insect relations with every part of {256} the globe, and the only rational
explanation of such facts is, that they are indications of very ancient and
once widespread groups, maintaining themselves only in a few widely
separated portions of what was at one time or another the area of their
distribution.

_Land-shells of the Azores._--Like the insects and birds, the land-shells
of these islands have a generally European aspect, but with a larger
proportion of peculiar species. This was to be expected, because the means
by which molluscs are carried over the sea are far less numerous and varied
than in the case of insects;[51] and we may therefore conclude that their
introduction is a very rare event, and that a species once arrived remains
for long periods undisturbed by new arrivals, and is therefore more likely
to become modified by the new conditions, and then fixed as a distinct
type. Out of the sixty-nine known species, thirty-seven are common to
Europe or the other Atlantic islands, while thirty-two are peculiar, though
almost all are distinctly allied to European types. The majority of these
shells, especially the peculiar forms, are very small, and many of them may
date back to beyond the glacial epoch. The eggs of these would be
exceedingly minute, and might occasionally be carried on leaves or other
materials during gales of exceptional violence and duration, while others
might be conveyed with the earth that often sticks to the feet of birds.
There are also, probably, other unknown means of conveyance; but however
this may be, the general character of the land-molluscs is such as to
confirm the conclusions we have arrived at from a study of the birds and
insects,--that these islands have never been connected with a continent,
and have been peopled with living things by such forms only as in some way
or other have been able to reach them across many hundred miles of ocean.

_The Flora of the Azores._--The flowering-plants of the Azores have been
studied by one of our first botanists, Mr. H. C. Watson, who has himself
visited the islands and made extensive collections; and he has given a
complete catalogue of the species in Mr. Godman's volume. As our {257}
object in the present work is to trace the past history of the more
important islands by means of the forms of life that inhabit them, and as
for this purpose plants are sometimes of more value than any class of
animals, it will be well to take advantage of th