to comprise 2 horsetails, 11 Conifers (including the living Pinus
Abies), 2 grasses, a sedge, 2 poplars, a willow, 2 birches, 2 hazels,
an elm, a Viburnum, a water-lily, and a lime. Such an assemblage
at the present day would suggest a latitude quite 25° farther south;
but it shows decidedly colder conditions than any of the European
Eocene, Oligocene, or Miocene strata. From lat. 78° in Spitsbergen
Heer records 136 species of fossil plants. More to the south, at
Disco Island in lat. 70°, the Tertiary wood seem to have been
principally composed of planes and Sequoias; but a large number
of other genera occur, the total number of plants already recorded
being 137. From various parts of Greenland they now amount
to at least 280. Among the plants from Disco, more than a quarter
are also found in the Miocene of central Europe. The plants of
Disco include, besides the plane and Sequoia, such warm-temperate
trees as Ginkgo, oak, beech, poplar, maple, walnut, lime and
magnolia. If these different deposits are contemporaneous, as is
not improbable, there is a distinct change in the flora as we move
farther from the pole, which suggests that difference of latitude then
as now was accompanied by a difference in the flora. But if this
process is continuous from latitude to latitude, then we ought not
to look for a flora of equivalent age in the warm-temperate Miocene
deposits of central Europe, but should rather expect to find that the
temperate plants of Greenland were contemporaneous with a tropical
flora in central Europe. As Mr Starkie Gardner has pointed out, it
does not seem reasonable to assume that the same flora could have
ranged then through 40° of latitude; it is more probable that an
Eocene temperate flora found in the Arctic regions travelled southwards
as the climate became cooler, till it became the Miocene
temperate flora of central Europe. Mr Gardner suggests, therefore,
that the plant-beds of Greenland and Spitsbergen represent the
period of greatest heat, and are therefore wrongly referred to the
Miocene. At present the evidence is scarcely sufficient to decide
the question, for if this view is right, we ought to find within the
Arctic circle truly Arctic floras equivalent to the cool Lower Eocene
and Miocene periods; but these have not yet been met with.
A steady decrease of temperature marked the Pliocene period throughout Europe, and gradually brought the climatic conditions Pliocene. into correspondence with those now existing, till towards the end of the period neither climate nor physical geography differed greatly from those now existing. Concurrently with this change, the tropical and extinct forms disappeared, and the flora approached more and more nearly to that now existing in the districts where the fossil plants are found, though in the older deposits, at any rate, the geographical distribution still differed considerably from that now met with. At last, in the latest Pliocene strata (often called “pre-Glacial”) we find a flora consisting almost entirely of existing species belonging to the Palaearctic regions, and nearly all still living in the country where the fossils are found. This flora, however, is associated with a fauna of large mammals, the majority of which are extinct.
The plants of the Older Pliocene period are unknown in Great Britain, and little known throughout Europe except in central France and the Mediterranean region. The forests of central France during this epoch showed, according to Saporta, a singular admixture of living European species, with trees now characteristic of the Canary Isles and of North America. For instance, of the living species found at Meximieux, near Lyons, one is American, eight at least belong to the Canaries (six being characteristic of those islands), two are Asiatic, and ten still live in Europe. Taking into account, however, the closest living allies of the fossil plants, we find about equal affinities with the floras of Europe, America, and Asia. There is also a decided resemblance to the earlier Miocene flora. Among the more interesting plants of this deposit may be mentioned Torreya nucifera, now Japanese; an evergreen oak close to the common Quercus Ilex; Laurus canariensis, Apollonias canariensis, Persea carolinensis, and Ilex canariensis; Daphne pontica (a plant of Asia Minor); a species of box, scarcely differing from the English, and a bamboo. To this epoch, or perhaps to a stage slightly later, and not to the Newer Pliocene period, as is generally supposed, should probably be referred the lignite deposits of the Val d'Arno. This lignite and the accompanying leaf-bearing clays underlie and are apparently older than the strata with Newer Pliocene mammals and mollusca. The only mammal actually associated with the plants appears to be a species of tapir, a genus which in Europe seems to be characteristically Miocene and Older Pliocene. The plants of the Val d'Arno have been described by Ristori; they consist mainly of deciduous trees, a large proportion of which are known Miocene and early Pliocene forms, nearly all of them being extinct. A markedly upland character is given to the flora of this valley through the abundance of pines (9 species) and oaks (16 species) which it contains; but this peculiarity is readily accounted for by the steep slopes of the Apennines, which everywhere surround and dominate the old lake-basin. Among the other noticeable plants may be mentioned Betula (3 species), Alnus (2 species), Carpinus, Fagus (4 species), Salix (4 species), Populus (2 species), Platanus, Liquidambar, Planera, Ulmus (2 species), Ficus (2 species), Persoonia, Laurus (5 species), Persea, Sassafras, Cinnamomum (5 species), Oreodaphne, Diospyros (2 species), Andromeda, Magnolia, Acer (3 species), Sapindus, Celastrus (2 species). Ilex (4 species), Rhamnus (3 species), Juglans (5 species), Carya (2 species), Rhus, Myrtus, Crataegus, Prunus, Cassia (3 species). These plants suggest a colder climate than that indicated by the plants of Meximieux—they might, therefore, be thought to belong to a later period. The difference, however, is probably fully accounted for when we take into consideration the biting winds still felt in spring in the valley of the Arno, and the probable large admixture of plants washed down from the mountains above. Somewhat later Pliocene deposits in the Val d'Arno, as well as the tuffs associated with the Pliocene volcanoes in central France, yield plants of a more familiar type, a considerable proportion of them still living in the Mediterranean region, though some are only now found at distant localities, and others are extinct. The flora, however, is essentially Palaearctic, American and Australian types having disappeared.
