Few kinds of animals are distributed worldwide; most are restricted to certain geographical areas. Thus each portion of the world has a characteristic fauna that differs from the others in varying degrees. For example, the animal population of each of the islands of the West Indies differs only slightly from that of any neighboring island, while the animals of the opposite sides of the North American continent, though showing some like characteristics, differ in many respects. Some elements of the fauna of the western United States bear closer relationships to the animal population of Europe than to that of the eastern coast, and the whole of the North American fauna is much more similar to the fauna of Europe than to that of its southern neighbor, South America. The fauna of Australia is, in general, totally unlike that of any other continent, although certain species have relatives in North and South America. Most famous of these are the marsupial opossums of the New World.
So long as the belief in special creation was adhered to, these facts of distribution presented no problem, but with the development of the concept of evolution they challenged the attention and imagination of biologists. It soon became apparent that related species had common origins and must have dispersed from certain centers.
History
Many early naturalists noted details of animal distribution and were struck by its peculiarities. Voyages such as Capt. James Cook's three-year circumnavigation of the globe (1768–1771) and Matthew Flinders' expedition to Australia (1801–1803) brought back to Europe ample evidence that different regions of the world possessed different faunas. Charles Darwin during the famous voyage of the Beagle (1831–1836) collected sufficient zoological materials and observations to enable him later to discuss the facts of animal distribution with authority. This voyage was the impetus for his later development of evolutionary theories that finally enabled scientists to explain some of the peculiar facts of animal distribution. One of the most important ideas in the development of the science of zoogeography was contributed by the 19th century geologist Charles Lyell, who pointed out that the present distribution of animals is conditioned by past changes involving major landmasses.
In 1858 Philip Lutley Sclater attempted to divide the world into zoogeographical regions, basing his work on studies of perching birds. Alfred Russel Wallace expanded Sclater's work in The Geographical Distribution of Animals (1876), basing his analysis on mammals. Wallace had worked as a field naturalist in the Malay Archipelago and South America and was impressed with the differences and similarities of these two distinct faunas. In his book, which is still one of the most important volumes on the subject, he divided the world into six zoogeographical regions, which he named Palaearctic, Nearctic, Ethiopian, Oriental, Australian, and Neotropical. Since his time zoogeographers have been engaged in further studies of the regions, more careful analysis of the fauna, and the incorporation of new geological facts, evolutionary theories, and ecological principles in the interpretation of zoogeographical data.
