movements which any country has undergone, and the
succession of such movements, can be determined with much accuracy;
but geology alone can tell us nothing of lands which have entirely
disappeared beneath the ocean. Here physical geography and the
distribution of animals and plants are of the greatest service. By
ascertaining the depth of the seas separating one country from another,
we can form some judgment of the changes which are taking place. If
there are other evidences of subsidence, a shallow sea implies a former
connexion of the adjacent lands; but if this evidence is wanting, or if
there is reason to suspect a rising of the land, then the shallow
sea may be the result of that rising, and may indicate that the two
countries will be joined at some future time, but not that they have
previously been so. The nature of the animals and plants inhabiting
these countries will, however, almost always enable us to determine this
question. Mr. Darwin has shown us how we may determine in almost every
case, whether an island has ever been connected with a continent or
larger land, by the presence or absence of terrestrial Mammalia and
reptiles. What he terms "oceanic islands" possess neither of these
groups of animals, though they may have a luxuriant vegetation, and a
fair number of birds, insects, and landshells; and we therefore conclude
that they have originated in mid-ocean, and have never been connected
with the nearest masses of land. St. Helena, Madeira, and New Zealand
are examples of oceanic islands. They possess all other classes of life,
because these have means of dispersion over wide spaces of sea, which
terrestrial mammals and birds have not, as is fully explained in Sir
Charles Lyell's "Principles of Geology," and Mr. Darwin's "Origin of
Species." On the other hand, an island may never have been actually
connected with the adjacent continents or islands, and yet may possess
representatives of all classes of animals, because many terrestrial
mammals and some reptiles have the means of passing over short distances
of sea. But in these cases the number of species that have thus migrated
will be very small, and there will be great deficiencies even in birds
and flying insects, which we should imagine could easily cross over.
The island of Timor (as I have already shown in Chapter XIII) bears this
relation to Australia; for while it contains several birds and insects
of Australian forms, no Australian mammal or r