ater invertebrates too have no doubt
reached us from the north. Some of them may have originated in
Scandinavia or within the Arctic Circle, but others probably came still
farther, either from America or even from Asia, and used the Arctic
land-connection _via_ Greenland in their migration to Europe. As I shall
give a number of additional instances of such migrants in the succeeding
chapters, I need not, perhaps, dwell upon them now any longer, except to
mention a few of the more typical ones. _Vertigo alpestris_, a minute
snail with an amber-coloured shell, and our freshwater pearl-mussel,
_Unio (Margaritana) margaritifer_, belong to this migration. Then among
butterflies we may cite the Marsh-ringlet (_Coenonympha typhon_), and
among beetles, _Pelophila borealis_ and _Blethisa multipunctata_. There
are a number of northern spiders, among which a few certainly indicate
an Arctic origin, or at any rate, that they have wandered to Europe
across Greenland and the old Arctic land-connections. _Bathyphantes
nigrinus_, _Linyphia insignis_, and _Drapetisca socialis_, for instance,
are three British species whose range indicates a northern origin, and
which also occur, according to Mr. Carpenter, in North America. Mr.
Carpenter also tells me that the Collembolan, _Isotoma littoralis_, is a
typical northern migrant. He has recently discovered it in the west of
Ireland, its only station in the British Islands.

Among the crustacea, the genus _Apus_ forms an exceedingly interesting
illustration of the northern migration, _Apus glacialis_ having been
discovered in a Scottish pleistocene freshwater deposit, whilst it is
now almost confined to the Arctic regions.

To the same group of animals also belong the three remarkable species of
freshwater sponges, _Ephydatia crateriformis_, _Heteromeyenia Ryderi_,
and _Tubella pensylvanica_, which Dr. Hanitsch has described from some
lakes in Western Ireland. None of these are known from Great Britain or
from the continent of Europe. A few North American plants grow wild in
the same district. That any of these should owe their existence in
Ireland to accidental introduction appears to me exceedingly improbable.
In a former contribution to this subject (_a_, p. 475) I assumed that
these American plants and animals had migrated to Europe at the same
time as the other northern forms referred to. My friend Mr. Carpenter,
however, takes exception to this (p. 383), and I quite recognise the
force