During Quaternary ice age cold periods, when the , both Asia and Australia were joined with their respective islands. The deep water between those two large continental shelf areas was — for over 50 million years — a barrier that kept the flora and fauna of Australia separated from that of Asia. Wallacea consists of islands that were never recently connected by dry land to either of the continental land masses, and thus was populated by organisms capable of crossing the straits between islands. 'Weber's Line' runs through this area (rather to the east of centre), at the tipping point between dominance by species of Asian vs. species of Australian origin.