plants Before the Appearance of Man
|SAPORTA'S WORLD OF PLANTS BEFORE THE APPEARANCE OF MAN|||
MEN of science, whose patient researches have accumulated the proofs of the theory of evolution, have perhaps found more facts in support of this great philosophical doctrine in the vegetable than in the animal world. When we say the vegetable world, we of course mean chiefly fossil vegetables. It is only by the study of extinct forms, and their comparison with the living flora, that the affinities between actual types and distant ancestors have been discovered, and their mode of evolution revealed. Vegetable paleontology, it is true, is yet in its infancy, and has many great gaps; still, the rapidity with which it is being developed, and the prodigious number of facts that have been already collected, give good ground for the hope that the day is not far distant when we shall have surely determined the ancestral lines of most of our plants. To this the efforts of paleontologists are tending, and their activity is beyond all praise. During the last twenty years their discoveries have furnished the matter for large volumes and for many memoirs, published in the reports of academies of science, in the bulletins of geological societies, etc. But the profound lessons derived from these discoveries have hitherto been almost the exclusive possession of scientific men. People of general intelligence, who are interested in all progress have known little of the results obtained. This injustice could be no longer tolerated. A complete treatise was required, written in a style that all could comprehend, and summing up the progress thus far accomplished; and M. de Saporta, one of the most eminent authorities in vegetable paleontology, has just published such a work.
He devotes his first chapter to the theory of evolution, passing successively in review the most important arguments in its favor. Notwithstanding the great interest of this subject, it will not detain us now, for we wish to examine the main body of the work. Besides, Saporta is now writing for the "International Scientific Series" a book devoted to the study of evolution in the vegetable kingdom, where he will show the line of descent of the great families of plants.
The study of fossil flora not only enables us to follow the evolution of plants from their remotest known ancestors to their present actual descendants, but it throws much light upon the past mysteries of the earth, and especially upon the climatic conditions which controlled its surface while the slow revolutions of organic life were going on. We know what numerous causes concur to form a climate; latitude and longitude, the direction of winds and of currents of water, the nature and relief of the soil, and the distance from the sea. All these causes have their respective known effects, and have acted in the past as they act now; yet we know that, if it were needful to determine the amount of influence due to each of these agencies in the different geological epochs, we could not do it; the difficulties are too great. But in one case, that of latitude, we can find out its ancient effects by analogy with what is passing under our eyes, and by abstracting all other influences. We know that the obliquity of the sun's rays increases with latitude, and that temperature diminishes in the same proportion; that the higher the latitude of a region the less heat has its climate. But we know also that vegetation marches with temperature, provided always that soil and moisture are favorable. The floras of the temperate and polar regions show clearly the decrease of temperature from the equator to the pole. There exists between a flora and the climate in which it lives a relation so close that, knowing the one, we can represent the other. Palms do not grow in Greenland nor fir-trees on the plains of equatorial Africa. Each climate has its flora, and each flora its climate.
Paleontology has established the permanence and universality of this law; but it has at the same time established a singular fact which remains inexplicable. It is this: the different climates of the earth have not always been what they are now, either as to temperature or distribution. We speak only of those epochs which have succeeded each other since the time of the most ancient known plants. If we transport ourselves in thought to a time toward the end of the Tertiary period, and then, leaving behind us the Quaternary epoch, follow the course of the ages, we find, as an increasing enlargement of the tropical zone, that which is equivalent to an increase of temperature for the whole earth. More extended in the Pliocene epoch than in our day, this zone was still greater in the Miocene epoch, and yet greater in the Eocene, and so on till we reach a time when it embraced the whole surface of the earth, bestowing everywhere an equal temperature, feebly oscillating between certain limits. This climatic equality, which, according to Saporta, reaches at least as far back as the time of the coal, would probably cease at the epoch of the inferior chalk. Such is the fact established by examination of the flora of different ages.
Let us proceed to details. The Quaternary epoch, contrary to the opinion of the majority of geologists, was not, in France, and probably also in other countries, a period of universal cold. The term glacial which has been applied to it is certainly appropriate, because of the enormous and unknown extension of the glaciers, the traces of which are the principal character of the epoch. But at certain distances from existing glaciers there doubtless existed valleys with a
warm or at least a temperate climate. The mixed fauna and flora of this period abundantly prove this. The remains of huge animals gathered in the ancient alluvium of the Seine and Somme, as determined by MM. Larted and Gaudry, demonstrate that many species indicating a very cold climate are found associated with those of a diametrically opposite character. Besides the mammoth, we encounter the ancient elephant, approaching that of India; the hippopotamus of African rivers peopled the waters of the Seine; while the hyena of the Cape frequented the meridian of France. The study of the forest flora, of which we find numerous remains in the contemporaneous tufa, leads to the same results; the vine, the laurel, the ivy, are found in abundance, not only in our southern regions, but also at Moret, near Paris. We find there also the much tenderer laurel of the Canaries. The northern trees of the same epoch were pines, lindens, maples, and oaks.
