iting these islands are either such as are now extinct in Europe,
or have become modified since their arrival there from the continent at
a time when extensive land-connections allowed a free migration by land.

The theory of accidental introductions is an extremely popular one. It
allows free scope to a host of speculations, and once the idea has taken
firm root that a certain species is introduced, especially among the
class of naturalists who by way of experiment are wont to create new
centres of dispersion in their own neighbourhood, evidence to the
contrary must be of the most convincing nature to shake the popular
belief. Thus, it is almost regarded as an established fact by
conchologists and others, that the fresh-water mussel (_Dreyssensia
polymorpha_) was introduced into England at the beginning of this
century. Though it has been proved that this species is quite unable to
live in pure sea-water, yet the view that it has been carried from the
Black Sea ports to this country attached to the bottom of ships is
maintained by many, whilst others incline to the theory that the shell
came with timber. But _Dreyssensia polymorpha_ was by no means always
confined to the Caspian and Black Sea areas; it occurs abundantly in the
lower continental boulder-clay (see p. 230), and no doubt it had at one
time a much wider geographical distribution. It appears to me possible,
that it was able to maintain itself in certain fresh-water lakes and
slow-flowing rivers in Northern Europe, from which it might have spread
since the introduction of canals into Europe at the beginning of the
century. As the larva of this fresh-water mussel is free-swimming, its
propagation is much favoured by canals. Quickly-flowing rivers are fatal
to its existence, since the delicate larvae are swept out to sea and
perish. Such an hypothesis as this is strengthened by the fact of its
recent discovery in a sandy layer fifteen feet below the present surface
under the streets of London in a deposit which probably, as Mr. Woodward
remarks (p. 8), was accumulated in the early days of the city's
existence. In spite of Mr. Woodward's interesting find, and Dr.
Jeffreys' opinion, who always maintained that this shell was indigenous
to England, popular belief still clings tenaciously to the introduction
theory.

Among man's intentional introductions into a new country, no instance is
better known than that of the rabbit to Australia. Rabbits are entirely
confi