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 givea 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.® 
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.” 
224 
