We can clearly trace
its gradual advance. In the Carboniferous and Jurassic the
vegetation of the Arctic regions had been the same as that of
England; in the Eocene palms can flourish in England, but not
further north; in the Pliocene the palms and bamboos and
semi-tropical species are driven out of Europe; in the
Pleistocene the ice-sheet advances to the valleys of the Thames
and the Danube (and proportionately in the United States), every
warmth-loving species is annihilated, and our grasses, oaks,
beeches, elms, apples, plums, etc., linger on the green southern
fringe of the Continent, and in a few uncovered regions, ready to
spread north once more as the ice creeps back towards the Alps or
the Arctic circle. Thus, in few words, did Europe and North
America come to have the vegetation we find in them to-day.
The next broad characteristic of our landscape is the spreading
carpet of grass. The interest of the evolution of the grasses
will be seen later, when we shall find the evolution of the
horse, for instance, following very closely upon it. So striking,
indeed, is the connection between the advance of the grasses and
the advance of the mammals that Dr.
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