All these facts prove that the quaternary animals and plants characteristic of a cold climate existed only in the neighborhood of glaciers; and that, close by, in the valleys, lived creatures whose presence indicated a climate softer and more humid than ours. The mean annual heat necessary to their existence would be, at least, 14° or 15° centigrade. But, if we place ourselves now in the full Pliocene period, say near Lyons, we encounter the same vegetables, with others of a more southern character. At this epoch, in fact, the laurel-rose flourished on the banks of the Saone in company with the laurel, the avocatier of the Canaries, the bamboo, the magnolia, and the evergreen oak. The well-known climatic needs of these species warrant us in assigning to the country a mean annual temperature of 17° or 18° centigrade, and, as the actual mean temperature of Lyons is only 11°, we can judge of the difference of temperature which separates our epoch from that of the Pliocene. Moreover, as Saporta remarks, the figures which express the climate of Lyons during the Pliocene epoch are not only higher than those which apply to the neighborhood of Marseilles in Quaternary time, but, in place of corresponding to the 43° of latitude, these higher figures coincide with the 46°. They mark a progression of heat corresponding with latitude, the effect of which is to raise the temperature of northern regions in proportion as we bury ourselves in the past.
These curious phenomena appear still more evident and more general, if we transport ourselves in thought to the Miocene epoch. The entire unbroken documents abound in the boreal hemisphere, and we can there exactly determine the climates of all latitudes from 40° to 80°. Admirably preserved fossil plants, brought from the polar regions by different travelers, show that glaciers have not always desolated the pole. One of the principal deposits of these vegetables was found on the western side of Greenland at Atanekerdluk in 70° of latitude in the adjacent island of Noursoak. On the steep sides of a ravine one thousand feet deep, there exist entire beds of petrified leaves and other debris imbedded in a very ferruginous rock. The vast accumulation of leaves is truly surprising—trunks yet in place; flowers, fruit, insects accompanying them. M. Heer, who has studied these precious remains, says that here arose a vast forest where abounded sequoias, poplars, oaks, magnolias, ebony, holly, walnut, and a host of other species. Still farther north, at 80° of latitude, were found aquatic
plants, pond-weed, water-lilies, rushes, etc., and terrestrial plants—bald cypress, thyme, fir, plane, linden, maple, mountain-ash, and even magnolias, forming a grand forest. The illustrious Professor of Zurich, M. Heer, regards many of these plants as miocene, and concludes that if these latitudes at this time were disposed as they are now—that is, if the earth's axis has not been displaced—all the earth received more heat, and the line of the tropics must have risen toward the north. The difference of the Miocene period may be valued at 25° or 30° of latitude—that is, we must at present descend 40° or 45° to find the temperature which then existed in Greenland.
The study of the more ancient floras brings new proof of this phenomenon of the extension of heat into the higher latitudes, and conducts us finally to that equality of climate of which we have spoken. "We are forced to conclude, however," remarks Saporta, "that when we reach the time of the coal and the most remote period in the history of organic beings, if there has been no change in the relations of the heat that falls upon our globe, there have doubtless been other changes profound enough to impress upon it a very different aspect from that which it has since presented, and to create conditions of existence about which we can form no idea." We are, in fact, ignorant of the conditions in which living beings first made their appearance and were developed. There have been many hypotheses about it, but the facts on which they rest are yet neither sufficiently numerous nor convincing. For the settlement of this question we must await the future.
As to the cause of climatic equality over all the earth in the Primary and Secondary epochs, we are equally in the dark. All the explanations that have been given have been successively rejected. The displacement of the axis of the earth, the inclination of that axis on the orbit of our planet, the precession of the equinoxes, etc., are some of the hypotheses put forth on this subject. We can not here enumerate them all; but there is one on which Saporta insists, not because it explains everything, but because it agrees more or less with the celebrated cosmogony of Laplace, and accords with the phenomena of the primitive world as revealed by science. This hypothesis was put forth some years ago by M. Blaudet. We know that, according to the theory of Laplace, the entire solar system was originally an immense nebula which has since condensed little by little, and successively given off rings of cosmical matter which have become planets. The central star is hence more and more reduced, has become more dense, more luminous and more ardent, until it has attained the dimensions and properties of our actual sun. In other words, if we could trace backward the course of the ages, we should find the sun progressively augmenting in volume, but its heat and light would diminish in intensity in the same proportion. We do not know what sun lighted the earth when life first appeared upon it, but, from the theory of Laplace, we may suppose that it was much larger than ours.
