has
not been the case. In the strata, whose length I have reckoned alone, I
have not found a single fragment of a foreign rock so large as a child's
head."[19]

Now it is quite impossible to ignore or evade the force of this testimony
as to the continuous warm climates of the north temperate and polar zones
throughout Tertiary times. The evidence extends over a vast area, both in
space and time, it is derived from the work of the most competent living
geologists, and it is absolutely consistent in its general tendency. We
have in the Lower Cretaceous period an almost tropical climate in France
and England, a somewhat lower temperature in the United States, and a mild
insular climate in the Arctic regions. In each successive period the
climate becomes somewhat less tropical; but down to the Upper Miocene it
remains warm temperate in Central Europe, and cold temperate within the
polar area, with not a trace of any intervening periods of Arctic cold. It
then gradually cools down and merges through the Pliocene into the glacial
epoch in Europe, while in the Arctic zone there is a break in the record
between the Miocene and the recent glacial deposits.[20]

{79}

Accepting this as a substantially correct account of the general climatic
aspect of the Tertiary period in the northern hemisphere, let us see
whether the principles we have already laid down will enable us to give a
satisfactory explanation of its causes.

_The Causes of mild Arctic Climates._--In his remarkable series of papers
on "Ocean Currents," the late Dr. James Croll has proved, with a wealth of
argument and illustration whose cogency is irresistible, that the very
habitability of our globe is due to the equalizing climatic effects of the
waters of the ocean; and that it is to the same cause that we owe, either
directly or indirectly, almost all the chief diversities of climate between
places situated in the same latitude. Owing to the peculiar distribution of
land and sea upon the globe, more than its fair proportion of the warm
equatorial waters is directed towards the western shores of Europe, the
result being that the British Isles, Norway, and Spitzbergen, have all a
milder climate than any other parts of the globe in corresponding
latitudes. A very small portion of the Arctic regions, however, obtains
this benefit, and it thus remains, generally speaking, a land of snow and
ice, with too short a summer to nourish more than a very scanty and
fugiti