There are two
reasons for this, one being that the islands decrease rapidly in size
from Timor to Lombock, and can therefore support a decreasing number
of species; the other and the more important is, that the distance of
Australia from Timor cuts off the supply of fresh immigrants, and has
thus allowed variation to have full play; while the vicinity of
Lombock to Bali and Java has allowed a continual influx of fresh
individuals which, by crossing with the earlier immigrants, has
checked variation.
To simplify our view of the derivative origin of the birds of these
islands let us treat them as a whole, and thus perhaps render more
intelligible their respective relations to Java and Australia.
The Timor group of islands contains:
Javan birds . . . . . . . 36 Australian birds . . . 13
Closely allied species . . 11 Closely allied species . . 35
Derived from Java . . . . 47 Derived from Australia . . . 48
We have here a wonderful agreement in the number of birds belonging to
Australian and Javanese groups, but they are divided in exactly a
reverse manner, three-fourths of the Javan birds being identical
species and one-fourth representatives, while only one-fourth of the
Australian forms are identical and three-fourths representatives.