Such conditions, however, would explain many phenomena. This great sun, occupying a good part of the horizon, would give a twilight so luminous and so prolonged as perhaps to annul the night. Sending his perpendicular rays much farther from the equator than now, the torrid zone would be thus enlarged. The calmer light, the more gentle and equalized heat, the thicker and more humid atmosphere, explain that equalization of temperature, those days half veiled and transparent nights, and that tepid climate of the polar regions, that we might consider as presiding at the development of primitive life. Finally, the primitive sun, by its slow condensation, passing insensibly into its present state, necessarily forced the retreat of the torrid zone, thus ending the anterior equality of the climate, permitting cold to become established at the pole, and concentrating heat at the equator. Such is the bold but attractive hypothesis of M. Blaudet. No doubt it leaves many points obscure, but the numerous partisans of the theory of Laplace will not hesitate to acknowledge its importance, for, in reality, it is part of the theory itself.
It remains now to review the remarkable chapter that Saporta has given to the study of vegetable periods. We may remark at the outset that this word "period" implies no such general convulsions as the first geologists believed in, who supposed the history of the globe broken into sharp periods, each of which was inaugurated by a distinct creation and terminated by a sudden and universal destruction. Saporta takes care to warn us against this error. "Nature, always active," says he, "has had no intermittence nor time of sleep. Life, since its first appearance, has not ceased to inhabit the earth. Depressed sometimes, interrupted never, there has circulated without respite a constantly fertile sap. The epochs and revolutions which geologists have named are valuable only as serving to introduce great dividing lines in the bosom of an incalculable duration, but a closer view reveals these beings always succeeding each other; the extinction of some among them would not prevent survivors from occupying their place. Physical revolutions, essentially accidental and unequal, have never been radically destructive. If some periods have been less favorable than others to the development of life, these relatively impoverished intervals have possessed organized beings that, afterward multiplying and diversifying, have easily repeopled the globe."
Saporta divides the world of fossil vegetables into four great periods: 1. The Primordial or eophytic, corresponding to the Laurentian, Cambrian, and Silurian; 2. The Carboniferous or palæophytic, comprehending the Devonian, Carboniferous, and Permian; 3. The Secondary period or mesophytic, commencing with the Trias and reaching to the end of the chloritic chalk; 4. Finally, the Tertiary or neopyhytic, embracing all the formations from the chalk of Rouen up to and including the Pliocene.
The flora of the eophytic period is unknown. The débris which represents it has in general a character so vague that there is yet no agreement upon its true nature. The graphite found in the Laurentian indicates, however, that from this epoch vegetables existed in great abundance. In the Cambrian and Silurian, fossils are found that are differently interpreted, and in which at present some think they see algæ. The famous bilobites, so abundant at the base of the Silurian, appear also to have been algæ of very great height. Finally, certain marine plants, as those that are represented by Fig. 3, are connected with a type of algæ so marked that it is difficult to mistake them.
Many of these plants are undeniably linked with more modern types, of which they bear the generic form, and prove that this primordial flora is not really separated from that which followed it. We can even affirm that certain Silurian algæ have had a duration so prodigious and a tenacity of character so pronounced that their last direct descendants were living in the European seas in the middle of Tertiary time. As to primordial land-plants they are excessively rare, and those that we have gathered seem to demonstrate that in the Silurian epoch to which they belong the vegetable forms represented types that we encounter in subsequent formations, and that are characteristic. In Fig. 4 are shown those that M. Lesquereux observed in the Upper Silurian of the United States. Among them are the Psilophyton, which disappeared with the Devonian, and the ambiguous characters of which approached at the same time the ferns by the Hymenophyles, the Lycopodiaceæ by Psilotum, and the Rhizocarpes by Pilularia.
