Its birth was geologically violent and its history has continued to be geologically violent. South American species migrated North and North American species migrated South. Some species passed all the way through and others stopped in Central America. During the past three million years the climate and the ecological regimes in both South and North America have varied widely. Central American life ecologies are a mosaic of species from the two continents.
Some species are remnants from a North American rain forest that ceased to exist long ago.
Migration paths varied. There are lands now drowned which were once major migration paths and rich fossil beds under a hundred feet of Caribbean water. Rain forests came and went, serving as barriers to migration whilst they were present. Wallace mixes in a good deal of history into his exposition in a haphazard sort of way. He shines a spotlight on interesting events that happened in places that he happened to visit.