They went naked and houseless, and lived on fish and grubs and worms and wild fruits, and were just plain savages, for all their smartness.
With a country as big as the United States to live and multiply in, and with no epidemic diseases among them till the white man came with those and his other appliances of civilization, it is quite probable that there was never a day in his history when he could muster 100,000 of his race in all Australia.

He diligently and deliberately kept population down by infanticide--largely; but mainly by certain other methods.

He did not need to practise these artificialities any more after the white man came.
The white man knew ways of keeping down population which were worth several of his.

The white man knew ways of reducing a native population 80 percent.

in 20 years.