Biogeography involves the study of the geographic distribution of species throughout the world. Study of geographical distribution of plants and animals has added further evidence in favour of the theory of organic evolution or common descent.  Generally, it is seen that the floras and fauna in the regions that are isolated for only a short period of geologic time contain similar but not identical plants and animals. The longer the period of isolation of one region from another, the greater is the degree of differences among the plants and animals of the two regions. These facts lead to the conclusion that the existing plant and animal species are descendants of the extinct forms which were of a more generalized type.