As we analyse more carefully the Triassic-Rhaetic floras of the world we shall no doubt find evidence of regional differences com parable, though less pronounced, to those in the vegetation at the present day. The close correspondence between the floras of east Greenland and the province of Scania (southern Sweden) is a re markable fact. When we think of the enormous difference be tween the present north Temperate vegetation of Sweden and the treeless and stunted Arctic flora it is very difficult to understand the extraordinary resemblance presented by the two fossil floras, not only in the number and variety of the plants but also in the size of the vegetative organs. It is very unlikely, so astronomers tell us, that the earth's axis has changed its position, or at least to an extent that would make any appreciable difference to cli matic conditions during that portion of geological history with which we are concerned. A possible explanation is offered by the Wegener hypothesis; in the Rhaetic period Greenland may have occupied a position relative to Europe and to the north pole dif ferent from that which it occupies to-day. There seems to be no satisfactory solution of this and other climatic problems raised by palaeobotanical researches if we reject, as we probably must, the suggestion that the axis of the earth has altered its position, and if we follow some of Wegener's critics and reject also the hypothesis which assumes changes in the relative positions of portions of the earth's crust.
There are few more attractive problems than those raised by the discovery of comparatively luxuriant floras in the Arctic re gions. It is clearly impossible to make any definite statement of scientific value as to the temperature necessary for the existence of a vegetation composed of extinct species.. We are apt to as sume that a plant from Rhaetic or other strata, if it bears a re semblance to one which still exists, must have required for its normal development climatic or other conditions approximately the same as those prevailing in the localities of its modern rep resentative. Such an assumption might be reasonable if based on a comparison of an assemblage of many extinct and recent forms, but it must be remembered that living plants closely related to one another are often able to exist in places with sharply con trasted temperatures. Making due allowance for the adaptability of closely allied species to widely different conditions, and ad mitting the danger of applying knowledge derived from observa tions on living plants to species which became extinct many mil lion years ago, there remains the problem of accounting for the occurrence in southern Sweden and in eastern Greenland of floras that are in general terms identical.
What then are the salient features of the Arctic Rhaetic flora? Among the Equisetales are species of Neocalamites in no way in ferior to those from Queensland, Tongking, South Africa and else where. The discovery of a large Lycopodiaceous cone, Lycostro
bus Scotti, in the Rhaetic flora of Sweden and the subsequent recognition of spores in the Greenland beds, which are almost cer tainly referable to Lycostrobus, affords one of the few instances of the survival into the Rhaetic period of a plant bearing a strik ing resemblance to the Lepidodendra of the Carboniferous for ests. Ferns are represented by large fronds of the genus Todites, that are so named from their likeness in form and in the struc ture of the spore-capsules (sporangia) to the living osmundaceous fern Todea barbara of South Africa and New Zealand, and by species with fronds of the type designated as Cladophlebis a genus that was widespread also in the Jurassic period. The oc currence in Greenland of pieces of fronds hardly distinguishable from a species of Gleichenites described from the Lower Jurassic flora of Franconia in Germany, affords some evidence of the ex istence in the Arctic flora of a member of a family that is now very widely spread in the Tropics and in the Cretaceous period was particularly prominent in the vegetation of western Green land. Among other ferns are Matonidium, similar in habit to the living Matonia (fig. 5), Laccopteris, Dictyophyllum, and Haus mannia, genera agreeing in habit and in the structure of the sporangia with what has been called by F. 0. Bower the Matonia Dipteris alliance.
One of the most clearly established facts in the history of ferns is the striking contrast between the present restricted geographi cal range of the two Malayan genera Matonia and Dipteris and the almost world-wide range of Mesozoic ferns, described under sev eral generic names, which are believed on good evidence to be nearly related to one or other of these plants. The two living gen era, figured by A. R. Wallace (fig. 5) side by side on Mt. Ophir in the Malay Peninsula, may be regarded as relics of a remote past, survivors of a line of ancient lineage, which after wandering over the world from Arctic lands to the far south and east per sist as impressive links with a vanished world.
Reference has already been made to the genus Thinnfeldia; the fronds of this plant and those of a somewhat similar genus Lepidopteris are widely scattered Rhaetic types. Lepidopteris, bearing small, thick leaflets with rounded tips, is one of the Rhae tic genera which though fern-like in habit is probably a Pterido sperm : it is recorded from Greenland, Sweden, Germany, Poland and Madagascar. Thinnfeldia and Lepidopteris differ from ferns in the more leathery texture of the leaflets, which are more re sistant than those of ferns to certain chemical reagents, and by the absence of any sporangia of the typical filicean type. Both may be Pteridosperms, anachronisms in the Mesozoic floras.