supposing that no Frog had
ever been seen in Ireland before, since Giraldus Cambrensis, in his
_Topography of Ireland_, mentions that a Frog was found in a meadow near
Waterford in the year 1187.

Certain British species of vertebrates are generally looked upon as
introduced species, though we cannot trace any record of their first
establishment, and it is quite possible that, though there was local
extinction and subsequent local re-introduction, they are truly
indigenous and may never have become totally extinct. Such are, for
instance, the Rabbit (_Lepus cuniculus_) and the Pheasant (_Phasianus
colchicus_). The latter certainly had become naturalised in England
before the Norman invasion.

But cases of introduction such as those above referred to are by no
means confined to the vertebrates, similar instances among invertebrates
being numerous enough. I am sure every naturalist is personally
acquainted with a good number, and it is hardly necessary that I should
quote in any detail after what has been said on the subject generally.
The two species of snails, _Helix pomatia_ and _Cyclostoma elegans_,
both of which occur in England, and which I had occasion to mention
among those experimented on by Darwin, were turned out in several
suitable localities in Ireland by Thompson, but failed to establish
themselves. The former, according to Mr. Kew, was also introduced into
Scotland and Norway, whilst fifty or sixty specimens were brought to
Petersfield in England, but none of these trials at acclimatisation were
successful. As among vertebrates, a large number of the so-called
successful introductions rest upon insufficient evidence.

When we once more carefully review the evidence as to the undoubted
difficulty attendant on intentional introduction of animals by human
agency, placed as they often were in most suitable localities, we must
feel that accidental introduction cannot play an important role in the
making of the fauna of any country. Especially is this the case with an
island fauna. Vertebrates are almost altogether excluded, and
invertebrates must arrive singly as a rule, often stranded on an
inhospitable and unsuitable shore. Their chances of surviving a passage
by sea, of finding suitable food and shelter and a mate in order to
procreate their species, appear to me infinitesimally small. Yet there
may be some such cases. However, I quite agree with Mr. Andrew Murray--a
high authority on geographical distribu