With the Devonian things changed. The bad state of preservation of fossil vegetables belonging to this formation has not permitted us to study them perfectly; but, from the aspect of those which we possess, we conclude that at this epoch the vegetable kingdom was already vigorous and varied, and that nature while in its infancy put forth the carboniferous flora, the almost inconceivable exuberance of
which has never since been equaled. This flora, to the description of which numerous works have been devoted, is still more interesting and important as furnishing the elements of the coal—the soul of industry, as it has so justly been called. We know that the conditions in which the coal-beds were formed very much resembled those in the midst of which the peat is now actually being formed. As Saporta has observed, in the Carboniferous epoch there were emersions upon a grand scale, emersions succeeding each other, flowing over and receding from the insular or continental space, until its recovery from the waters. This action of the waters would produce a low bank or shore around the primitive land, the relief of which would tend to become more accented, and at length would retain the waters coming from the interior and unite them at the bottom of extensive depressions. In this way were formed vast lakes, with vague banks and shallow waters, easily invaded by plants loving an aquatic station. If we join to this the humid warmth, the thickness of the atmosphere, charged with vapors, causing frequent and violent rains, we perceive how favorable were the conditions for the development of the carboniferous vegetation.
The plants of this flora belong exclusively to the two classes of vascular cryptogams and gymnospermous phanerogams. At the head of the cryptogams were the Calamarias, which recall on a gigantic scale the Equisetaceæ of our day; by their side Asteraceæ, Annularia, Sphenophyles; then come ferns of very varied form and structure, and Lycopodiaceæ of the type Lepidendroideæ. Certain plants, the Bornia, Calamodendreæ, and Sigillaria, form connecting links between cryptogams and phanerogams. These were gymnosperms, that is, plants assimilable by the class Cycadeæ to the conifers and the actual Gnetaceæ. The true phanerogams, the angiosperms, appeared much later. Further, the carboniferous flora comprehended some Cycadeæ, such as the Noeggerathia foliosa and a pterophyllum discovered recently by M. Grand' Eury; some true conifers, as the Walchia; some Taxineæ more or less like our Ginkgo; and, finally, a great number of Cordaïticæ, most of which were great trees so perfectly preserved that they could not only be placed at the head of the gymnosperms, but their affinities with the class of angiosperms could also be observed.
The Permian flora, which succeeded the Carboniferous, is only a pale reflection of it. The characteristic type of the preceding age has disappeared, while the others, the Cycadeæ, the conifers (Fig. 5), and the Taxineæ, tend to preponderate. The Permian is an epoch of transition, having ambiguous characters. The constituent elements of the coming vegetation were being developed. Saporta says of the Trias, which commences the Secondary or mesophytic period, that "it appears to correspond to one of those periods of revival where the
failing types finally disappear, while those which displace them are successively introduced. The first leave chasms because they are reduced to a decreasing number of individuals; the last are yet obscure and rare. Both old and young are equally feeble, and, when these two extremes meet, the apparel of nature seems poor and monotonous." At the beginning of the Jurassic period a transformation is already manifest, and we soon find ourselves in the presence of a new flora, where the carboniferous types have disappeared, but where, except some rare monocotyledons, the angiosperms are still wanting. Always cryptogams and gymnosperms, the first represented by ferns and Equesitaceæ, the second by Cycadeæ and conifers. From Spitzbergen to Hindostan, from Europe to Siberia, everywhere the same vegetable forms, so that the character of the Jurassic flora is monotonous, lifeless, and relatively indigent. However, we quickly perceive two sorts of vegetation: one peculiar to low and humid plains, including beautiful ferns and Cycadeæ (Fig. 6); and the other covering the hilly regions, and composed of different genera of the same families, but chiefly of tall conifers, which in great part composed the forests of that time.
We know not under the influence of what conditions organic evolution, and especially the appearance of dicotyledons, has taken place; but we do know that from the horizon of the cenomanne chalk commenced the neophytic period, these plants appear in a multitude of places and multiply with great rapidity. Wherever the cenomanien is found we find the remains of that age, proving the predominance of dicotyledons and the decrease of Cycadeæ and conifers. "This revolution," says Saporta, "has been as rapid in its progress as universal in its effects." It would certainly be interesting to follow the author in his enumeration of the ancestors of our common plants, and his description of the progenitors of the poplar, the beech, the ivy, the chestnut, the plane-tree and others, but it would extend this article beyond the limits of our space. Besides, we have followed vegetable evolution through its principal phases—that is to say, we have in some sort witnessed the successive appearance of different classes of plants, we have seen the rising and falling movements of vegetation, periods of activity alternating with periods of relative repose, and that succession, or better, that procession of phenomena which has enabled us, if not to comprehend, at least to prove the transformations of life that all together are called evolution.
