chs, and thus enables us to
prove that certain areas have been long isolated and the fauna and flora
allowed time for special development. Here, too, our knowledge is
exceedingly imperfect, though the blanks upon the geological map of the
world are yearly diminishing in extent. Lastly, as a most valuable
supplement to geology, we require to know approximately, the depth and
contour of the ocean-bed, since this affords an important clue to the
former existence of now-submerged lands, uniting islands to continents, or
affording intermediate stations which have aided the migrations of many
organisms. This kind of information has only been partially obtained during
the last few years; and it will be seen in the latter part of this volume,
that some of the most recent deep-sea soundings have afforded a basis for
an explanation of one of the most difficult and interesting questions in
geographical biology--the origin of the fauna and flora of New Zealand.

Such are the various classes of evidence that bear directly on the question
of the distribution of organisms; but there are others of even a more
fundamental character, and the importance of which is only now beginning to
be recognised by students of nature. These are, firstly, the wonderful
alterations of climate which have occurred in the temperate and polar
zones, as proved by the evidences of glaciation in the one and of luxuriant
vegetation in the other; and, secondly, the theory of the permanence of
existing continents and oceans. If glacial epochs in temperate lands and
mild climates near the poles have, as now believed by men of eminence,
occurred several times over in the past history of the earth, the effects
of such great and repeated changes, both on the migration, modification,
and extinction, of species, must have been of overwhelming importance--of
more importance perhaps than even the geological changes of sea and land.
It is therefore necessary to consider the evidence for these climatal
changes; {10} and then, by a critical examination of their possible causes,
to ascertain whether they were isolated phenomena, were due to recurrent
cosmical actions, or were the result of a great system of terrestrial
development. The latter is the conclusion we arrive at; and this conclusion
brings with it the conviction, that in the theory which accounts for both
glacial epochs and warm polar climates, we have the key to explain and
harmonize many of the most anomalous bio