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Rh preclude the possibility of solving difficulties in geographical distribution by the construction of hypothetical land-surfaces, an expedient which Darwin always stoutly opposed (Life and Letters, ii. 74–78) The furrowed surface of the earth gives the land-area a star-shaped figure, which may from time to time have varied in outline, but in the main has been permanent. It is excentric as regards the pole and sends tapering extensions towards the south. Sir George Darwin finds a possible explanation of these in the screwing motion which the earth would suffer in its plastic state. The polar regions travelled a little from west to east relatively to the equatorial, which lagged behind.

The great primary divisions of the earth’s flora present themselves at a glance. The tropics of Cancer and Capricorn cut off with surprising precision (the latter somewhat less so) the tropical from the north and south temperate zones. The north temperate region is more sharply separated from the other two than the south temperate region from the tropical.

I. (Holarctic).—This is the largest of all, circumpolar, and but for the break at Bering Straits, would be, as it has been in the past, continuous in both the old and new worlds. It is characterized by its needle-leaved Coniferae, its catkin-bearing (Amentaceae) and other trees, deciduous in winter, and its profusion of herbaceous species.

II. .—This occupies widely separated areas in South Africa, Australia, New Zealand and South America. These are connected by the presence of peculiar types, Proteaceae, Restiaceae, Rutaceae, &c., mostly shrubby in habit and on the whole somewhat intolerant of a moist climate. Individual species are extremely numerous and often very restricted in area.

III. .—This is characterized by the presence of gigantic Monocotyledons, palms, Musaceae and bamboos, and of evergreen polypetalous trees and figs. Herbaceous plants are rare and mostly epiphytic.

A consideration of these regions makes it apparent that they are to a large extent adaptive. The boreal is cold, the austral warm, and the tropical affords conditions of heat and moisture to which the vegetation of the others would be intolerant. If we take with Drude the number of known families of flowering plants at 240, 92 are generally dispersed, 17 are more restricted, while the remainder are either dominant in or peculiar to separate regions. Of these 40 are boreal, 22 austral and 69 tropical. If we add to the latter figure the families which are widely dispersed, we find that the tropics possess 161 or almost exactly two-thirds of the large groups comprised in the world’s vegetation. M. Casimir de Candolle has made an independent investigation, based on Hooker and Bentham’s Genera plantarum. The result is unfortunately (1910) unpublished, but he informs the present writer that the result leads to the striking conclusion: “La végétation est un phénomène surtout intertropical, dont nous ne voyons plus que restes affaiblis dans nos régions tempérées.” In attempting to account for the distribution of existing vegetation we must take into account palaeontological evidence. The results arrived at may be read as a sequel to the article on.

The vegetation of the Palaeozoic era, till towards its close, was apparently remarkably homogeneous all over the world. It was characterized by arborescent vascular Cryptogams and Gymnosperms of a type (Cordaiteae) which have left no descendants beyond it. In the southern hemisphere the Palaeozoic flora appears ultimately to have been profoundly modified by a lowering of temperature and the existence of glacial conditions over a wide area. It was replaced by the Glossopteris flora which is assumed to have originated in a vast continental area (Gondwana land), of which remnants remain in South America, South Africa and Australia.

The Glossopteris flora gradually spread to the northern hemisphere and intermingled with the later Palaeozoic flora which still persisted. Both were in turn replaced by the Lower Mesozoic flora, which again is thought to have had its birth in the hypothetical Gondwana land, and in which Gymnosperms played the leading part formerly taken by vascular Cryptogams. The abundance of Cycadean plants is one of its most striking features. They attained the highest degree of structural complexity in the Bennettiteae, which have been thought even to foreshadow a floral organization. Though now on the way to extinction, Cycadeae are still widely represented in the southern hemisphere by genera which, however, have no counterpart in the Mesozoic era. Amongst Conifers the archaic genera, Ginkgo and Araucaria still persist. Once widely distributed in the Jurassic period throughout the world, they are now dying out: the former is represented by the solitary maiden-hair tree of China and Japan; the latter by some ten species confined to the southern hemisphere, once perhaps their original home. With them may be associated the anomalous Sciadopitys of Japan.

