ocene Arctic
Flora--Mild Arctic Climates of the Cretaceous Period--Stratigraphical
Evidence of Long-continued Mild Arctic Conditions--The Causes of Mild
Arctic Climates--Geographical Conditions Favouring Mild Northern
Climates in Tertiary Times--The Indian Ocean as a Source of Heat in
Tertiary Times--Condition of North America During the Tertiary
Period--Effect of High Excentricity on Warm Polar Climates--Evidences
as to Climate in the Secondary and Palaeozoic Epochs--Warm Arctic
Climates in Early Secondary and Palaeozoic Times--Conclusions as to the
Climates of Secondary and Tertiary Periods--General View of Geological
Climates as Dependent on the Physical Features of the Earth's
Surface--Estimate of the Comparative Effects of Geographical and
Physical Causes in Producing Changes of Climate.

If we adopt the view set forth in the preceding chapter as to the character
of the glacial epoch and of the accompanying alternations of climate, it
must have been a very important agent in producing changes in the
distribution of animal and vegetable life. The intervening mild periods,
which almost certainly occurred during its earlier and later phases, may
have been sometimes more equable than even our present insular climate, and
severe frosts were probably then unknown. During the four or five {59}
thousand years that each specially mild period may have lasted, some
portions of the north temperate zone, which had been buried in snow or ice,
would become again clothed with vegetation and stocked with animal life,
both of which, as the cold again came on, would be driven southward, or
perhaps partially exterminated. Forms usually separated would thus be
crowded together, and a struggle for existence would follow, which must
have led to the modification or the extinction of many species. When the
survivors in the struggle had reached a state of equilibrium, a fresh field
would be opened to them by the later ameliorations of climate; the more
successful of the survivors would spread and multiply; and after this had
gone on for thousands of generations, another change of climate, another
southward migration, another struggle of northern and southern forms would
take place.

But if the last glacial epoch has coincided with, and has been to a
considerable extent caused by, a high excentricity of the earth's orbit, we
are naturally led to expect that earlier glacial epochs would have occ