48 A JfEW FLORA OF 



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 lati- 

 tudes and climates, to speak in round numbers, a lowering of one 

 degree of mean temperature ; and for this reason it is often pos- 

 sible, 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° ; bnt 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 Cyhele Britannica,'^^- 

 our principal authority on botanical geography, Mr. H. C. "Watson, 

 has divided the surface of the island into two regions of tempera- 

 ture, as modified partly through latitude and partly through alti- 

 tude, 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 ip. 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 buliosus, Nasturtium officinale, Lolium 

 perenne, and Geranium molle are examples. The six zones he 

 calls Super, Mid, and Inferarctic, Super, Mid, and Inferagrarian. 



* Cyhele Britannica; or British Plants and their Geographical Relations, in four Vols. 

 London: Longmanns, 1847-1859. 



