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.
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. This is the most
important fact which we can elicit from a study of the birds of these
islands, since it gives us a very complete clue to much of their past
history.
Change of species is a slow process--on that we are all agreed, though
we may differ about how it has taken place. The fact that the Australian
species in these islands have mostly changed, while the Javan species
have almost all remained unchanged, would therefore indicate that the
district was first peopled from Australia. But, for this to have been
the case, the physical conditions must have been very different from
what they are now. Nearly three hundred miles of open sea now separate
Australia from Timor, which island is connected with Java by a chain of
broken land divided by straits which are nowhere more than about twenty
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