Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/66

48 there is a regular temperature, all the year round, of from 75° to 80° in the shade. An elevation of 100 yards causes, in all latitudes and climates, to speak in round numbers, a lowering of one degree of mean temperature; and for this reason it is often possible, in mountainous intertropical regions, to pass through, in a single day's excursion, a range in temperature equal to that which there is normally between a tropical and arctic station. At the base of the Himalayas, for instance, we may begin the day amongst tree-ferns and palms, climb through a belt of oaks, and chestnuts, and magnolias, and a higher belt of pines and rhododendrons, to a region where no trees can exist, and only mosses, lichens, saxifrages, and gentians grow on the edge of the fields of perpetual snow, and then return again in the evening or next day to the palms and tree-ferns. Even within the compass of Britain we have more than one-third of this whole range of 55°. The difference in mean temperature at sea-level, along the east coast, is not more than 5°; but between the extreme points of the island, say the Lands End and the peak of Ben-na-muic-dhui, the difference is not much under 20°, the mean temperature being 52° for the one station, and not much over freezing-point for the other. In his elaborate work, called Cylele Britannica our principal authority on botanical geography, Mr. H. C. Watson, has divided the surface of the island into two regions of temperature, as modified partly through latitude and partly through altitude, and subdivided each of them into three zones, each of which covers a range of about 3° of mean annual temperature. The upper region he calls the Arctic, and the lower one the Agrarian region, and the boundary between them, which is at an elevation of 600 yards in Wales and the North of England, and descends to 450 yards in the Central Scotch Highlands, is marked by the line of limit of the possible cultivation of grain, which corresponds to the line of upper limit of several familiar wild plants, of which Pteris aquilina, Ranunculus bulbosus, Nasturtium officinale, Lolium perenne, and Geranium molle are examples. The six zones he calls Super, Mid, and Inferarctic, Super, Mid, and Inferagrarian.