The next succeeding period, that of the Pliocene, continues the conditions of the last, but with signs of decadence. Many of the old gigantic pachyderms have disappeared; and in their stead some familiar modern genera were introduced. The Pliocene was terminated by the cold or glacial period, in which a remarkable lowering of temperature occurred over all the northern hemisphere, accompanied, at least in a portion of the time, by a very general and great subsidence, which laid all the lower parts of our continents under water. This terminated much of the life of the Pliocene, and replaced it with boreal and Arctic forms, some of them, like the great hairy Siberian mammoth and the woolly rhinoceros, fit successors of the gigantic Miocene fauna. How it happened that such creatures were continued during the Post-pliocene cold, we cannot understand till we have the Tertiary vegetation before us. It must suffice now to say, that as the temperature was modified, and the land rose, and the Modern period was inaugurated, these animals passed away, and those of the present time remained.
Perhaps the most remarkable fact connected with this change, is that stated by Pictet, that all the modern European mammals are direct descendants of Post-pliocene species; but that in the Post-pliocene they were associated with many other species; and these, often of great dimensions, now extinct. In other words, the time from the Pliocene to the Modern, has been a time of diminution of species, while that from the Eocene to the Miocene was a time of rapid introduction of new species. Thus the Tertiary fauna culminated in the Miocene. Yet, strange though this may appear, Man himself, the latest and noblest of all, would seem to have been a product of the later stages of the time of decadence. I propose, however, to return to the animals immediately preceding man and his contemporaries, after we have noticed the Tertiary flora and the Glacial period.
THE NEOZOIC AGES (continued).
Plant-life in the Tertiary approaches very nearly to that of the Modern World, in so far as its leading types are concerned; but in its distribution geographically it was wonderfully different from that with which we are at present familiar. For example, in the Isle of Sheppey, at the mouth of the Thames, are beds of “London clay,” fall of fossil nuts; and these, instead of being hazel nuts and acorns, belong to palms allied to species now found in the Philippine Islands and Bengal, while with them are numerous cone-like fruits belonging to the Proteaceæ (banksias, silver-trees, wagenbooms, etc.), a group of trees now confined to Australia and South Africa, but which in the Northern Hemisphere had already, as stated in a previous paper, made their appearance in the Cretaceous, and were abundant in the Eocene. The state of preservation of these fruits shows that they were not drifted far; and in some beds in Hampshire, also of Eocene age, the leaves of similar plants occur along with species of fig, cinnamon, and other forms equally Australian or Indian. In America, especially in the west, there are thick and widely-distributed beds of lignite or imperfect coal of the Eocene period; but the plants found in the American Eocene are more like those of the European Miocene or the Modern American flora, a fact to which we must revert immediately.
In Europe, while the Eocene plants resemble those of Australia, when we ascend into the Miocene they resemble those of America, though still retaining some of the Australian forms. In the leaf-beds of the Isle of Mull,—where beds of vegetable mould and leaves were covered up with the erupted matter of a volcano belonging to a great series of such eruptions which produced the basaltic cliffs of Antrim and of Staffa,—and at Bovey, in Devonshire, where Miocene plants have accumulated in many thick beds of lignite, the prevailing plants are sequoias or red-woods, vines, figs, cinnamons, etc. In the sandstones at the base of the Alps similar plants and also palms of American types occur. In the Upper Miocene beds of [OE]ningen in the Rhine valley, nearly five hundred species of plants have been found, and include such familiar forms as the maples, plane-trees, cypress, elm, and sweet-gum, more American, however, than European in their aspect. It thus appears that the Miocene flora of Europe resembles that of America at pre sent, while the Eocene flora of Europe resembles that of Australia, and the Eocene flora of America, as well as the modern, resembles the Miocene of Europe. In other words, the changes of the flora have been more rapid in Europe than in America and probably slowest of all in Australia. The Eastern Continent has thus taken the lead in rapidity of change in the Tertiary period, and it has done so in animals as well as in plants.
The following description of the flora of Bovey is given, with slight alteration, in the words of Dr. Heer, in his memoir on that district. The woods that covered the slopes consisted mainly of a huge pine-tree (sequoia), whose figure resembled in all probability its highly-admired cousin, the giant Wellingtonia of California. The leafy trees of most frequent occurrence were the cinnamon and an evergreen oak like those now seen in Mexico. The evergreen figs, the custard apples, and allies of the Cape jasmine, were rarer. The trees were festooned with vines, beside which the prickly rotang palm twined its snake-like form. In the shade of the forest throve numerous ferns, one species of which formed trees of imposing grandeur, and there were masses of under-wood belonging to various species of Nyssa, like the tupelos and sour-gums of North America. This is a true picture, based on actual facts, of the vegetation of England in the Miocene age.
But all the other wonders of the Miocene flora are thrown into the shade by the discoveries of plants of this age which have recently been made in Greenland, a region now bound up in what we poetically call eternal ice, but which in the Miocene was a fair and verdant land, rejoicing in a mild climate and rich vegetation. The beds containing these specimens occur in various places in North Greenland; and the principal locality, Atane-Kerdluk, is in lat. 70 N. and at an elevation of more than a thousand feet above the sea. The plants occur abundantly in sandstone and clay beds, and the manner in which delicate leaves and fruits are preserved shows that they have not been far water-borne, a conclusion which is confirmed by the occurrence of beds of lignite of considerable thickness, and which are evidently peaty accumulations containing trunks of trees. The collections made have enabled Heer to catalogue 137 species, all of them of forms proper to temperate, or even warm regions, and mostly American in character. As many as forty-six of the species already referred to as occurring at Bovey Tracey and [OE]ningen occur also in the Greenland beds. Among the plants are many species of pines, some of them of large size; and the beeches, oaks, planes, poplars, maples, walnuts, limes, magnolias, and vines are apparently as well represented as in the warm temperate zone of America at the present day. This wonderful flora was not a merely local phenomenon, for similar plants are found in Spitzbergen in lat. 78° 56'. It is to be further observed, that while the general characters of these ancient Arctic plants imply a large amount of summer heat and light, the evergreens equally imply a mild winter. Further, though animal remains are not found with these plants, it is probable that so rich a supply of vegetable food was not unutilised, and that we shall some time find that there was an Arctic fauna corresponding to the Arctic flora. How such a climate could exist in Greenland and Spitzbergen is still a mystery. It has, however, been suggested that this effect might result from the concurrence of such astronomical conditions in connection with the eccentricity of the earth’s orbit as would give the greatest amount of warmth in the Northern Hemisphere with such distribution of land and water as would give the least amount of cold northern land and the most favourable arrangement of the warm surface currents of the ocean.[AI]