came to be generally adopted, and it
was seen that each animal could only have come into existence in some area
where ancestral {13} forms closely allied to it already lived, a real and
important relation was established between an animal and its native
country, and a new set of problems at once sprang into existence. From the
old point of view the _diversities_ of animal life in the separate
continents, even where physical conditions were almost identical, was the
fact that excited astonishment; but seen by the light of the evolution
theory, it is the _resemblances_ rather than the diversities in these
distant continents and islands that are most difficult to explain. It thus
comes to be admitted that a knowledge of the exact area occupied by a
species or a group is a real portion of its natural history, of as much
importance as its habits, its structure, or its affinities; and that we can
never arrive at any trustworthy conclusions as to how the present state of
the organic world was brought about, until we have ascertained with some
accuracy the general laws of the distribution of living things over the
earth's surface.

_Areas of Distribution._--Every species of animal has a certain area of
distribution to which, as a rule, it is permanently confined, although, no
doubt, the limits of its range fluctuate somewhat from year to year, and in
some exceptional cases may be considerably altered in a few years or
centuries. Each species is moreover usually limited to one continuous area,
over the whole of which it is more or less frequently to be met with, but
there are many apparent and some real exceptions to this rule. Some animals
are so adapted to certain kinds of country--as to forests or marshes,
mountains or deserts--that they cannot, permanently, live elsewhere. These
may be found scattered over a wide area in suitable spots only, but can
hardly on that account be said to have several distinct areas of
distribution. As an example we may name the chamois, which lives only on
high mountains, but is found in the Pyrenees, the Alps, the Carpathians, in
some of the Greek mountains and the Caucasus. The variable hare is another
and more remarkable case, being found all over Northern Europe and Asia
beyond lat. 55deg, and also in Scotland and Ireland. In central Europe it
is unknown till we come to the Alps, the Pyrenees, and the Caucasus, where
it again appears. This is one of the best cases known of the {14}
discontinuo