It is worth while to consider in rather more detail the nature of the vegetation which flourished in Greenland in the earlier stage of the Cretaceous period. We have already commented on the luxuriance of the Rhaetic flora of Scoresby Sound in east Green land ; the Lower Cretaceous flora has been partially reconstructed from specimens obtained from beds on approximately the same latitude (Lat. 70-71° N.) on the opposite coast. Another question of great importance and of no little difficulty is the apparently sudden transformation of the older Mesozoic vegetation, in which flowering plants played little or no part, into what we may call a vegetation that is entirely modern in its dominant characters. The only indication in Rhaetic and Jurassic floras of plants fore shadowing angiosperms in the nature of their reproductive or gans is furnished by the Caytoniales. From Lower Cretaceous rocks in England above the Wealden series Dr. Marie Stopes de scribed some highly specialized types of dicotyledonous wood which cannot be regarded as primitive. It is theref ore certain that angiosperms had reached an advanced state of development at a comparatively early stage in the Cretaceous period.
The salient features of the west Greenland flora may be briefly summarised as follows: Species of Gleichenites were the chief representatives of the ferns ; they agree closely in the forking of the frond axis, in the structure of the spore-capsules, and in the anatomy of the leaf-stalk with existing species of Gleichenia. Among other ferns are Laccopteris, a genus that in the form of the fronds and in the structure of the sporangia bears a striking likeness to Matonia; Hausmannia closely allied to Dipteris; spe cies of Sphenopteris and Cladophlebis which have not as yet been assigned to a definite position in relation to living species. The Cycadophyta are represented by plants bearing leaves having the characters of Pseudocycas, Ptilophyllum and Otozamites genera which played a prominent part in Jurassic floras. The fronds known as Ptilophyllum are characterized by two rows of linear segments giving them an appearance similar to that of the Mexi can cycad Dioon; Otozamites fronds are distinguished by the eared base of the segments which are occasionally relatively broad. Trees with leaves like those of Ginkgo were abundant in the Arctic forests and with them were other members of the Gink goales. Conifers were represented by species believed to be akin to Araucaria and Agathis of the southern hemisphere, by mem bers of the cypress family, relatives of the sequoias of California, at least one species of Abietineae, and by several conifers with leaves resembling in structure those of the umbrella pine (Scia dopitys) of Japan. With these were associated several species of dicotyledons surprisingly modern in the pattern of the foliage : in addition to the genera previously mentioned special attention is drawn to Artocarpus, the tropical bread-fruit tree. The occur rence of leaves agreeing closely in venation with those of the cinnamon tree (Cinnamomum) and of members of the Menis permaceae, a family of flowering plants that is now characteristic of tropical and warm temperate countries, affords further evi dence of unusual climatic conditions in the Arctic regions (Map, fig. 12). We cannot believe that the evolution of the flowering plants was a sudden event. The oldest known angiosperms so far discovered must be the descendants of a line of ancestors stretch ing far back into the earlier stages of the Mesozoic era.
shows that the character of the vegetation was fundamentally changed during the later stages of the Palaeozoic era ; a few of the older types survived, but most of the plants which flourished in the Coal age disappeared. By degrees as the Triassic period advanced, ferns of modern aspect increased rapidly in number and displaced the Pteridosperms from their position of dominance. The seed-bearing fern-like plants were by no means extinguished but they were overshadowed by the gradual rise to power of the Cycadophyta, the Ginkgoales, and other groups. The next great change in the plant-world was at the end of the Jurassic period and at the beginning of the Cretaceous period when the present dominant class, the angiosperms, began to assert its ascendancy and the floras became modified "as by a new creation." It is impossible to follow in detail the progress of evolution during the Cretaceous period as a whole. The important point is that at the beginning of the period flowering plants were small in number or entirely absent ; this is the conclusion based on the very incom plete data at present available. In the Lower Cretaceous floras of Greenland, North America, Bohemia and Sakhalin Island in which dicotyledons first appear in quantity there were also sev eral representatives of gymnosperms and ferns which had sur vived from the Jurassic period. As we ascend to the higher divisions of the Cretaceous period the floras become more modern and the chief differences between them and those of our own day are geographical rather than botanical. The floras of North Amer ica and Europe, for example, agree much more closely with present-day floras in sub-tropical or even tropical lands than with the plant associations which now occupy the territory where the fossils have been found. The problem of climatic change raised by the Cretaceous vegetation of Greenland has still to be solved. On the accompanying map (fig. 12) the northern limit of dis tribution of some of the families and genera is roughly shown. If we assume that the Greenland Cretaceous species grew under conditions similar to those which govern the life of their present day relatives, it follows that the climate must have been at least as warm as that in the Mediterranean region at the present time. It is certain that the Cretaceous vegetation could not have endured the hardships imposed on the present Arctic floras ; but it is by no means certain that extinct species of living genera could not have existed under conditions which would be fatal to their modern descendants. The past is in many respects the key to the present ; but it is dangerous to carry this principle too far.
From the broken fragments of plants preserved in the sedi mentary rocks of the Mesozoic periods we have been able to reconstruct a few of the links in the middle portion of the chain of life and to follow the varying fortunes of certain groups and families through successive ages. The history of the plant-world may be compared with that of human races ; for a time one race holds sway over a widening territory unchecked by serious com petitors until, with apparent suddenness, a more efficient stock asserts itself and the balance of power is disturbed. In the con trast between the vegetation of the Cretaceous period and that characteristic of the earlier stages of the Mesozoic era we have one of the most impressive illustrations of the revolutionary aspect of evolution presented by the records of the rocks