Continental and Island Life [pp. 1-29]

THE PRIiVCE T0N RE VIEW.
be irrational to suppose that these peculiar insects either still
inhabit or did once inhabit some part of the continents, and may
be portions of "ancient and wide-spread groups" once widely
diffused, but now restricted to a few insular spots. Among the
land snails, if anywhere, we should find evidence either of autochthonous evolution or of specific change. These animals
have existed on the earth since the Carboniferous period, and,
notwithstanding their proverbial slowness and sedentary habits,
they have contrived to colonize every habitable spot of land on
the globe-that is, unless in some of these places they have
originated de novo. In the Azores there are sixty-nine species
of land snails, of which no less than thirty-two, or nearly one
half, are peculiar, tho nearly all are closely allied to European
types. What, then, is the origin of these thirty-two species,
admitting for the sake of argument that they are really distinct and not merely varietal forms, tho it is well known that
in this group species are often unduly multiplied. Three suppositions are possible. (I) These snails may have originated
in the islands themselves, either by creation or evolution from
lower forms, say from sea snails. (2) They may have been
modified from modern continental species. (3) They may be
unmodified descendants of species of Miocene or Pliocene age
now existing on the continents only as fossils. As the islands
appear to have existed since Miocene times, it is no more improbable that species of that or the Pliocene age should have
found their way to them than that modern species should; and
as we know only a fraction of the Tertiary species of Europe
or Africa, it is not likely that we shall be able to identify all of
these early visitors. Unfortunately no Miocene. or Pliocene
deposits holding remains of land snails are known in the Azores
themselves, so that this kind of evidence fails us. In Madeira
and Porto Santo, however, where there are numerous modern
snails, there are Pliocene beds holding remains of these animals.
In Madeira there are, according to Lyell, 36 Pliocene species,
and in Porto Santo 35, and of these only eight are extinct.
Thus we can prove that many of the peculiar species of these
islands have remained unchanged since Pliocene times. While
differing from modern European shells, several of these species
are very near to European Miocene species. Thus we seem to
I8