occurred in Europe during the latest geological epochs. The study of the
geographical distribution of the European fauna, as expounded in this
work, will in many instances confirm the theories as to geographical
changes based upon geological foundations. But in every case the views
herein advocated are founded upon the geographical distribution of
living and extinct organisms alone.

A terrestrial mammal like the deer can, under ordinary circumstances,
only reach one part of a country from another by walking or running to
it; but a beetle, such as the cockchafer, has two different modes of
progression. It may walk or fly. In both, however, there is a third mode
of transport--an involuntary one. The deer may be suddenly seized by a
flood whilst crossing a river, and carried far away without necessarily
coming to grief. The beetle in a similar manner could be transported to
a distant country, or it might be caught in a whirlwind and blown
hundreds of miles off.

We may thus distinguish between the natural or active and the accidental
or passive means of distribution of animals. The active mode of
dispersal again may be only migratory, as in most animals, or periodic
and migratory, as in some birds and fishes. It is of course the tendency
of every species to spread in all directions from its original home,
provided it does not encounter obstacles, such as want of food,
unsuitability of climate or soil, or barriers such as mountains, rivers,
or the sea. Birds might be thought to be little interfered with by any
of these barriers, but, as Dr. Wallace has shown, they are almost as
much affected by them in their distribution as mammals are.

This then is the ordinary migratory distribution. Periodic distribution
obtains with migratory birds and fishes. The annual flight of swallows
to their northern summer residence comes under the heading of periodic
migration or distribution, but apart from this, the swallow must seek to
extend its range by the ordinary method, like every other animal.
Similarly, the herring migrates periodically into shallow water to
spawn, only to return again to its deeper home, where, as its numbers
increase, there must be a tendency to spread. We have in these cases,
therefore, both a periodic and an ordinary movement of migration.

Now, in studying the composition of a fauna, and especially its origin,
it is of the utmost importance to be able to determine approximately the
percentage of accidental