The number of living species is so great, and the required knowledge so detailed, that rarely does a worker attempt more than one branch of the study. All workers have one initial need in common—as complete a collection as possible of those parts of the living plant they wish to study. Ideally, all living species should find place in the collection, and the material should be arranged so as best to suit the particular branch of study. The arrangement of public herbaria is usually not very suitable for palaeobotany. The reason lies in the method of classification of living plants. This is based principally on the arrangement of the floral organs, although other organs play a subsidiary part. In consequence all collections of dried plants, and all books dealing with classification, are arranged on this basis, and afford no ready key to the palaeobotanist, as they do to the botanist. For the former the organs he studies have always to be re-classified. All great herbaria provide abundant material for the study of leaves, but not for that of other organs. Fruits and seeds are badly repre sented, and there is great need for better collecting of these organs.
Either from collections or illustrations, the student must gather all possible material for comparison. But the living material is the most important. He must examine the fossil and living part in detail, using the microscope. Stress is laid on the comparison with living material, for sometimes palaeobotanists have been content to follow the easier course of comparing with previous determinations only. But on the exact comparison with living plants depends our knowledge of the relation of fossil species to living ones.
Scope of the Study of Tertiary Plants.—Tertiary plants furnish some of the later chapters in the history of living plants, and of the world ; the latest of all being found in the Quaternary. They give the most reliable information obtainable about the ancestors of the living forms ; where they lived and what their various organs were like. That is to say, what changes in dis tribution and structure they have undergone, or, on the other hand, what stability they may show. They give also the best in formation obtainable as to past climate.
In the early stages of the period we can trace no living species, but some species are sufficiently close to living ones to be placed in living genera. Others which cannot be placed in living genera may yet be placed in living families. Nevertheless, until the close of the Tertiary period, plants constantly occur whose living rela tions have not been discovered. Whether this is because the plants have become extinct and left no link with the present, or because our knowledge of living plants is inadequate to trace their relations is difficult to tell. Probably both causes operate. It is certain, however, that old forms more remote from the living have yielded place to newer forms closer to the living; so that by the end of the Tertiary period (latest Pliocene) all known fossil floras are composed, almost exclusively, of living species, although the geographical distribution of these may have changed in the interval.
The study of a succession of floras in a given region, say western Europe, shows clearly that changes occur in their com ponents from age to age. Further, that the components of the successive floras are related to those of living localized floras; now to a tropical, now to a warm-temperate flora, and so on.
But living plants are grouped into localized floras largely under the influence of climate. Adverse climatic conditions will kill developed, i.e., rooted, plants. Whole species, or even genera would be killed out, were it not that plants have an inherent power of movement in their embryo stage—the seeding stage. If the members of a species can cast their seed beyond the range of the adverse conditions, that species may survive. If not, it must die. But for survival, the change of climate must not overtake its rate of travel, and there must be a suitable habitat within reach. That plants continually try to occupy fresh ground is shown by the rapidity with which they spring up on waste land (Krakatoa after the earthquake, Flanders during the war). That they fail to establish themselves, if the ground is already occupied by a healthy population, is shown by the rarity with which intro duced plants are able to establish themselves among a native flora. It is largely by movement under changing climate that in process of time plants have been grouped into the existing local floras. Under a change from heat to cold the movement will be from higher to lower latitudes and from higher to lower altitudes. Under a change from cold to heat it will be in the reverse directions.
In considering the results which have accrued from the study of Tertiary plants, we propose to divide the subject under three heads belonging to the three great geographical regions in con nection with which most of the work on Tertiary plants has been carried out. These regions are Europe, North America and the Arctic. The work on the Tertiary plants of all other parts of the world is, as yet, too scattered and too scanty to admit of treat ment in a short article. Space does not permit of more than a general inflication of the characters of the floras which succeeded one another in the three regions during the long Tertiary ages— probably some tens of millions of years. For fuller knowledge readers must consult the bibliography. Much of the older work needs revision, chiefly because the knowledge of the plants of the Far East—just those with which Tertiary plants are most closely allied—has increased so greatly during this century. In view of this increase in knowledge we illustrate the fossil floras of the various periods, where we can, by the latest work, provided the floras are large and their age well established. There is an addi tional advantage in so doing because the later works give refer ences to the more important older works.