TERTIARY PLANTS IN RELATION TO WORLD HISTORY Summary.—The Tertiary floras of Europe, America and the Arctic, and the living floras of the Far East and North America, are all intimately related. What little is known of the Tertiary floras of Russia and Asia indicates that they, too, were related. It follows that Tertiary floras of an east Asian-American alliance lived on to form the source of much of the living floras of east Asia and America, but were killed in Eurasia. In those regions where they lived, there are no transcontinental mountain-chains forming barriers between the Pole and the Equator, whereas in the regions where they died, there are. Further, the mountain chains were being formed whilst the extermination was in progress, and the extermination was greatest after they were formed. Hence it may be inferred that the mountain-chains helped to kill. There is another set of facts which points to the same conclusion. In the early Tertiary, the European flora showed considerable alliance with Indian and African plants, indicating passage between these regions. Later, at the time when the bar rier was in existence, the alliance died out. Again, therefore, the barrier would seem to have helped to kill. Alliances must indicate either linkages through some common source, or direct linkage. Also, if the allies are far removed, they must indicate migration. The interruption of the alliances by east-west barriers must indicate that the migration was north-south. Finally the presence of ever cooler and cooler forms must indicate that it was from north to south.
Explanation.—The phenomena involved are of world-wide significance, not localized. The best explanation is that originally suggested by Saporta. It meets all the facts, and is based on the assumption that whilst the temperature of, at least, the northern portion of the Northern Hemisphere underwent change, no change, or but very slight, took place in the position of the Poles. As the full measure of the relationship with plants of the Far East has become better known, the evidence supporting this explanation has become more full and definite. The explanation is that in Cretaceous and early Tertiary times the North Polar regions supported a warm type of vegetation, allied to living plants.
In the Cretaceous these were of a sub-tropical or warm-tern perate type; in the Eocene of a cooler type, at which time the flora of Britain and France was tropical. Slowly the climate cooled, and as it did so, plants migrated southward throughout Europe, Asia and North America, their places being taken by others of a cooler type. These new forms must have been evolved in the north since the linkage of floras continued throughout the Tertiary. The migrants must have suffered loss by the way and undergone evolutionary changes even in America and east Asia, for, although allied to one another, the living floras of these regions differ. Some elements were lost here, others there. In Europe and western Asia the whole were ultimately destroyed. With the ever increasing cold behind, and the impassable moun tains in front, they perished, leaving scarcely a trace.
Space will not permit us to enter into any full discussion of the recurrence of Glacial and inter-Glacial periods and the influence they may have had on the flora. It is evident, however, that if climatic alterations, such as those just described, are part of the normal routine that has gone on through all geological periods, and are not merely confined to the latest, then such changes must evidently have had great influence on the evolution and geo graphical distribution both of species and of floras. Whether this was so is a question still to be decided, for in dealing with extinct floras it is difficult to decide, except in the most general way, to what climatic conditions they point. We seem to find indications of long-period climatic oscillations in Tertiary times, but none of the sudden invasion of an Arctic flora, like that which occurred during more recent times.
It might appear from the above that the Eurasian continent was gradually depleted of plants. But this was not so. The flora was greatly impoverished, and has remained so, in marked contrast to that of America, and especially to that of the Far East. But new plants came in to form the living flora of Eurasia. Where they came from is not certain. Some may have come from the north, but many, possibly most, appear to have come from the vast highlands of central Asia. Their history has yet to be discovered.