A SOIL ARCHIPELAGO 171 
found to be roughly north-western as opposed to 
south-eastern. The further change due to elimina- 
tion of species has been already referred to. Most 
plants no doubt have spread in our islands as far as 
prevailing climatic and soil conditions allow, but in 
other cases the sea-barriers seem to have put a period 
to their natural advance. Considering the wide range 
of conditions of climate and soil under which, for 
instance, the Hairy Crowfoot (Ranunculus sardous), 
the Common Rock-rose (Helianthemum Chamecis- 
tus), the Needle Furze (Genista anglica), and the 
Small Marsh Valerian (Valeriana dioica), occur in 
England, Wales, and Scotland, it is difficult to impute 
their absence from Ireland to climate. 
Thirdly, we find (as we have already seen in the 
first chapter) varying conditions of soil intruding 
themselves and producing such local changes in the 
grouping of the plants as may quite obscure the 
broader differences just dealt with. Were our islands 
a plain formed of uniform materials, the gradual 
changes from south to north or east to west might 
be traced step by step. But their surface is most 
diversified; their rocks contain an epitome of the 
whole geology of Europe; the soils are consequently 
various: from the point of view of the plant world 
the area is an archipelago: for some plants a desert 
with occasional oases, for others an oasis enclosing 
occasional deserts. Certain species are confined to the 
Chalk—for instance, the Box (Buxus sempervirens) 
and the Stinking Hellebore (Helleborus fatidus)— 
while to others a limy soil is a barrier comparable to 
that formed by the English Channel. It will be seen, 
then, that when we speak of the flora in general being 
