190 
CROMER FOREST-BED. 
and we can apply to these the same reasoning as is used above, 
merely changing north for south. The southern species would 
be able, under these circumstances, to spread down the valley of 
the Rhine in a continuous stream. Their march would not be 
checked by the previous occupation of the country by arctic 
forms, for there does not appear to have been any land-connexion 
between Britain and Scandinavia till a later period. The 
southern fauna could thus occupy the whole of those portions of 
Britain where the climate was suitable. The local extermination 
of any species by an exceptionally severe winter would be of 
little consequence, for it could be immediately replaced by other 
individuals, and there were no boreal forms to take possession of 
its district. 
Thus it is that in the Forest-bed we find a distinctly southern 
land fauna contemporaneous with an equally marked arctic 
marine fauna; the plants at the same time showing that the 
climate was much the same as that of Norfolk at the present 
day, though perhaps more continental, i.e., hotter in summer 
and colder in winter. 
