e verge of extinction, whilst
many not only maintain their position in the constant struggle for
existence, but are even extending their range.

The problem of tracing the origin of the British fauna, or at least that
of some of the more characteristic members of every section or element,
appears at first a somewhat difficult task. Indeed, the means of
dispersal of the various groups of animals are so different that it
occurred to me it might be better to deal with the mammals, the birds,
the reptiles, and so forth, all separately. This idea I have attempted
to follow to some extent, with most satisfactory results. The British
fauna of the present day is no doubt complex, but no more so than the
fauna of the most recent of our geological deposits--the Pleistocene.
However, when we go back still further and look at the earlier Tertiary
remains, we find the fauna becoming less complex. Northern species
disappear, and the strata are entirely filled with the remains of
southern animals and plants. Geologists indeed are quite unanimous in
their belief, that the fauna of the British Islands during the earlier
epochs of the Tertiary Era was a southern one; that it then gradually
became more temperate, until at last, in more recent times, decidedly
northern forms invaded the country. These seem to have driven out--to
some extent at least--the southern species; but more recently again, the
southerners, reinforced by an eastern contingent, appear to have gained
territory and are advancing into the area held by the northerners. The
eastern invasion does not seem to have affected Ireland at all, and we
find the country there divided between the southern and northern
animals. We can thus roughly construct a map as I have done here,
showing, by means of horizontal and sloping lines, the principal areas
inhabited at the present time by the species of northern, southern, and
eastern origin (Fig. 1).

[Illustration: Fig. 1.--Map of the British Islands, indicating
approximately the areas inhabited by the northern, southern, and eastern
animals. The horizontal lines represent the areas of northern species,
the sloping lines those of southern and eastern ones.]

In the problems which are being discussed in this work I have often
found it of advantage, in order to facilitate the comprehension of the
arguments used, to give maps. Some of these represent the geographical
conditions at the particular epoch referred to in the text, but they