A somewhat later Pliocene flora is represented by the plants found at Tegelen, near Venloo, on the borders of the Netherlands and Germany. This deposit is of especial interest for the light it throws on the origin of the existing flora of Britain. The Tegelen plants are mainly north European; but there occur others of central and south Europe, and various exotic and extinct forms, nearly all of which, however, belong to the Palaearctic region, though some may now be confined to widely separated parts of it. For instance, Pterocarya caucasica does not grow nearer than the Caucasus, where it is associated with the wild vine—also found at Tegelen; Magnolia Kobus is confined to the north island of Japan; another species of Magnolia cannot be identified and may be extinct. An extinct water-lily, Euryale limburgensis, belongs to a monotypic genus now confined to Assam and China; an extinct sedge, Dulichium vespiforme, belongs to a genus only living in America, though the only living species once flourished also in Denmark; an extinct species of water-aloe (Stratiotes elegans) makes a third genus, represented only by a single living species, which was evidently better represented in Pliocene times. A large proportion of the plants, however, may still be found living in Holland and Britain; but there is a singular scarcity of Composites, though this order is fairly well represented in British strata of slightly later date.
The latest Pliocene, or pre-Glacial, flora of northern Europe is best known from the Cromer Forest-bed of Norfolk and Suffolk, a fluvio-marine deposit which lies beneath the whole of the Glacial deposits of these counties, and passes downwards into the Crag, many of the animals actually associated with the plants being characteristic Pliocene species which seem immediately afterwards to have been exterminated by the increasing cold. The plants contained in the Cromer Forest-bed, of which about 150 species have now been determined, fall mainly into two groups—the forest-trees, and marsh and aquatic plants. We know little or nothing at present of the upland plants, or of those of dry or chalky soils. Forest trees are well represented; they are, in fact, better known than in any of the later English deposits. We find the living British species of Rhamnus, maple, sloe, hawthorn, apple, white-beam, guelder-rose, cornel, elm, birch, alder, hornbeam, hazel, oak, beech, willow, yew and pine, and also the spruce. This is an assemblage that could not well be found under conditions differing greatly from those now holding in Norfolk; there is an absence of both Arctic and south European plants. The variety of trees shows that the climate was mild and moist. Among the herbaceous plants we find, mingled with a number that still live in Norfolk, Hypecoum procumbens, the water-chestnut (Trapa natans), and Najas minor, none of which is now British.
On the Norfolk coast another thin plant-bed occurs locally above the Forest-bed and immediately beneath the Boulder Clay. This deposit shows no trace of forest-trees, but it is full of remains of Arctic mosses, and of the dwarf willow and birch; in short, it yields the flora now found within the Arctic circle.
The incoming of the Glacial epoch does not appear to have been accompanied by any acclimatization of the plants—the Pleistocene. species belonging to temperate Europe were locally exterminated, and Arctic forms took their places. The same Arctic flora reappears in deposits immediately above the highest Boulder Clay, deposits formed after the ice had passed away. These fossil Arctic plants have now been found as far south as Bovey Tracey in Devonshire, where Pengelly and Heer discovered the bear-berry and dwarf birch; London, where also Betula nana occurs; and at Deuben in Saxony, which lies nearly as far south as lat. 50°, but has yielded to Professor Nathorst's researches several Arctic species of willow and saxifrage. The cold period, however, was not continuous, for both in Great Britain and on the continent of Europe, as well as in Canada, it was broken by the recurrence of a milder
