THE main factors influencing plant distribution over long periods of time are first climatic change and secondly those movements of the earth’s crust which result in mountain building or in extensive alterations in land and sea areas. (Mountain ranges are often more effective barriers to plant migration than arms of the sea.) There are, of course, other minor influences, such as the relation of the plants to a changing animal population. The interaction of all these factors,
extinguishing some species, driving others from their original habitats, or leaving tracts of country open for re-colonisation, has gradually led to the establishment of the present floral regions, and to the apparent anomalies of discontinuous distribution.
A combined study of the geology and of the numerous relics of fossil floras enables us to trace in a general way the changes which have taken place throughout the Tertiary period—at any rate in the northern hemisphere, where the fossils have been more adequately studied. In early Tertiary times the flora of what is now the north-temperate and cir-cumpolar region seems to have been fairly uniform in general character, and until after the close of the Miocene the vegetation changed but slowly. The oncoming of the Ice Age drove many plants southwards, and in Europe and western Asia the east-and-west mountain ranges which had meanwhile been uplifted effectively barred their escape to warmer latitudes. In North America and eastern Asia, however, the north-and-south direction of the main ranges made no such barrier; hence the survival in Central America and the Sino-Malayan region of many Tertiary relics. In warmer post-glacial times some of these exiles were able to spread northwards; this is especially noticeable in eastern America, but in the western part of that continent the uplift of the Rocky Mountains had so changed the climate byoff moisture-bearing winds that the region was no longer suitable for the ‘Miocene ‘flora.
PLANTS DRIVEN FROM EUROPE BY THE COLD IN Europe many of the Tertiary species were extinguished because they had no southward refuge from the cold, nor could there be any later northward re-colonization. Consequently the present European flora is comparatively poor in species; thus, plants like Azolla (a tiny floating water-fern) and Brasenia (a water-lily) are now native in every continent except Europe, though both of them were abundant in Oligo-cene beds of the Isle of Wight, for example. Nevertheless, the present climate of Europe is perfectly favourable to many of the missing forms; Azolla, reintroduced by man after the lapse of ages, flourishes once more in theand streams of southern England, and to look through a nurseryman’s catalogue of trees and shrubs is to find a whole group of plants, such as Gingko, Abelia, Actinidia, Dipelta and Koelreuteria and many another, all of which have been introduced into our gardens from eastern Asia, and all of which lived in Europe during the Tertiary period.