e forced to migrate
frequently owing to scarcity of food or temporary changes of climate,
many of the invertebrates remain practically unaffected by either. Most
of our land mollusca, for instance, are satisfied with meagre provender,
and stand extremes of climate well, as long as there is sufficient
moisture. As a result of their peculiar disposition, many of them, no
doubt, have survived through several geological epochs, and have
witnessed vast geographical revolutions in their immediate surroundings,
whilst mammals are comparatively short-lived. Being driven from one
country to another, and exposed to innumerable enemies, new types appear
and old ones rapidly vanish; in fact, there are almost constant changes
in the mammalian fauna as we pass from one epoch to another.

I have until now referred more particularly to the British fauna and the
North European in general, because the history of our own animals
interests us all more than those of any other European area. It is,
moreover, preferable to commence our investigations into the origin of
the European fauna by the study of a small district. This should, if
possible, be an island. If we took a slice of the continent like France
or Germany, we should find the problem more complex. Instead of choosing
the British Islands, we might, however, take an island like Corsica or
Sardinia. In either of these we should discover peculiarities in the
composition of their fauna precisely similar to those which I have
indicated to be present in the British fauna. We should find probably a
more striking endemic[3] element, which with us is so meagre that it
can almost be left unnoticed; the main features, however, remain nearly
the same. The fauna of both of these islands is composed of a strong
southern element, of an eastern and a northern one, and in addition we
have here species whose ancestors lived in Western Europe.

Before investigating more minutely the problems suggested by the
composition of the faunas of these insular and also of some continental
areas, it is necessary that we should thoroughly understand all about
the migrations of animals. One of the principal objects of this work is
to show how the autochthonous animals of Europe, _i.e._, those which
have originated there, may be distinguished from the immigrants, and to
trace the latter to the home of their ancestors. But in doing so, it is
necessary to refer to the many important geographical changes which have