species of Pinus, and Miss Stopes and Dr Fujii have made important
contributions on the structure of Cretaceous plants from
Japan. Cones of Lower Cretaceous age have been described by
Fliche from Argonne, which bear a close resemblance to the female
flowers of recent species of Cedrus. The two surviving species of
Sequoia afford an illustration of the persistence of an old type, but
unfortunately most of the Mesozoic species referred to this genus
do not possess sufficiently perfect cones to confirm their identification
as examples of Sequoia. Some of the best examples of cones
and twigs referred to Sequoia are those described by Heer from
Cretaceous rocks of Greenland, and Professor D. P. Penhallow of
Montreal has described the anatomical structure of the stem of
Sequoia Langsdorfii, a Tertiary species occurring in Europe and
North America.
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. The imperfection of the geological record, considered from the point of view of evolution, has been rendered familiar by Darwin’s remarkable chapter in the Origin of Species. Breaks in the chain of life, as represented by gaps in the blurred and incomplete documents afforded by fragmentary fossils, are a necessary consequence of the general plan of geological evolution; they mark missing chapters rather than sudden breaks in an evolutionary series. On the other hand, a study of the plant-life of past ages tends to the conviction that too much stress may be laid on the imperfection of the geological record as a factor in the interpretation of palaeontological data. The doctrine of Uniformitarianism, as propounded by Lyell, served to establish geology on a firmer and more rational basis than it had previously possessed; but latterly the tendency has been to modify the Lyellian view by an admission of the probability of a more intense action of groups of forces at certain stages of the earth’s history. As a definite instance a short review may be given of the evidence of palaeobotanical records as regards their bearing on plant-evolution. Starting with the Permo-Carboniferous vegetation, and omitting for the moment the Glossopteris flora, we find a comparatively homogeneous flora of wide geographical range, consisting to a large extent of arborescent lycopods, calamites, and other vascular cryptogams, plants which occupied a place comparable with that of Gymnosperms and Angiosperms in our modern forests; with these were other types of the greatest phylogenetic importance, which serve as finger-posts pointing to lines of evolution of which we have but the faintest signs among existing plants. Other types, again, which may be referred to the Gymnosperms, played a not unimportant part in the Palaeozoic vegetation. No conclusive proof has so far been adduced of the existence in those days of the Cycads, nor is there more than partial evidence of the occurrence of genera which can be placed with confidence in any of the existing families of Conifers. There are, moreover, no facts furnished by fossil plants in support of the view that Angiosperms were represented either in the low-lying forests or on the slopes of the mountains of the Coal period. Passing higher up the geological series, we find but scanty records of the vegetation that existed during the closing ages of the Permian period, and of the plants which witnessed the beginning of the Triassic period we have to be content with the most fragmentary relics. 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. Arborescent Pteridophytes are barely represented, and such dominant types as Lepidodendron, Sigillaria, Calamites and Sphenophyllum have practically ceased to exist; Cycads and Conifers have assumed the leading role, and the still luxuriant fern vegetation has put on a different aspect. This description applies almost equally to the floras of the succeeding Jurassic and Wealden periods. The change to this newer type of vegetation was no doubt less sudden than it appears as read from palaeobotanical records, but the transition period between the Palaeozoic type of vegetation and that which flourished in the Lower Mesozoic era, and continued to the close of the Wealden age, was probably characterized by rapid or almost sudden changes. In the southern hemisphere the Glossopteris flora succeeded a Lower Carboniferous vegetation with a rapidity similar to that which marked the passage in the north from Palaeozoic to Mesozoic floras. This apparently rapid alteration in the character of the southern vegetation took place at an earlier period than that which witnessed the transformation in the northern hemisphere. The appearance of a new type of vegetation in India and the southern hemisphere was probably connected with a widespread lowering of temperature, to which reference has already been made. It was from this Glossopteris flora that several types gradually migrated across the equator, where they formed part of the vegetation of more northern regions. The difference between the Glossopteris flora and those which have left traces in the Upper Gondwana rocks of India, in the Wianamatta and Hawkesbury beds of Australia, and in the Stormberg series of South Africa is much less marked than that between the Permo-Carboniferous flora of the northern hemisphere and the succeeding Mesozoic vegetation. In other words, the change took place at an earlier period in the south than in the north. To return to the northern hemisphere, it is clear that the Wealden flora, as represented by plants recorded from England, France, Belgium, Portugal, Russia, Germany and other European regions, as also from Japan and elsewhere, carries on, with minor differences, the facies of the older Jurassic floras. It was at the close of the Wealden period that a second evolutionary wave swept over the vegetation of the world. This change is most strikingly illustrated by the inrush of Angiosperms, in the equally marked decrease in the Cycads, and in the altered character of the ferns. It would appear that in this case the new influence, supplied by the advent of Angiosperms, had its origin in the north. 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. As to the nature of the chief factors concerned in the two revolutions in the vegetable kingdom, if it is admissible to use so strong a term, only a guess can be hazarded. Physical conditions no doubt played an important part, but whatever cause may have had the greatest share in disturbing the equilibrium of evolutionary forces, it would seem that the apparently sudden appearance of Cycads and other types at the close of the Palaeozoic period made a widespread and sudden impression on the whole character of the vegetation. At a later stage—in post-Wealden days—it was the appearance of Angiosperms, probably in northern latitudes, that formed the chief motive power in accelerating the transition in the facies of plant-life from that which marked what we have called the Mesozoic floras, to the vegetation of the Upper Cretaceous and Tertiary periods. With the advent of Angiosperms began, as the late marquis of Saporta expressed it, “Une révolution, ainsi rapide dans sa marche qu’universelle dans ses effets.” 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.
Authorities.—Glossopteris Flora: Blanford, H. F., “On the age and correlation of the Plant-bearing Series of India, &c.,” Quarterly Journal Geol. Soc. xxxi. (1875); Feistmantel, “Fossil Flora of the Gondwana System,” Mem. Geol. Surv. India, vols. iii., &c. (1879, &c.); Seward, Fossil Plants as Tests of Climate (Cambridge, 1892), with bibliography; “The Glossopteris Flora,” Science Progress, with bibliography; “On the Association of Sigillaria and Glossopteris in South Africa,” Q.J.G.S., vol. liii. (1897); E. A. N. Arber, Catalogue of the Fossil Plants of the Glossopteris Flora in the Department of Geology (British Museum, Nat. Hist., Brit. Mus. Catalogue (London, 1905), with full bibliography; Medlicott and Blanford, Manual of the Geology of India (2nd ed., Oldham, R. D., Calcutta, 1893); David, “Evidences of Glacial Action in Australia in Permo-Carboniferous time,” Q.J.G.S., vol. lii. (1896); Zeiller, Éléments de paléobotanique (Paris, 1900); Potonié, “Fossile Pflanzen
