The geographic distributions of organisms are not static, but dynamic. Over time the ranges of species shift, expand, and contract. The distributions of higher taxonomic groups such as genera, families, orders, and classes also change because their geographic ranges simply reflect the cumulative distributions of all the included species.
Brown Jr., J.H., and A.C. Gibson. 1983. Biogeography. St. Louis, Missouri: C.V. Mosby Company.