THE final great transformation in the plant-life of the earth—the fourth we have described—introduced the vegetation of to-day. The old Mesozoic flora lasted, as we have seen, well into the Cretaceous period, but when we come to the Upper Cretaceous a complete change is manifest. The cycadophytes have disappeared almost entirely; the Maidenhair-trees are insignificant in numbers and variety; there are no strange problematical relics ofnow unknown. Instead there is an abundance of belonging to families with which we are familiar at the present
day. Palms, reeds, and other Monocotyledons (plants which have only one) make their appearance, and there is a profusion of Dicotyledons (plants which have two )—walnuts, willows, oaks, laurels, breadfruits, tulip-trees, planes, and maples among others—mainly trees, for their deciduous leaves are rather more likely to be preserved, but with a good sprinkling of herbaceous plants too.
In fact,’ as D. H. Scott has said, ‘if we judge by present evidence, it would not be surprising to find that by about the middle of the Cretaceous period the angiosperms generally were developed very much as they are now so far as the families and even some of the genera are concerned.’
For this same general type of flora has persisted throughout the whole of the Tertiary period to the present day, and it may be said that, botanically speaking, we are still living in the Cretaceous epoch.
DARWIN’S ‘ABOMINABLE MYSTERY’: THE TRIUMPH OF FLOWERS ALTHOUGH we may follow certain definite lines along which individual plant organs may have evolved, we do not yet know any fossil plants which can with certainty be regarded as ancestors of the flowering plants. Nor do we find that the first fossil forms are particularly primitive. So far no undoubted angiosperms have been found earlier than the Cretaceous. A few rare and dubious Jurassic records may be disregarded, and the lowest Cretaceous rocks still contain an abundant cycadophyte flora without a trace of Dicotyledons or Monocotyledons. But in the Lower Greensand rocks of this country (that is, in the later part of the Lower Cretaceous), among numerous fossil woods which are mainly conifers with a sprinkling of cycadophytes, there are several distinct kinds of broad-leaved trees. Leaves andare not known from these rocks, but so far as their anatomy is concerned, the Dicotyledons were ‘already highly differentiated in various directions.’ From this time on the flowering plants gained in importance at the expense of the older gymnosperms. Darwin referred to the rapidity with which they developed and overspread
the world as ‘an abominable mystery.’ In a letter to Hooker, Darwin wrote : ‘Nothing is more extraordinary in the history of the Vegetable Kingdom, as it seems to me, than the apparently very sudden or abrupt development of the higher plants. I have sometimes speculated whether there did not exist somewhere during the long ages an extremely isolated continent, perhaps near the South Pole.’ As we have seen, the ancient continent of Gondwanaland included the south polar region (either as a continuous continent or as a series of closely grouped island masses), and, moreover, there is some reason for thinking that the stock which gave rise to the flowering plants may have evolved partly in Gondwanaland. But further than that we cannot go. Polar regions have frequently been suggested as the original homes of various groups, but on general grounds the teeming tropics seem more likely to have cradled a new race. The more favourable conditions of tropical or subtropical climates would permit the survival of novelties which under severer circumstances might be destroyed as useless.
THE MYSTERY OF FLOWERS’ ANCESTRY
IT is clear that the actual ancestry of the flowering plants, which is still shrouded in mystery, must be sought far back in the Mesozoic era. Their apparently rapid spread is a different matter, and to begin with we must note that in any case it took a few million years at least, which though geologically rapid, is slow compared with the rapidity of man’s own evolution and spread. In the case of man, however, we are dealing with a single species, whereas there are now more than a hundred thousand different species of flowering plants. A second point to remember is that we are less likely to find records of rare and restricted plants than of those which are common and widely distributed.
Two different sets of factors may be me: tioned as influencing the rise of the modern flora : first, changes in the physical aspect of land and sea due to crustal movement; and secondly, the interaction of plant and animal life. About the middle of the Cretaceous period there was, as A. C. Seward puts it,
‘a remarkably widespread marine transgression; vast regions which had previously been dry land were flooded
by an invading sea. May we not see in this sinking and flooding a possible influence on the course of evolution in the organic world, an almost world-wide interference with the physical environment which had its repercussion in the altered trend of plant development?’
Great changes such as this must have been accompanied by alterations of climate, and the two together may have been largely responsible for the extermination of the older flora. As the land re-emerged a newer and more vigorous stock peopled it, suggesting that the evolutionary periods, which seem to alternate with periods of quiescence, are stimulated by great physical changes.
The interrelationship of plants and animals is a large subject, but one aspect of it is very important in connection with the rise of the flowering plants. There is little doubt that the mutual adaptation of pollinating insects and honey-bearing flowers was an important factor in the success of both groups. In species the insects now outnumber all the rest of the animal Kingdom, just as the angiosperms form the most numerous group of plants. Insects were already in existence in the Coal Measures, but they do not seem to have become really important until the later Mesozoic. Although, of course, many flowering plants are still wind-pollinated, the marvellous variety and subtle mechanisms of all the most conspicuous flowers of to-day have obviously been evolved for the attraction of insect visitors and the insurance of cross-.
FLORA UNIFORM ALL OVER THE WORLD
THE history of plant life throughout this last ‘modern era,’ that is, from the Middle or Upper Cretaceous to the present day, is intimately connected with changes of climate, and with the differentiation of genera and species rather than with the rise of new groups. Some of the old groups, such as the conifers, still retain a measure of importance, but this is definitely the age of flowering plants, and on the whole, in spite of the existence of distinct botanical provinces, it may be called a world-wide flora. Some botanists have claimed that the era of world-wide floras began to pass away after the Cretaceous age, but there is no doubt that if a geologist could look at the fossil relics of to-day’s vegetation from a distance of a few million years he would regard it as the most uniform flora that the world has ever seen.
Differences in detail should not blind us to the essential similarity of the vegetation all over the world for the last sixty or seventy million years. The various floral migrations and changes do, of course, make an intensely interesting study. Thus we may refer once more to the presence of abundant fossil plants in Arctic regions where to-day the vegetation is scanty or nil. At the beginning of the Tertiary period the flora of Greenland might be described as warm-temperate, and temperate plants have been found fossilised as far north as Grinnell Land. In southern England, the Eocene flora was distinctly tropical. The London Clay has yielded a very extensive series of well-preserved fossils and, particularly abundant on the foreshore at Sheppey; these had accumulated in the estuary or in the sea near the mouth of a great river, comparable to the tropical rivers of to-day which bring down quantities of driftwood and various fruits and , depositing them in the mud at their mouths.
The commonest and most characteristic fossil of the London Clay is the fruit of a species of swamp-palm called Nipa, which is to-day found only on Indo-Malayan coasts, but several other palms are known, and a whole series of fruits and seeds belonging mainly to tropical families. Most of these have not got common English names, but the general conclusion is that the London Clay flora was mainly of a tropical rain-forest type, allied principally with the living Indo-Malayan flora. In those times a great sea—a sort of extended Mediterranean—stretched from England to further India, and the fossil Nipa has been found at various points which were then on the shores of this sea. Subsequent geographical and climatic changes have resulted in this Eocene tropical flora being now confined to the Indo-Malayan region. It will be interesting to follow some of these general changes during the Tertiary epoch.