54 Origin of the British Flora. 
persal of the species, and a short discussion of the principal 
changes that can be shown to have taken place may assist 
in explaining some of the anomalies in geographical 
distribution. 
It is useless for our present purpose to go back to any 
distant geological period, for in Britain there exists so vast 
a break in the series of Tertiary strata that we are unable 
to bridge it. Our Middle Tertiary flora, which can be 
studied in the Oligocene strata of Hampshire, is a sub- 
tropical one, not allied to that now occupying the country. 
The history of the succeeding Miocene Period in these 
islands is a complete blank, for we have no fossiliferous 
deposits of that age, and all we can say is, that the 
Miocene appears to have been a period of great earth- 
movement and folding, under which the surface con- 
figuration of Britain was completely changed. Whether 
Britain was then under water or was mainly dry land we 
do not know. Certain of the Miocene plants found on 
the Continent are living European species—probably none 
of them now British—and the flora as a whole begins to 
show a distinct affinity with that now occupying the 
southern parts of the Continent. 
Throughout the Pliocene Period there is evidence of 
the slow refrigeration which culminated in the Glacial 
Epoch; but unfortunately, as far as the botany is con- 
cerned, this climatic change cannot be followed, for plants 
only occur in the newest stage of the period. The whole 
of the strata of Older Pliocene age yet discovered in Britain 
are of marine origin, and were laid down at some distance 
from land in a warm sea. The Coralline Crag of Suffolk 
yields, however, a few drifted land-shells, and at its base 
contains bones of land animals, washed out of some older 
deposit; but there are in it no determinable plant-remains. 
A few pieces of much decayed worm-eaten drift-wood are 
