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Dividing the world into regions of similar faunistic or floristic composition and shared evolutionary history is a major aim of biogeography.

The purpose of this part is to present a full account of the species of amphibians and reptiles known to inhabit the state of Michoacán; the accounts of the species are accompanied by a brief description of the natural landscape and of the various assemblages of species comprising the major faunistic groups within the region.

In connection with this theory some interesting faunistic data are given which seem to support it.

Many zoologists who have investigated the fauna of that region, and have attempted to explain the faunistic relations, had to acknowledge that the migrations must have taken place under geographical conditions entirely different from those obtaining at present.

If it was all land, or more likely great changing archipelagoes, faunistic exchange between North America, Europe and Siberia would present no difficulties, but there is one connection which engages much attention, namely a land where now lies the North temperate and Northern part of the Atlantic ocean.