A writer asks: “The question is not how the escaped from Australia etc, but how they got back there. But the evolutionist faces this problem as well. Would one of you care to volunteer your answer?”
The marsupials and placentals diverged from a pantotheran stem stock in the late Cretaceous. The first marsupials appeared in North America approximately 80 million years ago, e.g. Alphadon (marsupials can be distinguished from placentals by their dentition – marsupials have 3 premolars and 4 molars whilst placentals have 3-5 premolars and 3 molars). Towards the end of the Late Cretaceous, marsupials start appearing in South America (Peru and Bolivia). In the Eocene marsupials radiated into Europe, North Africa and reached Asia by the Oligocene. However these groups rapidly became extinct. South America parted company with North America in the Eocene, effectively blocking the rapid radiation of placentals in North America at this time from spreading to South America. During the Eocene, marsupials reached Antarctica, which was attached to South America and Australia at this time. Marsupials could follow a belt of Northophagus vegetation all the way around from southern South America, across Antarctica into southern Australia. The first marsupials appear in Australia in the Oligocene via this route. Australia parted company from Antarctica in the Miocene, effectively isolating the marsupial fauna here.
If you want to read in plain English, here’s the Wikipedia version. But, frankly, I like the other … it makes me feel smart.