(geologically speaking) within recent or more remote times. The
better the fauna is known, both recent and fossil, the more precisely
can the period of connection be indicated, and its duration determined.

FOOTNOTES:

[1] The numbers in brackets throughout this work refer to the
page-number in the Bibliography at the end.

[2] A map giving its exact distribution in Ireland will be found on p.
300, and a figure of the slug on p. 298.

[3] The term _endemic_ will be employed throughout this work as applied
to species peculiar to a country and not found elsewhere.
_Autochthonous_ will be used in speaking of a species which has
originated in a country to which, however, it is not peculiar; _e.g._,
the Chamois is an autochthonous Alpine species, but occurs also in the
Pyrenees and Caucasus. An _indigenous_ species is one native to a
country, as opposed to the term "introduced," and is applicable to all
species which have reached it by ordinary migration.

[4] I might refer any one more specially interested in these
introductions to an article on this subject in the _Irish Naturalist_ of
March 1898, by Mr. Barrett-Hamilton.

[5] I should recommend those who are particularly interested in the full
history of the Irish frog to read the notes on this subject contained in
vol. ii. of the _Irish Naturalist_.

CHAPTER II.

PRELIMINARY CONSIDERATIONS.

I intend to give in this chapter a general outline of the subject which
will be discussed in the subsequent ones. This will include a brief
history of the great events, in recent geological times, which have
modified the evolution of the European fauna by the influence which they
have exerted on the course of the successive streams of migration.

The composition of the European fauna is the first item which will have
to be taken into consideration. But not only must the existing species
of animals be dealt with: the extinct ones, too, at least those which
have lived in Europe during late Tertiary times, will be useful for our
inquiries. A knowledge of the past faunas is a most important factor in
tracing the original home of the European animals.

Where a species first originated, whether this was in one or several
places, or, in other words, where it first had its home, cannot be
determined with absolute certainty in the present state of our
knowledge, but as a rule it can be indicated approximately with a fair
amount of precision. In a few instances, species ma