The divisions of the earth into faunal regions by Dr P.L. Sclater have been found to hold good for a large number of groups of animals as different in their mode of life as birds and mammals, and they may thus be accepted as based on nature. They are six in number: (1) _Palaearctic_, including Europe, Asia north of the Himalaya, and Africa north of the Sahara; (2) _Ethiopian_, consisting of Africa south of the Atlas range, and Madagascar; (3) _Oriental_, including India, Indo-China and the Malay Archipelago north of Wallace's line, which runs between Bali and Lombok; (4) _Australian_, including Australia, New Zealand, New Guinea and Polynesia; (5) _Nearctic_ or North America, north of Mexico; and (6) _Neotropical_ or South America. Each of these divisions is the home of a special fauna, many species of which are confined to it alone; in the Australian region, indeed, practically the whole fauna is peculiar and distinctive, suggesting a prolonged period of complete biological isolation. In some cases, such as the Ethiopian and Neotropical and the Palaearctic and Nearctic regions, the faunas, although distinct, are related, several forms on opposite sides of the Atlantic being analogous, e.g. the lion and puma, ostrich and rhea. Where two of the faunal realms meet there is usually, though not always, a mixing of faunas. These facts have led some naturalists to include the Palaearctic and Nearctic regions in one, termed _Holarctic_, and to suggest transitional regions, such as the _Sonoran_, between North and South America, and the _Mediterranean_, between Europe and Africa, or to create sub-regions, such as Madagascar and New Zealand. Oceanic islands have, as a rule, distinctive faunas and floras which resemble, but are not identical with, those of other islands in similar positions. Biological distribution as a means of geographical research. The study of the evolution of faunas and the comparison of the faunas of distant regions have furnished a trustworthy instrument of pre-historic geographical research, which enables earlier geographical relations of land and sea to be traced out, and the approximate period, or at least the chronological order of the larger changes, to be estimated. In this way, for example, it has been suggested that a land, "Lemuria," once connected Madagascar with the Malay Archipelago, and that a northern extension of the antarctic land once united the three southern continents. The distribution of fossils frequently makes it possible to map out approximately the general features of land and sea in long-past geological periods, and so to enable the history of crustal relief to be traced.[45] Reaction of organisms on environment.