THE age of the earth, according to the most probable astronomical estimate, is in the neighbourhood of two thousand million years. Geological time, as measured by the evidence from radio-active minerals in the rocks, extends back some fifteen hundred million years, and the first unequivocal fossils are considerably more than five hundred million years old. Man, as such, cannot have been on the globe for more than a few million years—the last few moments of a long pageant. It is essential, when picturing the plant life of the past, to bear in mind this background of Time.
In order to follow the story of fossil plants it is necessary to be familiar with terms like ‘Devonian ‘and ‘Jurassic,’ but it is well to remember that the divisions of the stratified rocks have been based in the main on the successive faunas of marine animals; had fossil plants been used for dividing up the geological time-scale, some of the lines would have been drawn in different places. Thus the flora of the Upper Devonian is much more closely allied to that of the succeeding Lower Carboniferous period than to the preceding Middle Devonian, and that of the Upper Jurassic is almost indistinguishable from that of the lowest Cretaceous beds. Moreover, as will be seen later, the three primary divisions of the stratified series, into Palaeozoic, Mesozoic, and Cainozoic or Tertiary, do not coincide with the great transformations of plant life.
TELLING THE AGE OF THE EARTH BY PLANTS THE division of plant history into a number of eras each with its own general type of vegetation is, of course, simply an abstraction based on our present knowledge of the floras of the past. There are no sharp lines between the different eras; the whole story was really continuous. On the other hand, it is clear that at various periods differentof plants have been dominant, and with a more detailed stud’-
of past floras it would be possible to show that almost every successive geological horizon is characterised by its own particular genera or species of plants. Sometimes one particular plant which may have been widely spread in space, had a very restricted range in time, and is therefore a useful index of the geological age of the beds in which it is found. Sometimes these beds may be distinguished rather by the assemblage of fossil plants in them, just as the flora of two countries might be distinguished although some of the species in each were the same.
In studying the succession of floras, the botanist must work hand in hand with the geologist. The broad outlines of the story may be fairly clear, but there are many difficulties in detail. Thus in rocks of approximately the same age apparent differences in the flora may be due to variety in the original habitat of the fossil plants; one set of rocks may contain only swamp plants, for example, and the other may contain relics of highland plants washed down by a river. But such questions touch the geological rather than the biological side of fossil botany.
The observed facts of the succession of different types of life on the globe, apparently ranging from simpler to more complex along a network of lines or branches, which is what we call evolution, naturally raises questions not merely of why (which is quite beyond us) but of how it all happened. The fossils give us numerous hints as to how the various plant organs may have been elaborated from simple structures, and how plantmay be linked together, either directly or, as we sometimes infer, by a common ancestry. To these hints we add the spice of speculation, which has its value in stimulating further research to verify or disprove new hypotheses. Moreover, there are always investigators who check the excessive speculations of their colleagues with new speculations of their own. Even the ‘facts ‘of one generation may be half-forgotten fancies in the next. ‘Ferns ‘turn out to be -plants, ‘corals ‘are found to be seaweeds, ‘seaweeds ‘are recognised as land plants. In short, as Bernard Shaw has said, one of the chief glories of science is that it is always wrong, though he might have added that science is indeed an orderly and unceasing method of rectifying errors —its own as well as that of others. Nevertheless those who want dogmatic and immutable certainty must turn away from the halls of science.
The account here given of successive eras of plant life spread over hundreds of millions of years, with suggestions as to the relationship and descent of certain groups, is a mere sketchy oudine based on the present state of knowledge. One general conclusion may perhaps be emphasised : progressive change is not a constant inherent feature of living beings. Many a group of plants has reproduced its kind with little change in structure or habit for many million years; in other groups great changes seem to have taken place comparatively rapidly. ‘Persistence of type,’ writes A. C. Seward, ‘and from time to time the apparently sudden influx of new types, are among the outstanding features of the history of plant evolution.’
One of the greatest palaeobotanists of our time, Dukinfield M. Scott, once remarked that we know a good deal about extinct plants, but not enough, as yet, to throw much light on the exact course of their evolution. ‘It is impossible,’ he wrote on another occasion, ‘to construct a genealogical tree of the Vegetable Kingdom, but the tree symbol is probably still the best scheme for picturing the course of evolution to our imagination. The symbolic tree must have had very long parallel branches—perhaps something like a Lombardy Poplar.’
When we ponder further on the vast sea of our ignorance concerning Lombardy Poplars and other living organisms —although we can watch their development from seed to tree, study their habits, experiment on them, and dissect them in our laboratories—we shall not be surprised that our knowledge of the past history of plants, based on casual assemblages of fragmentary fossils, is so incomplete.