y possibly have had
a dual origin. The majority of naturalists doubt that there are any
such, but it seems to me that almost the same forces may have acted in
different localities on certain forms so as to produce, in very
exceptional circumstances, similar species. The vast majority of
animals, however, have no doubt originated in one locality; or, we might
say, almost all species have but one home.

We may assume that every animal gradually extends its range by
migration, as the result of the natural increase of the species
necessitating a search for fresh feeding grounds. Every species thus
tends to slowly take possession of all the habitable parts of the globe
to which it has access. They would all naturally spread from their
original homes in every direction, unless prevented by an impassable
barrier. We have already learned that to all land animals, the sea acts
as such a barrier. Mountains and rivers act also in a similar way, but
not to the same extent. It is not difficult to understand also that a
forest may be a formidable barrier to a typical inhabitant of the open
country and _vice versa_, whilst a desert is impassable to almost all
terrestrial organisms. Some species are scarcely affected by climate,
and flourish equally well in the tropics and in temperate or cold
countries; the majority, however, are greatly influenced by it. "No more
striking illustration," remarks Merriam (p. 38), "could be desired of
the potency of climate compared with the inefficiency of physical
barriers, than is presented by the almost total dissimilarity of the
North American Tropical and Sonoran Regions, though in direct contact,
contrasted with the great similarity of the Boreal Regions of North
America and Eurasia, now separated by broad oceans, though formerly
united, doubtless, in the region of Behring Sea."

To return to the composition of the European fauna, we now know
positively that a number of the mammals and birds inhabiting Central and
Eastern Europe are of Siberian origin. How they came, and when, will
form the subject for discussion in Chapter V. At present it will suffice
to mention that in the superficial deposits belonging to the Pleistocene
series of the North European plain have been discovered the remains of
many typical members of the Siberian Steppe-fauna. Some of these, such
as the Saiga-Antelope (_Saiga tartarica_), Fig. 2, still inhabit
portions of Eastern Europe, whilst others have retreated to their na