CHAPTER XIV. GEOGRAPHICAL VICISSITUDES OF PLANTS. ONLY those familiar with the geological history of Great Britain are aware of the numerous, extreme, and extensive geographical and climatal changes which have taken place in these latitudes since our familiar terrestial plants first appeared. With slow, but certain movements, extending over hundreds of thousands of years, pendulum-like the climate has swung from one extreme to another-from tropical heat to Arctic cold. The period in which we live is, as regards our northern hemisphere, half-way between these extremes, or temperate in its climate. We are passing from the last extreme condition of Arctic cold (during the Glacial Period), slowly, to an infinitesimally increasing state of warmth, which may culminate ages hence, in a tropical warmth similar to that which geologists are perfectly assured existed in this country during the Eocene Period of Tertiary time, when the familiar blue clay cut into during the too frequent metropolitan excavations was being slowly accumulated on a shallow sea-floor. For ages before that—indeed ever since the dry land received its first lowly-organised vegetation in Cambrian times-this slow beat of climatic changes has repeatedly taken place. What strange extremes the land-plants must have been subjected to! It is as if the Esquimaux had been repeatedly transported to Calcutta, and the Hindoos forced to take up their abode amidst the eternal frosts and snows of the Arctic regions! It must be further remembered that these climatic variations variations have been accompanied by equally slow but certain geographical changes, which have converted dry land into sea-bed, and upheaved the bottom of the sea to form new dry lands-which have crumpled up previously horizontal strata into tablelands and mountain-chains; or depressed those already existing until they were let down in the middle like a bellying chain, as is the case with the underground connection between the Mendip and Ardennes Hills, beneath London. We can hardly overstate the influence such climatal and physiographical changes must have had upon terrestial floras. As soon as a comparative equilibrium had been set up, and the plants had relatively fought out their battles and yielded to the strongest—as soon as all had more or less adapted themselves to the surrounding physical conditions a change commenced, which ultimately required the whole process to be done over again. A warmth-loving flora, like that we know existed in England during the Eocene Period, had settled down pleasantly, and seemed to be irremovable. This state of things must have existed for many hundreds of thousands of years, when the climate gradually toned down, more and more, during the Miocene Period; and, when the succeeding Pliocene era set in, it inaugurated a temperate climate like that now prevailing. This was followed by the long, bitter climatal winter of "The Great Ice Age." Even if the warmth-loving Eocene plants had been able to adapt themselves to the lower temperature, they could not have lived in Great Britain during the latter period, for during one stage, at least, of that bitter time, these islands were swathed in an ice-sheet just as Greenland is now, and this would effectually prevent plants from maintaining their ancient habitats. The case must have gone equally bad with them, even if the climate remained practically unchanged, when one of the numerous oscillations of level occurred which lowered the dry land until it became the bed of a sea. In both these instances there would be only one way in which plants could perpetuate their species when their ancient sites had been taken from them -they would have to migrate. The British plants of the Eocene Period, under stress of altered climate, had moved farther south, and we accordingly find them now forming part of the flora of South Africa, and Brazil, etc., such as the Proteacea - the flora occupying the low lands, which were first submerged during a change of level, would be forced to retire farther inland, and grow at higher elevations. In both such cases as these there would be a tendency to overcrowd the surface of the dry land, and to cause a keener struggle for existence, resulting in the extinction of some kinds, and the complete alteration of others, under the influence of natural selection. Large islands and parts of continental areas have been broken up, by changes of level, into small islands. These would still retain some of the old plants of the mainland, whose altered surroundings would have to be responded to in changed and adapted structures. Suppose such a geological change to take place when the climate was very warm-all the plants on the newly-formed islands would then necessarily be warmth-loving. If a climatal change occurred, the increased cold would not go so hardly with the island plants as with those of the adjacent mainland, because an island climate is always more regular and warmer than the continental climate in the same latitude. So that whilst the plants on the latter would change, or their places would be taken by cold-loving plants from higher latitudes, those on the islands would be better circumstanced, and also free from invasion. Now let ages elapse, and an upward movement once more connect the island with the mainland, so that it formed an integral part of it—we should then have the phenomenon of two distinct floras living side by side. Or let the opposite fact be imagined. Islands are formed during a Glacial Period, and the characteristic flora remains with them. The climate alters; some of the more sensitive species die out (although plants used to cold adapt themselves sooner to increased warmth than those which love heat do to cold); others creep farther up the hillsides to the summits. When the intervening seabed is upheaved, these hilltops will have been raised still higher above the sea-level and into a colder region of the atmosphere. Or suppose a country to be very cold and occupied by an Arctic flora, and its geographical connections such that it is part and parcel of an extensive continent, or series of continents (like the land-masses of the northern hemisphere). Then, if the climate slowly changes to warmer conditions, as the snows cease to lie permanently on the mountain-slopes, and to fill up the valleys with their glaciers, the cold-loving plants of the plains, finding the climate changing, would creep up to higher and therefore colder levels, and occupy the ground previously covered with snow and ice. Eventually the tops of all the high hills and U |