The present-day distribution of animals is the combined result of many factors. Among them are continental drift and volcanic activity, which constantly reshape the surface of the earth. By splitting up groups of animals, and creating completely new habitats, these geological processes have had a profound impact on animal life. One of the most important effects can be seen on remote islands, such as Australia and Madagascar, which have been isolated from the rest of the world for millions of years. Until humans arrived, their land-based animals lived in total seclusion, unaffected by competitors from outside.
The result is a whole range of indigenous species, such as kangaroos and lemurs, which are found nowhere outside their native homes. Animals are separated when continents drift apart, and they are brought together when they collide. The distribution of animals is evidence of such events long after they occur. For example, Australasia and Southeast Asia became close neighbors long ago, but their wildlife remains entirely different: it is divided by “Wallace’s line”, an invisible boundary that indicates where the continents came together.