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.
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