We shall find analogous phenomena in the series of Tertiary time, to which Saporta has given his longest chapter. This part of his work contains many descriptions of plants. They show us more than the simple succession of flora. The growing differentiation of the divers types and the consequent multiplication of species conduct us insensibly to the actual vegetable world. We see the growth of local flora, some of which are so clearly defined that we are able, by the aid of the imagination and of some well-preserved fragments, to reconstruct the principal genera and species of which they were composed. Fig. 1 represents a group of these plants so restored. The numerous modifications undergone by the vegetable kingdom during the Tertiary age, the formation of local floras, etc., are easily explained if we recall what was before said, of the influence of the medium upon living beings, and especially upon plants, that can not escape. Climatic equality no longer exists. The European Continent, up to this time made up of islands, tends now to aggregate and take on its present form; the soil
is subject to movements of oscillation, which often change the configuration and relief of various countries. Lakes of fresh water are formed, and then disappear. The nature of the soil varies as marine deposits recapture it from deposits of fresh water; and reciprocally. This instability of the environment has produced an instability of the flora, and caused those differences which have resulted in the European vegetation of our time.
As before remarked, in speaking of ancient climates, when we go back in time, and particularly Tertiary time, we see the vegetation taking more and more of a tropical character. Hence in these epochs there existed in Europe a multitude of forms which can not live there now. Palms and Cycadeæ (Fig. 2) and large, beautiful ferns were long ago exiled. Other forms, as the laurel, the vine, the ivy, have never quitted the region where they were born, or, at least, where they appeared for the first time.
The number of figures that Saporta has interspersed with his text, representing the principal vegetable types of the past, offer us the still further advantage of comparing species of the same type, and verifying by inspection the respective modifications of these species, and their passage from one to the other. Without doubt, we are far from possessing all the terms of all the series; but what we know of some enables us to judge by analogy that what has happened with one genus may happen with others. See, for example, the forms of Pliocene and Eocene oak (Fig. 7), which show clearly how climate has affected this species from the formation of Gelinden at the base of the Pliocene to the gypsum of Aix, that is, the superior Eocene. The forms represented
here belong to the group of oaks with entire leaves; but there is another group with leaves toothed or lobed, in which we discover analogous modifications. We see that leaves at first oval tend to become more and more slender, and these lanceolate forms express very truly the action of the warm, dry climate of the Eocene, which succeeded the warm but humid climate of the Pliocene.
Other striking examples of these affinities of species are furnished by the laurel type (Fig. 8) and that of the ivy (Fig. 9). The large
varieties of Laurus primigenia pass insensibly into Laurus Canariensis. "It seems," says M. de Saporta, "that the narrow forms of this same Laurus primigenia, which at the same time are the most ancient, mark the existence of a race due to the Eocene climate. This influence is gradually lessened, as seen in the expansion of the leaf as we advance toward the Aquitanian, and in the Armissan at first, and Manosque afterward. The relation between the amplified leaves of the Laurus primigenia and those of Laurus Canariensis and Laurus nobilis is more and more pronounced. The Laurus princeps of the superior Miocene approaches still nearer to our laurel; while, finally, the Canarian race has all the characters of Meximieux in the inferior Pliocene.
As to the ivy, its most distant ancestor is a species of the cenomanienne chalk of Bohemia, the Hedera primordialis, whose large leaves bear witness to the moistness of the climate under which it lived. The Pliocene species, Hedera prisca, found at Sezanne, is sensibly removed from the preceding species by the salient angles of its leaves and its much smaller dimensions. The Hedera Philibertii, recently discovered in the gypsum of Aix by Professor Philiberti, testifies clearly, by its narrow and pointed form, to the influence of the Eocene climate. It recalls to our astonishment the most slender forms of the ivy of Algiers, and also the forms that the European ivy takes when it runs on the ground, so that these two races may well have had the Hedera Philibertii for their common point of departure. The Hedera Kargii, characterized by its very small leaves, seems to be derived from the Hedera prisca. The Hedera acutelobata scarcely differs from the actual species; in the same way the Hedera Mac-Cluri is confounded with the ivy of Ireland. Upon the whole, if we consider the varieties presented by our actual ivy we are tempted to believe that the ancient forms have only been races of the same species.
We must here close this analysis. Readers wishing a better knowledge of this important subject than we have been able to give must be referred to the work of M. de Saporta, which will be found as agreeable as it is instructive.—Revue Scientifique.
- The World of Plants before the Appearance of Man. By Count de Saporta, Correspondent of the Institute. 8vo. 416 pages, with Thirteen Plates, of which five are colored, and One Hundred and Eighteen Figures in the Text. Paris: G. Masson. 1879.