t are now barren wastes, covered for ten months of the year with snow
and ice. The arctic zone has, therefore, been in past times capable of
supporting almost all the forms of life of our temperate regions; and we
must take account of this condition of things whenever we have to
speculate on the possible migrations of organisms between the old and
new continents.

_The Conditions which have determined Distribution._

When we endeavour to explain in detail the facts of the existing
distribution of organic beings, we are confronted by several preliminary
questions, upon the solution of which will depend our treatment of the
phenomena presented to us. Upon the theory of descent which we have
adopted, all the different species of a genus, as well as all the genera
which compose a family or higher group, have descended from some common
ancestor, and must therefore, at some remote epoch, have occupied the
same area, from which their descendants have spread to the regions they
now inhabit. In the numerous cases in which the same group now occupies
countries separated by oceans or seas, by lofty mountain-chains, by wide
deserts, or by inhospitable climates, we have to consider how the
migration which must certainly have taken place has been effected. It is
possible that during some portion of the time which has elapsed since
the origin of the group the interposing barriers have not been in
existence; or, on the other hand, the particular organisms we are
dealing with may have the power of overpassing the barriers, and thus
reaching their present remote dwelling-places. As this is really the
fundamental question of distribution on which the solution of all its
more difficult problems depends, we have to inquire, in the first place,
what is the nature of, and what are the limits to, the changes of the
earth's surface, especially during the Tertiary and latter part of the
Secondary periods, as it was during those periods that most of the
existing types of the higher animals and plants came into existence;
and, in the next place, what are the extreme limits of the powers of
dispersal possessed by the chief groups of animals and plants. We will
first consider the question of barriers, more especially those formed by
seas and oceans.

_The Permanence of Oceans._

It was formerly a very general belief, even amongst geologists, that the
great features of the earth's surface, no less than the smaller ones,
were subject to contin