As a step towards such hypothesis it has been noted that the Antarctic, the South African, and the Australian floras have many types in common.
To quote further: Existing floras exhibit only one moment in the history of the earths vegetation.
The remark may conveniently find its place here that plants which have reached a high degree of adaptive specialization have come to the end of their tether: a too complicated adjustment has deprived them of the elasticity which would enable them to adapt themselves to any further change in their surroundings, and they would pass away with conditions with which they are too inextricably bound up. Vast floras have doubtless thus found their grave in geologic change.
As Clement Reid remarked: World-wide floras, such as seem to characterize some of the older periods, have ceased to be, and plants are distributed more markedly according to geographical provinces and in climatic zones.
Starkie Gardner has argued with much plausibility that the Tertiary floras which have been found in the far north must have been of Eocene age.
Thus comparing the Nearctic and Palaearctic floras we find striking differences overlying the points of agreement already indicated.
A detailed examination of mountain floras shows that a large local element is present in each besides the arctic. The one is in tact the result of similar physical conditions to that which has produced the other.
At the same time the then existing alpine floras descended to lower levels, though we may agree with Ball that they did not necessarily become extinct at higher ones as long as any land-surface remained uncovered by ice.
At the close of the glacial period the alpine floras retreated to the mountains accompanied by an arctic contingent, though doubtless many species of the latter, such as Salix polaris, failed to establish themselves.
High mountain levels supplied paths of communication for stocking the South Temperate region, the floras of which were enriched by adapted forms of tropical types.
This has brought about superficial resemblance in the floras of different countries.
Oceanic islands have, as a rule, distinctive faunas and floras which resemble, but are not identical with, those of other islands in similar positions.
Their flora is far closer akin to the floras of N.
In central Russia the species become still more numerous, and, though the local floras are not yet complete, they number 850 to 1050 species in the separate governments, and about 1600 in the best explored parts of the S.W.
The central region is a transition ground where these floras find representation generally in deteriorated and dwarfed species.
The question of the mode in which the floras of islands and of continents have been formed gave rise to important speculations by such eminent botanical travellers as Charles Darwin, Sir J.
Hooker, A Lecture on Insular Floras (London, 1868); E.
As for the explanation of the community between the alpine and arctic floras, all authorities are agreed that the key to the problem is furnished by the occurrence of the glacial period.
In the ice-free belt, between the northern ice-sheet and the vastly extendedglaciers of the Alps, the two floras must have found a common refuge and congenial conditions of existence; and this view is confirmed by direct palaeontological evidence.
In many parts of the world there is no sharp line of demarcation between the Devonian and the Carboniferous rocks; neither can the fossil faunas and floras be clearly separated at any well-defined line; this is true in Britain, Belgium, Russia, Westphalia and parts of North America.
The groups of organisms utilized for zoning and correlation by different workers include brachiopods, pelecypods, cephalopods, corals, fishes and plants; and the results of the comparison of the faunas and floras of different areas where Carboniferous rocks occur are generalized in the table below.
But even within the limits of the Lower Gondwana series there are great diversities of vegetation, three distinct floras occurring in the three great divisions of that formation.
Although during the Gondwana period the flora of India differed greatly from that of Europe, it was strikingly similar to the contemporaneous floras of South America, South Africa and Australia.
Its vegetation is in point of fact of a composite character, and is constituted by the meeting and more or less blending of adjoining floras, - those of Persia and the south-eastern Mediterranean area to the north-west, of Siberia to the north, of China to the east, and of Malaya to the south-east.
At the two ends of Cajon Pass, only four or five kilometres apart, are the two utterly distinct floras of the Mohave desert and the San Bernardino valley.
He was distinguished for his researches on the Tertiary floras of various parts of Europe, and on the fossil floras of Australia and New Zealand.
The separation of the coast and interior floras is almost complete; only along the mountain passes and river valleys, and rarely there, is there an exchange of species.
Leguminosae, the two most numerous orders of phanerogams, but in number of individual plants it probably far exceeds either; whilst from the wide extension of many of its species, the proportion of Gramineae to other orders in the various floras of the world is much higher than its number of species would lead one to expect.
See also accounts of the family in the various great floras, such as Ascherson and Graebner, Synopsis der mitteleuropdischen Flora; N.
There is, however, abundant evidence that the Ferns were represented in the most ancient floras known, though they were not such a dominant group as has hitherto been supposed.
It has floras of the plains, the hills and the mountains; an alpine flora, and an arctic flora; a flora of marshes, and a flora of steppes; floras peculiar to the clay, the chalk, the sandstone and the slate formations.
The various groups of plants represented in the Palaeozoic rocks will first be considered in systematic order, after which some account will be given of the succession and distribution of the various floras during the period.
Our knowledge of vegetation older than the Carboniferous is still far too scanty for any satisfactory history of the Palaeozoic Floras to be even attempted; a few, however, of the facts may be advantageously recapitulated in chronological order.
The Mesozoic era, as defined in geological textbooks, includes the Triassic, Jurassic and Cretaceous epochs; but from the point of view of the evolution of plants and the succession of floras, this division is not the most natural or most convenient.
Our aim is not simply to give a summary of the most striking botanical features of the several floras that have left traces in the sedimentary rocks, but rather to attempt to follow the different phases in the development of the vegetation of the world, as expressed in the contrasts exhibited by a comparison of the vegetation of the Coal period forests with that of the succeeding Mesozoic era up to the close of the Wealden period.