So far the evolution of the vegetable kingdom has proceeded without any conspicuous break. Successive types have arisen in ascending sequence, taken the lead, and in turn given way to others. But

the later period of the Mesozoic era saw the almost sudden advent of a fully developed angiospermous vegetation which rapidly occupied the earth’s surface, and which it is not easy to link on with any that preceded it. The closed ovary implies a mode of fertilization which is profoundly different, and which was probably correlated with a simultaneous development of insect life. This was accompanied by a vegetative organization of which there is no obvious foreshadowing. 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.” The field of evolution has now been transferred to the northern hemisphere. Though Angiosperms become dominant in all known plant-bearing Upper Cretaceous deposits, their origin dates even earlier. In Europe Heer’s Populus primaeva from the Lower Cretaceous in Greenland was long accepted as the oldest dicotyledonous plant. Other undoubted Dicotyledons, though of uncertain affinity, of similar age have now been detected in North America. The Cenomanian rocks of Bohemia have yielded remains of a sub-tropical flora which has been compared with that existing at present in Australia. Upper Cretaceous formations in America have yielded a copious flora of a warm-temperate climate from which it is evident that at least the generic types of numerous not closely related existing dicotyledonous trees had already come into existence. It may be admitted that the identification of fragmentary leaf-remains is at most precarious. Even, however, with this reservation, it is difficult to resist the mass of evidence as a whole. And it is a plausible conjecture that the vegetation of the globe had already in its main features been constituted at this period much as it exists at the present moment. There were oaks, beeches (scarcely distinguishable from existing species), birches, planes and willows (one closely related to the living Salix candida), laurels, represented by Sassafras and Cinnamomum, magnolias and tulip trees (Liriodendron), myrtles, Liquidambar, Diospyros and ivy. This is a flora which, thinned out by losses, practically exists to this day in the southern United States. And one essentially similar but adapted to slightly cooler conditions existed as far north as the latitude of Greenland.

The tertiary era opens with a climate in which during the Eocene period something like existing tropical conditions must have obtained in the northern hemisphere. The remains of palms (Sabal and Nipa) as well as of other large-leaved Monocotyledons are preserved. Sequoia (which had already appeared in the American Upper Cretaceous) and the deciduous cypress (Taxodium distichum) are found in Europe. 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. That of Grinnell Land in lat. 81° consisted of Conifers (including the living spruce), poplars and willows, such as would be found now 25° to the south. The flora of Disco Island in lat. 70° contained Sequoia, planes, maples and magnolias and closely agrees with the Miocene flora of central Europe. Of this copious remains have been found in Switzerland and have been investigated with great ability by O. Heer. They point to cooler conditions in the northern hemisphere: palms and tropical types diminish; deciduous trees increase. Sequoia and the tulip-tree still remain; figs are abundant; laurels are represented by Sassafras and camphor; herbaceous plants (Ranunculaceae, Cruciferae, Umbelliferae) are present, though, as might be expected, only fragmentarily preserved.

We may draw with some certainty the conclusion that a general movement southward of vegetation had been brought about. While Europe and probably North America were occupied by a warm temperate flora, tropical types had been driven southward, while the adaptation of others to arctic conditions had become accentuated. A gradual refrigeration proceeded through the Pliocene period. This was accompanied in Europe by a drastic weeding out of Miocene types, ultimately leaving the flora pretty much as it now exists. This, as will be explained, did not take place to anything like the same extent in North America, the vegetation of which still preserves a more Miocene facies. Torreya, now confined to North America and Japan, still lingered, as did Ocotea, now profusely developed in the tropics, but in north temperate regions only existing in the Canaries; the evergreen oaks, so characteristic of the Miocene, were reduced to the existing Quercus ilex. At the close of the Pliocene the European flora was apparently little different from that now existing, though some warmer types such as the water chestnut (Trapa natans) had a more northern extension. The glacial period effected in Europe a wholesale extermination of temperate types accompanied by a southern extension of the arctic flora. But its operation was in great measure local. The Pliocene flora found refuges in favoured localities from which at its close the lowlands were restocked while the arctic plants were left behind on the mountains. During the milder interglacial period some southern types, such as Rhododendron ponticum, still held their own, but ultimately succumbed.

The evidence which has thus been briefly summarized, points unmistakably to the conclusion that existing vegetation originated in the northern hemisphere and under climatic conditions corresponding to what would now be termed sub-tropical. It occupied a continuous circumpolar area which allowed free communication between the old and new worlds. The gradual differentiation of their floras