formed in cold as well
as in warm periods; and it is quite impossible to believe that in every
place and at all epochs all records of the former have been destroyed,
while in a considerable number of instances those of the latter have been
preserved. When to this uniform testimony of the palaeontological evidence
we add the equally uniform absence of any indication of those ice-borne
rocks, boulders, and drift, which are the constant and necessary
accompaniment of every period of glaciation, and which must inevitably
pervade all the marine deposits formed over a wide area so long as the
state of glaciation continues, we are driven to the conclusion that the
last glacial epoch of the northern hemisphere was exceptional, and was not
preceded by numerous similar glacial epochs throughout Tertiary and
Secondary time.

But although glacial epochs (with the one or two exceptions already
referred to) were certainly absent, considerable changes of climate may
have frequently occurred, and these would lead to important changes in the
organic world. We can hardly doubt that some such change occurred between
the Lower and Upper Cretaceous periods, the floras of which exhibit such an
extraordinary contrast in general character. We have also the testimony of
Mr. J. S. Gardner, who has long worked at the fossil floras of the Tertiary
deposits, and who states, that {93} there is strong negative and some
positive evidence of alternating warmer and colder conditions, not glacial,
contained not only in English Eocene, but all Tertiary beds throughout the
world.[28] In the case of marine faunas it is more difficult to judge, but
the numerous changes in the fossil remains from bed to bed only a few feet
and sometimes a few inches apart, may be sometimes due to change of
climate; and when it is recognised that such changes have probably occurred
at all geological epochs and their effects are systematically searched for,
many peculiarities in the distribution of organisms through the different
members of one deposit may be traced to this cause.

_General View of Geological Climates as dependent on the Physical Features
of the Earth's Surface._--In the preceding chapters I have earnestly
endeavoured to arrive at an explanation of geological climates in the
temperate and Arctic zones, which should be in harmony with the great body
of geological facts now available for their elucidation. If my conclusions
as here set forth diverge considerably