[168]
We shall find on examination that this view of the general permanence of
the oceanic and continental areas, with constant minor fluctuations of
land and sea over the whole extent of the latter, enables us to
understand, and offer a rational explanation of, most of the difficult
problems of geographical distribution; and further, that our power of
doing this is in direct proportion to our acquaintance with the
distribution of fossil forms of life during the Tertiary period. We
must, also, take due note of many other facts of almost equal importance
for a due appreciation of the problems presented for solution, the most
essential being, the various powers of dispersal possessed by the
different groups of animals and plants, the geological antiquity of the
species and genera, and the width and depth of the seas which separate
the countries they, inhabit. A few illustrations will now be given of
the way in which these branches of knowledge enable us to deal with the
difficulties and anomalies that present themselves.
_The Distribution of Marsupials.
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