MAP II.-Permo-Carboniferous Floras. parts of South America, and Australia remain as comparatively insignificant remnants, was exposed to climatal conditions favourable to the accumulation of snow and to the formation of glaciers. One possible explanation, therefore, of the existence of a distinct vegetation in the southern area is that the climate was such as to render impossible the existence of those coal-forest plants that exhibited so vigorous South Africa are in close association with boulder-beds of considerable extent. In some places, as for example in India and Australia, the boulder-beds rest on rocks bearing unmistakable signs of the grinding action of ice. There can be no reasonable doubt that the huge continental area of which India, South Africa, Glossopteris (Southern Flora). = Northern Flora. a development in northern latitudes. There is, moreover, another consideration, and that is the effect on the vegetation of an enormous continental mass; in North America and Europe it is probable that the forests grew on low-lying land penetrated by lagoons and in part submerged under shallow brackish water, a disposition of land and sea very different from that in the so-called Gondwana Land of the South. Possibly the apparently uniform vegetation of the Devonian and Lower Carboniferous period was unable, through stress of climatal conditions, to prolong its existence in the southern area, while in the north it continued to flourish, and as the evolution of new types proceeded in rapid succession it was not slow to colonise new areas stretching in South America and South Africa to the confines of the Glossopteris flora. There seems good reason for assuming that the Glossopteris flora originated in the South and before the close of the Permian period, as well as in the succeeding Triassic era, pushed northward over a portion of the area previously occupied by the northern flora. This northward extension is shown by the existence of Glossopteris in Upper Permian rocks of Russia, by the occurrence of several southern types in plant-bearing beds of the Altai mountains, and by the existence in Western Europe during the early stages of the Triassic era of such southern genera as Neuropteridium and Schizoneura. Triassic, Jurassic, and Wealden Floras. It is unfortunate that the records of plant-life towards the close of the Paleozoic and during the succeeding Triassic period are very fragmentary; the documents are few in number, and instead of the fairly continuous chapters in which the records of the Coal age have been preserved, we have to be content with a few blurred pages. During the Triassic period the vegetation of the world gradually changed its character; the balance of power was shifted from the Vascular Cryptogams, the dominant group of the Paleozoic era, to the Gymnosperms. It is not until we pass up the geologic series as far as the Rhætic formation, that we come to palæobotanical records at all comparable in their completeness with those of the Permo-Carboniferous era; but before considering the Rhætic vegetation we must glance at such scattered relics as remain of the vegetation belonging to the period of transition between the Paleozoic and Mesozoic facies. It is regrettable that this transitional period is unusually poor in documentary evidence that might throw light on the gradual change in the facies of Paleozoic vegetation. The new order, when once established, persisted for many succeeding ages without undergoing any essential alteration. One of the few floras of early Triassic age of which satisfactory relics have been preserved is that described in 1844 by Schimper and Mougeot from the Bunter Sandstones of the Vosges. The genus Neuropteridium, a plant which may be a true fern, or possibly a surviving member of the Cycadofilices, is represented by a species which can hardly be distinguished from that which flourished in South America, South Africa, and India in the Permo-Carboniferous period. This genus and another southern type, Schizoneura, both of which are met with in the Triassic rocks of the Vosges, would seem to point to a northern migration of certain members of the Glossopteris flora, which took place at the close of the Paleozoic era. In the Lower Triassic flora Conifers are relatively more abundant than in the earlier periods; such genera as Albertia (resembling in its vegetative features some recent species of Araucaria), Voltzia (with cones that cannot be closely matched with those of any existing members of the Conifera), and other representatives of this class are common fossils. Lepidodendra have apparently ceased to exist; Sigillaria may be said to survive in one somewhat doubtful form, Sigillaria oculina. The genus Pleuromeia, which makes its appearance in Triassic rocks, is known only in the form of casts exhibiting a strong likeness to some Paleozoic Lycopods, and is perhaps more akin to Isoetes than to any other existing plant. The Calamites are now replaced by large Equisetaceous plants, which are best described as Horsetails with much thicker stems than those of their modern descendants. From Recoaro in Northern Italy some of the Vosges genera have been recorded, and a few other European localities have furnished similar relics of a Triassic vegetation. Passing to the peninsula of India, we find the genus Glossopteris abundantly represented in strata which there is good reason for regarding as homotaxial with the European Trias, and the occurrence in the same beds of some other genera of Permo-Carboniferous age shows that the change in the character of the southern vegetation at the close of the Paleozoic era was much more gradual than in the north. The comparative abundance of plant remains in the northern hemisphere in rocks belonging to the Rhætic formation, a series of sediments so named from their development in the Rhætian Alps, is in welcome contrast to the paucity of the records from the underlying Triassic strata. From Virginia and adjacent districts in the United States a rich flora has been described, which by some authors is assigned to the Keuper or Upper Triassic series, while others class it as Rhætic. A similar assemblage of plants is known also from the Lettenkohle beds of Austria, which, as Stur has shown, clearly belong to the same period of vegetation as the American flora. We need not, however, concern ourselves with discussions as to the precise stratigraphical position of these American and European plant-beds, but may conveniently group together floras of Upper Triassic and Rhætic age since they exhibit but minor differences from one another. Plants of Upper Triassic or Rhætic age are known from Scania and Franconia in Europe, Virginia and elsewhere in North America, Honduras, Tonkin, Australia, South Africa, Chili, and other parts of the world. The geographical distribution of plants of approximately Rhætic age is shown in the following table on p. 838, which demonstrates an almost worldwide range of a vegetation of uniform character. The character of the plant-world is entirely different from that which we have described in speaking of the Paleozoic floras. Gymnosperms have ousted Vascular Cryptogams from their position of superiority; ferns, indeed, are still very abundant, but they have undergone many and striking changes, notably in the much smaller representation of the Marattiaceæ. The Palæozoic Lycopods and Calamites have gone, and in their place we have a wealth of Cycadean and Coniferous types. As we ascend to the Jurassic plantbeds the change in the vegetation is comparatively slight, and the same persistence of a well-marked type of vegetation extends into the Wealden period. It is a remarkable fact that after the Paleozoic floras had been replaced by those of the Mesozoic era, the vegetation maintained a striking uniformity of character, from the close of the Triassic up to the dawn of the Cretaceous era. This statement is open to misconception; I do not wish to convey the idea that a palæobotanist would be unable to discriminate between floras from Rhætic and Wealden rocks; but I wish to emphasise the fact that in spite of specific, and to a less extent of generic, peculiarities, which enable us to determine, within narrow limits, the age of a Mesozoic flora, the main features of the vegetation remained the same through a long succession of ages. The accompanying tables (pp. 839, 840) illustrate the geographical distribution of some of the leading types of Mesozoic plants during the Jurassic and Wealden periods, and demonstrate not only the striking differences between the Mesozoic and Paleozoic floras, but also the much greater uniformity in the vegetation of the world during the Secondary era than in the preceding Permo-Carboniferous epoch. Mesozoic Floras. It may be of interest to glance at some of the leading types of Mesozoic floras with a view to comparing them with their modern representatives. We are so familiar with the present position of the flowering plants in the vegetation of the world, that it is difficult for us to form a conception of a state of things in the history of the plant-kingdom in which Angiosperms had no part. a. Conifers. How may we describe the characteristic features of Rhætic and Jurassic floras? Gymnosperms, so far as we know, marked the highest level of plantevolution. Conifers were abundant, but the majority were not members of that group to which the best known and most widely distributed modern forms belong. |