 Two rival theories were developed to explain Buffon’s Law and its occasional exceptions. According to the first theory, due to Darwin and Wallace, both facts can be explained by the combined effects of two causal mechanisms—dispersal, and evolution by natural selection. The explanation for Buffon’s Law is as follows. Species gradually migrate into new areas, a process which Darwin calls “dispersal.” As natural selection acts over time on the contingent initial distribution of species in different areas, completely distinct species eventually evolve. The existence of cosmopolitan species is explained by “improbable dispersal,” Darwin’s term for dispersal across seemingly impenetrable barriers by “occasional means of transport” such as ocean currents, winds, and floating ice. Cosmopolitan species are explained as the result of improbable dispersal in the relatively recent past.