The Origin of our British Flora 



top and smother any young birches that are growing 

 amongst them.* 



So that this gradual succession of these four associa- 

 tions seems very plausible, and would give a clear picture 

 of the conquest of Britain by the plant world. 



The upper limit of the oak lies between 750 feet and 

 1250 feet, that of the Scotch fir between 1250 feet and 

 2400 feet ; whilst the birch goes distinctly higher than 

 the pine, and reaches 2700 feet in the Eden, Wear, and 

 Tyne district. 



But as every one knows who has been in the High- 

 lands or on the Yorkshire moors, we do not find to-day 

 any such distribution of pines and oaks as one would 

 expect from the preceding data. 



Between the scanty alpine or subalpine flora of the 

 mountain tops or hill summits and the plantations in 

 the valleys, there are almost always enormous areas of 

 desolate, whaup-haunted moorlands or cotton-grass or 

 stretches of peat-bog. Sometimes grass pastures, fit 

 only for blackfaced sheep or cheviots, are scattered 

 amongst these desolate and uninhabited haggs and 

 mosses, but this is not often the case in Scotland. 



The late Dr. Robert Smith initiated a very valuable 

 work, which has been continued since his untimely death 

 by his brother, Professor W. G. Smith, and Messrs. Lewis, 

 Rankin, Peall, Moss, and others. This is the Botanical 

 Survey which has been already carried out for Forfar 

 and Fife, Perthshire (North), Edinburghshire, Yorkshire, 

 the Eden, Wear, and Tyne valleys, and part of the 

 Pennines. 8 



On these survey maps the particular vegetation is 

 marked by special colours. One can detect at once 



* At Lake Hielmar in Sweden, birch, aspen, alder, and willow first estab- 

 lished themselves on the islands formed by the lowering of the water. Then 

 Scotch fir and spruce occupied the islands, replacing the birch association. 7 



224 



