The Origin of our British Flora 
sidered as consisting mainly of the four great invading 
armies of plants. 
In the North, in some parts of West Ireland, and upon 
the Scotch, Welsh, and a few English mountains, we still 
find Dryas and its little willows, and other characteristic 
arctic or alpine plants. 
The birch association is but seldom represented, but 
shrubs of birch, rowan, larger willows and junipers, occur 
on steep corrie sides and other places where they cannot 
be destroyed by sheep. 
Lower down most of the heather moorland, diversified 
by the silvery sheen of the cotton-grass, peaty mosses, or 
rushy grass pastures, represent the old Scotch pine forests. 
In the fertile, low-lying parts of Scotland, oak forest 
used to cover most of the country, but it is now 
replaced by arable land or permanent pasture. The 
valley floors or flat river holms seem, as we have shown 
elsewhere, to be due to the reeds (Phragmites and others), 
which choked unnecessary water channels and inter- 
cepted the river silt. 
Besides these main groups, which form a very large 
proportion of the Scotch flora, there are several other 
and distinct divisions, The number of weeds is very 
great but can never be accurately estimated, for with 
changes of cultivation some die out and strangers enter 
in. All over Britain one finds the Canadian weed 
Elodea, which seems to have first been noticed about 
1843. The Chilian Mimulus has got a firm footing by 
the southern Scottish rivers and burns, and occurs 
abundantly even in the marshes of the Tay. Claytonia 
siberica is establishing itself in the West of Scotland, and 
seems to be extending its range annually. 
Many other interesting cases will be found in almost 
any county flora, or in the pages of the Journal of 
Botany, and which show that the flora of Britain is 
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