We must begin by briefly considering this southern Palaeozoic province if we would trace the Mesozoic floras to their origin, and obtain a connected view of the vegetation of the globe as it existed in late Palaeozoic times and at the beginning of the succeeding era.
While a few similar or even identical types may be recognized in both floras, there can be no doubt that, during a considerable period subsequent to that represented by the Lower Carboniferous or Culm rocks, there existed two distinct floras, one of which had its headquarters in the northern hemisphere, while the other flourished in a vast continental area in the south.
Recent discoveries have shown that representatives of the two floras coexisted in certain regions; there was, in fact, a dovetailing between strata in Europe.
The Coal-bearing strata which occupy a considerable area in China (Map A, II.), contain abundant samples of a vegetation which appears to have agreed in their main features with the Permo-Carboniferous floras of the northern hemisphere.
Evidence of the same northern extension is supplied by floras described by Schmalhausen from Permian rocks in the Pechora valley (Map A, VI.), the Siberian genus Rhiptozamites being very similar to, and probably generically identical with, Naeggerathiopsis of the Glossopteris flora.
The Permo - Carboniferous beds of South Africa, India and Australia are succeeded by other plant-bearing strata, containing numerous species agreeing closely with members of the Rhaetic and Jurassic floras of the northern hemisphere.
These post-Permian floras, as represented by the Upper Gondwana beds of India and corresponding strata in Australia, South Africa, and South America, differ but slightly from the northern floras, and point to a uniformity in the Rhaetic and Jurassic vegetation which is in contrast to the existence of two botanical provinces during the latter part of the Palaeozoic period.
Having seen how the Glossopteris flora of the south gradually spread to the north in the Permian period, we may now take a brief survey of the succession of floras in the northern hemisphere, which have left traces in Mesozoic rocks of North America, Europe and Asia.
There is, however, a marked difference, as regards the floras as a whole, between the uppermost Palaeozoic flora of the northern hemisphere and such species as have been recorded from Lower Triassic beds.
There is evidence of a distinct break in the succession of the northern floras which is not apparent between the Permian and Trias floras of the south.
Representatives of the Ginkgoales constitute characteristic members of the later Triassic floras, and these, with other types, carry us on without any break in continuity to the Rhaetic floras of Scania, Germany, Asia, Chile, Tonkin and Honduras (Map A, VIII.), and to the Jurassic and Wealden floras of many regions in both the north and south hemispheres.
From the close of the Permian period, which marks the limit of the Upper Palaeozoic floras, to the .period immediately preceding the apparently sudden appearance of Angiosperms, we have a succession of floras differing from one another in certain minor details, but linked together by the possession of many characters in common.
There is no doubt that the Cycadophyta, using the term suggested by Nathorst in 1902, was represented in the Mesozoic period by several distinct families or classes which played a dominant part in the floras of the world before the advent of the Angiosperms. In addition to the bisporangiate reproductive shoots of Bennettites, distinguished by many important features from the flowers of recent Cycads, a few specimens of flowers have been discovered exhibiting a much closer resemblance to those of existing Cycads, e.g.
In Rhaetic, Jurassic and Wealden floras, the Ginkgoales were exceedingly abundant (Map B, G i -G 17); in addition to A, Ginkgodium, Japan (Jurassic).
There are a few points suggested by a general survey of the Mesozoic floras, which may be briefly touched on in conclusion.
In following the progress of plant-life through those periods in the history of the earth of which records are left in ancient sediments, seams of coal or old land-surfaces, we recognize at certain stages a want of continuity between the floras of successive ages.
It is in rocks of Upper Triassic and Rhaetic age that abundant remains of rich floras are met with, and an examination of the general features of the vegetation reveals a striking contrast between the Lower Mesozoic plants and those of the Palaeozoic period.
This description applies almost equally to the floras of the succeeding Jurassic and Wealden periods.
Unfortunately, our knowledge of the later floras in the southern hemisphere is very incomplete, but a similar transformation appears to have characterized the vegetation south of the equator.
At a later stage - in postWealden days - it was the appearance of Angiosperms, probably in northern latitudes, that formed the chief motive power in accelerating the transition in the fades of plant-life from that which marked what we have called the Mesozoic floras, to the vegetation of the Upper Cretaceous and Tertiary periods.
From the floras of the Tertiary age we pass by gradual stages to those which characterize the present phase of evolutionary progress.
Among modern floras we find here and there isolated types, such as Ginkgo, Sequoia, Matonia, Dipteris and the Cycads, persisting as more successful survivals which have held their own through the course of ages; these plants remain as vestiges from a remote past, and as links connecting the vegetation of to-day with that of the Mesozoic era.
Ward, " Status of the Mesozoic Floras of the United States," Twentieth Ann.
From Cretaceous times onwards local distribution may change; yet the successive floras can be analysed in the same way as, and compared with, the living floras of different regions.
World-wide floras, such as seem to characterize some of the older periods, have ceased to be, and plants are distributed more markedly according to geographical provinces and in climatic zones.
This being the case, it will be most convenient to discuss the Tertiary floras in successive order of appearance, since the main interest no longer lies in the occurrence of strange extinct plants or of transitional forms connecting orders now completely isolated.
Therefore, without a knowledge of the physical geography of any particular period, we cannot know whether like or unlike floras might be expected in neighbouring areas during that period.
If, however, we discover plantbearing strata interstratified with deposits containing marine fossils, we can fix the period to which the plants belong, and may be able to correlate them in distinct areas, even though the floras be unlike.