SPECIFIC AREAS 
i47 
be quite likely to become broken into separate portions 
but this too is not an explanation of many of the cases 
quoted. The results of geological science must be sum- 
moned to our aid. By their help many of the problems can 
be solved, and botanical geography can in its turn render 
useful assistance to the geologist, by pointing out former 
connecting lines, along which species were formerly con- 
tinuous or were at least able to migrate. Subsidence may 
break a continent up into islands, and thus a species that 
formerly occupied the large area may become disconnected 
and form the starting point of new species, which taken 
together form a genus. Or again, elevation may unite islands 
to continents and cause a mingling of floras, or may form 
mountain-ranges, which act as barriers. Smaller geological 
changes than these are sufficient to cause changes of the 
climate, and thus of the flora, of a region. 
The higher groups of plants appear to have developed 
in comparatively recent times— during the Tertiary period 
of the earth’s history. At first the climate seems to have 
been warm, and subtropical forms extended even into the 
northern polar regions. Gradually the climate grew colder, 
and species suited to these conditions appeared in the north 
and gradually followed the tropical plants southwards, ulti- 
mately giving rise to our present temperate flora ; as the cold 
increased, actual arctic species appeared. In the far north 
the land is continuous or nearly so right round the globe, 
and many arctic species are circumpolar in distribution at 
the present day. The similarity of the floras of temperate 
Europe and North America above-mentioned is usually 
ascribed to the earlier period when these plants were cir- 
cumpolar ; they were then gradually driven south and gave 
rise there to new forms. Later in the Tertiary period came 
the Glacial Era ; the cold spread southwards, driving the 
tropical plants mostly into the southern hemisphere ; in 
what is now the northern torrid zone a subtropical flora 
probably occurred, whilst arctic species came southwards 
over America and Europe to the latitude of the Pyrenees. 
Afterwards, as the cold retreated, these various northern 
plants were driven back again by the advance of the southern 
vegetation, but besides going northwards they would also 
go up the mountains as new and suitable territory became 
10 — 2 
