Such groups as the bower birds (Ptilonorhynchus), the black and red cockatoos (Calyptorhynchus), the blue wrens (Malurus), the crowshrikes (Cracticus), the Australian shrikes (Falcunculus and Colluricincla), and many others, which abound all over Australia, would certainly have spread into Timor if it had been united to that country, or even if for any long time it had approached nearer to it than twenty miles.

Neither do any of the most characteristic groups of Australian insects occur in Timor; so that everything combines to indicate that a strait of the sea has always separated it from Australia, but that at one period this strait was reduced to a width of about twenty miles.
But at the time when this narrowing of the sea took place in one direction, there must have been a greater separation at the other end of the chain, or we should find more equality in the numbers of identical and representative species derived from each extremity.

It is true that the widening of the strait at the Australian end by subsidence, would, by putting a stop to immigration and intercrossing of individuals from the mother country, have allowed full scope to the causes which have led to the modification of the species; while the continued stream of immigrants from Java, would, by continual intercrossing, check such modification.

This view will not, however, explain all the facts; for the character of the fauna of the Timorese group is indicated as well by the forms which are absent from it as by those which it contains, and is by this kind of evidence shown to be much more Australian than Indian.
No less than twenty-nine genera, all more or less abundant in Java, and most of which range over a wide area, are altogether absent; while of the equally diffused Australian genera only about fourteen are wanting.
This would clearly indicate that there has been, until recently, a wide separation from Java; and the fact that the islands of Bali and Lombock are small, and are almost wholly volcanic, and contain a smaller number of modified forms than the other islands, would point them out as of comparatively recent origin.

A wide arm of the sea probably occupied their place at the time when Timor was in the closest proximity to Australia; and as the subterranean fires were slowly piling up the now fertile islands of Bali and Lombock, the northern shores of Australia would be sinking beneath the ocean.