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 reptile is found in it,
and a great number of the most abundant and characteristic forms of Australian
birds and insects are entirely absent. Contrast this with the British Islands, in,
which a large proportion of the plants, insects, reptiles, and Mammalia of the
adjacent parts of the continent are fully represented, while there are no
remarkable deficiencies of extensive groups, such as always occur when there is
reason to believe there has been no such connexion. The case of Sumatra,
Borneo, and Java, and the Asiatic continent is equally clear; many large
Mammalia, terrestrial birds, and reptiles being common to all, while a large
number more are of closely allied forms. Now, geology has taught us that this
representation by allied forms in the same locality implies lapse of time, and we
therefore infer that in Great Britain, where almost every species is absolutely
identical with those on the Continent, the separation has been very recent; while
in Sumatra and Java, where a considerable number of the continental species are
represented by allied forms, the separation was more remote.
From these examples we may see how important a supplement to geological
evidence is the study of the geographical distribution of animals and plants, in
determining the former condition of the earth's surface; and how impossible it is
to understand the former without taking the latter into account. The productions
of the Aru Islands offer the strangest evidence, that at no very distant epoch they
formed a part of New Guinea; and the peculiar physical features which I have
described, indicate that they must have stood at very nearly the same level then
as they do now, having been separated by the subsidence of the great plain
which formerly connected them with it.
Persons who have formed the usual ideas of the vegetation of the tropics who
picture to themselves the abundance and brilliancy of the flowers, and the
magnificent appearance of hundreds of forest trees covered with masses of
coloured blossoms, will be surprised to hear, that though vegetation in Aru is
highly luxuriant and varied, and would afford abundance of fine and curious
plants to adorn our hothouses, yet bright and showy flowers are, as a general
rule, altogether absent, or so very scarce as to produce no effect whatever on the
general scenery. To give particulars: I have visited five distinct localities in the