(emphasis mine; I will never write anything half as well-crafted as the phrase below, and Dr. Horrible is to be lauded for it.)
If you look at a map of the Indonesian archipelago, you can draw a line running roughly southwest to northeast and cut it through the channel separating the Indonesian islands of Kalimantan and Sulawesi. This is called the Wallace Line (after explorer Alfred Russel Wallace, who first proposed it) and it is the line dividing the continent of Asia from the rough collection of islands, atolls and shoals known as Oceania. Oh, and Australia, New Guinea, etc. Those are there, too, but they didn't fit the big self-important sentence I was working on so I used artistic license and left them out.
Why this line? Because everything northwest of it contains what Colin McEvedy calls “an up-to-date fauna of placental mammals,” while everything to the southeast does not. Australia and some of the larger islands are an enormous zoo hosting the addled and goofy sister-taxon to the placentals, marsupials, who have all-but died out everywhere else on Earth; the smaller islands and those volcanic islands which have never touched a major continent are home only to birds. […]