In Timor.
Javan birds . . . . 33 23 11
Australian birds . . 4 5 10
Here we see plainly the course of the migration which has been going
on for hundreds or thousands of years, and is still going on at the
present day. Birds entering from Java are most numerous in the island
nearest Java; each strait of the sea to be crossed to reach another
island offers an obstacle, and thus a smaller number get over to the
next island. [The names of all the birds inhabiting these islands are
to be found in the "Proceedings of the Zoological Society of London"
for the year 1863.] It will be observed that the number of birds that
appear to have entered from Australia is much less than those which
have come from Java; and we may at first sight suppose that this is
due to the wide sea that separates Australia from Timor. But this
would be a hasty and, as we shall soon see, an unwarranted
supposition. Besides these birds identical with species inhabiting
Java and Australia, there are a considerable number of others very
closely allied to species peculiar to those countries, and we must
take these also into account before we form any conclusion on the
matter. It will be as well to combine these with the former table thus:
In Lombock. In Flores. In Timor.
Javan birds . . . . . . . . . . .33 23 11
Closely allied to Javan birds . . 1 5 6
Total . . . . . . . . . . . . . . 34 28 17
Australian birds . . . . . . . . . 4 5 10
Closely allied to Australian birds 3 9 26
Total . . . . . . . . . . . . . . 7 14 36
We now see that the total number of birds which seem to have been
derived from Java and Australia is very nearly equal, but there is
this remarkable difference between the two series: that whereas the
larger proportion by far of the Java set are identical with those
still inhabiting that country, an almost equally large proportion of
the Australian set are distinct, though often very closely allied
species. It is to be observed also, that these representative or
allied species diminish in number as they recede from Australia, while
they increase in number as they recede from Java. 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.
