In the early Tertiary period, as we have seen, the climate of the
northern hemisphere, as shown by the Eocene animals and plants,
was very much hotter than it is at present--partaking, indeed, of a
sub-tropical character. In the Middle Tertiary or Miocene period,
the temperature, though not so high, was still much warmer than
that now enjoyed by the northern hemisphere; and we know that the
plants of temperate regions at this time flourished within the
Arctic circle. In the later Tertiary or Pliocene period, again,
there is evidence that the northern hemisphere underwent a further
progressive diminution of temperature; though the climate of Europe
generally seems at the close of the Tertiary period to have been
if anything warmer, or at any rate not colder, than it is at
the present day. With the commencement of the Quaternary period,
however, this diminution of temperature became more decided; and
beginning with a temperate climate, we find the greater portion
of the northern hemisphere to become gradually subjected to all
the rigours of intense Arctic cold. All the mountainous regions
of Northern and Central Europe, of Britain, and of North America,
became the nurseries of huge ice-streams, and large areas of the
land appear to have been covered with a continuous ice-sheet.
The Arctic conditions of this, the well-known "Glacial period,"
relaxed more than once, and were more than once re-established
with lesser intensity.
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