inents, we shall not be surprised that many widespread forms in
either continent have not crossed into the other; and that while the
skunks (Mephitis), the pouched rats (Saccomyidae), and the turkeys
(Meleagris) are confined to America, the pigs and the hedgehogs, the
true flycatchers and the pheasants are found only in the Euro-Asiatic
continent. But, just as there have been periods which facilitated
intermigration between America and the Old World, there have almost
certainly been periods, perhaps of long duration even geologically, when
these continents have been separated by seas as wide as, or even wider
than, those of the present day; and thus may be explained such curious
anomalies as the origination of the camel-tribe in America, and its
entrance into Asia in comparatively recent Tertiary times, while the
introduction of oxen and bears into America from the Euro-Asiatic
continent appears to have been equally recent.[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._

This singular and lowly organised type of mammals constitutes almost the
sole representative of the class in Australia and New Guinea, while it
is entirely unknown in Asia, Africa, or Europe. It reappears in America,
where several species of opossums are found; and it was long thought
necessary to postulate a direct southern connection of these distant
countries, in order to account for this curious fact of distribution.
When, however, we look to