Two lands with the same climate, soils and forests can hold completely different animals, while lands that look unlike each other share them. What decides who lives where is the history of the water gaps between places, not how similar the places look. One strait only a few miles wide can mark where whole families of birds stop.
When islands hold the same land animals that cannot swim even a single mile of sea, the only sufficient explanation is that the islands were once joined to the mainland by land now drowned. By mapping which animals are shared, you can rebuild a coastline that no longer exists. This lets you reconstruct a continent's former reach.
The more creatures an island has that are found nowhere else, the longer it has been cut off from its neighbours. So relatedness of wildlife measures the order in which lands split apart, even when the map suggests otherwise. An island can sit right beside another and still be zoologically further from it than from a more distant one.