c. In the case of marine animals, we are as yet very far from
knowing the exact limits of distribution of many species within
our present seas; so that conclusions drawn from living forms
as to extinct species are apt to prove incorrect. For instance,
it has recently been shown that many shells formerly believed to
be confined to the Arctic Seas have, by reason of the extension
of Polar currents, a wide range to the south; and this has thrown
doubt upon the conclusions drawn from fossil shells as to the
Arctic conditions under which certain beds were supposed to have
been deposited.
d. The distribution of animals at the present day is certainly
dependent upon other conditions beside climate alone; and the causes
which now limit the range of given animals are certainly such as
belong to the existing order of things. But the establishment of
the present order of things does not date back in many cases to
the introduction of the present species of animals. Even in the
case, therefore, of existing species of animals, it can often
be shown that the past distribution of the species was different
formerly to what it is now, not necessarily because the climate
has changed, but because of the alteration of other conditions
essential to the life of the species or conducing to its extension.
Still, we are in many cases able to draw completely reliable
conclusions as to the climate of a given geological period, by
an examination of the fossils belonging to that period.
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