Fig. 99. Fig. 101.
Fig. 99. Alnus gracilis, an ovate-oblong leaf, like that of the Alder, found in Bohemia.
Figures 100, 101, 102, exhibit fragments of plants which occur in the leaf-bed at Ardtun Head, in Mull, and which is referred to the Miocene epoch. The figures are from the Duke of Argyll's paper.
Fig. 100. Taxites, or perhaps Taxodites Campbellii, a branch with leaves resembling those of the Yew, or rather those of Taxodium.
Fig. 101. Rhamnites multinervatus, a leaf resembling that of Rhamnus.
Fig. 102.
Fig. 102. Equisetum Campbellii, a stem like that of an Equisetum of the present day.
Dr. Oswald Heer[23] has examined the plants preserved in the lignite beds of Bovey Tracey, in Devonshire, and he finds that they belong to the Miocene formation. There is a remarkable coincidence between this and several of the continental fossil floras, such as those of Salzhauser in the Wetterau, Manosque in Provence, and of some parts of Switzerland. Bovey Tracey has no species in common with Iceland, although the Tertiary flora of Iceland belongs to the same period. Two of its species (Corylus MacQuarrii and Platanus aceroides) have been found in the Miocene of Ardtun Head. Even the genera are distinct, with the exception of Sequoia and Quercus. The Bovey Tracey flora has a much more southern character, corresponding entirely with that of the Lower Miocene of Switzerland. It contains three species of Cinnamon, one Laurel, evergreen Figs, one Palm, and large Ferns, thus manifesting a subtropical climate. One of the most important plants is Sequoia Couttsiæ, a Conifer which supplies a link between S. Langsdorfii and S. Sternbergi, the widely-distributed representatives of S. sempervirens and S. gigantea (Wellingtonia), which are Californian trees. Among other characteristic plants may be mentioned Cinnamomum lanceolatum and C. Scheuchzeri; Quercus Lyellii, an evergreen oak; species of evergreen fig (Ficus Falconeri and F. Pengellii), Palmacites Dæmonorops, a prickly twining Rotang-palm; species of Vine (Vitis Hookeri and V. Britannica); Pecopteris lignitum, a large tree-fern; species of Nyssa, at present confined to North America. Among other plants recorded by Heer in his paper are the following:—Laurus primigenia, Daphnogene Ungeri, species of Dryandroides, Andromeda, Vaccinium acheronticum, Echitonium cuspidatum, Gardenia Wetzleri, species of Anona, Nymphæa Doris, Carpolithes Websteri, C. Boveyanus, and other species. In the post-tertiary white clay of Bovey Tracey, Salix cinerea, and a species allied to S. repens, as well as Betula nana, are found.
The Arctic fossil flora (Miocene), according to Heer, amounts to 162 species: Cryptogamia, 18 species, of which 9 are large ferns; Phanerogamia, Coniferæ, 31; Monocotyledons, 14; Dicotyledons, 99. Among the Coniferæ are—Pinus M'Clurii, Sequoia Langsdorffii, Sternbergi, and Couttsiæ, Taxodium dubium, Glyptostrobus europæus, Thujopsis europæa. Among leafy trees are—Fagus Deucalionis, Quercus Olafseni, Platanus aceroides, willows, beeches, Acer, Otopteryx, tulip-tree, walnuts, Magnolia Inglefieldi, Prunus Scottii, Tilia Malmgreni, Corylus M'Quarrii, Alnus Kefersteinii, Daphnogene Kannii, probably one of the Lauraceæ; and among Proteaceæ, MacClintockia? and Hakea. In Greenland are found species of Rhamnus, Paliurus, Cornus, Ilex, Cratægus, Andromeda, Myrica, Ivy, and Vine. From the flora of Spitzbergen, in the Miocene epoch, we may conclude that under 79° N. lat. the mean temperature of the year may have been 41° Fahr., while at the same epoch that of Switzerland was 69°·8 Fahr.; judging from the analogy of floras, it appears that the mean temperature has fallen 6°·9. From this it follows that at Spitzbergen, at 78° N. lat., the mean temperature was perhaps 41°·9 Fahr. In Greenland, at 70°, it would be 49°·1 Fahr., and in Iceland and on the Mackenzie, in lat. 65°, it would be 52°·7 Fahr. At the Miocene epoch the temperature seems to have been much more uniform, the mean heat diminishing much more gradually in proportion as the pole was approached. The isothermal line of 32° Fahr. might have fallen upon the pole, while now it is situated under 58° N. (See Heer's conclusions as to changes of temperature depending on proportion of sea and land, eccentricity of the earth, and the earth moving through warm and cold spaces in the universe—Ann. Nat. Hist. 4th ser. i. 66.)
In speaking of the Polar flora of former epochs, Heer says that every plant executes a slow and continuous migration. These migrations, the starting-point of which is the distant past, are recorded in the rocks; and the interweaving of the carpets of flowers which adorn our present creation retraces them for us in its turn. For the vegetation of the present day is closely connected with that of preceding epochs; and throughout all these vegetable creations reigns one thought, which not only reveals itself around us by thousands upon thousands of images, but strikes us everywhere in the icy regions of the extreme north. Organic nature may become impoverished there, and even disappear when a cold mantle of ice extends over the whole earth; but where the flowers die the rocks speak, and relate the marvels of creation; they tell us that even in the most distant countries, and in the remotest parts, nature was governed by the same laws and the same harmony as immediately around us.[24]