ZOOLOGICAL REGIONS. Zoological distribution, also known as zoo-geography, is the science dealing with the distribu tion of animals over the surface of the globe. It is concerned not only with present conditions but also with those of former geo logical periods and with the mode in which the present arrange ment has arisen. The study of the present distribution of animals may be of two extreme types, whose fields overlap. It is possible to investigate the details of the occurrence of a species in a small district, relating the facts discovered not only to the physical and chemical conditions of its environment but also to its neighbours both plant and animal. Such a study (still very undeveloped) is called ecology (q.v. and see also DISTRIBUTION OF ANIMALS). On the other hand, it has long been known that it is possible to divide the land of the world into regions separated from other land masses either entirely, as in the case of Australia and New Zealand, or very nearly, as in the case of Africa. Such regions may possess characteristic faunas and the investigation of the nature of these and an elucidation of the resemblances and differ ences which they present, form the subject matter of that aspect of zoological distribution dealt with here.
To those early naturalists who believed in the independent creation of all the species of animals, their irregular distribution over the world must either have presented no problem or have been inexplicable. When, however, Charles Darwin showed that the existing distribution received a ready explanation on the theory of the evolutionary origin of species, and that its details might be used to throw light on the mechanism that had brought about evolution, the whole subject acquired a new interest and within a few years the main lines of the accepted division of the world into zoological regions were established by P. L. Sclater and his followers.
The differences which separate the faunas of the zoological regions are not dependent on climate or temperature. For ex
ample, the mammal fauna of North America is, on the whole, very similar to that of northern Asia and Europe, whilst the faunas of Patagonia and Australia, which present a similar range of climatic conditions, differ completely not only from those of the northern areas but also from each other. Furthermore, the mammal fauna of South America is a unit, although that con tinent stretches throughout the Tropics and into the Antarctic regions of Tierra del Fuego. Thus the factors which have led to the distribution of existing animals into regions are not discover able in present day conditions but must lie in the past, and they can only be discovered by an investigation of the history of the animals which composed the fauna, and of the changes in the distribution of land and water, which have taken place during geological time.
The zoo-geographical regions at present recognized are as follows : I. Palaearctic, including Europe with Iceland; Asia, including Japan. north of the Himalayas and of the Yangtze-kiang water shed ; Persia and Asia west of the Indus ; and Africa north of the Sahara, including the Azores.
These two regions are conveniently grouped as Holarctica.
To the south of Palaearctica lie two distinct regions, each a peninsula projecting from the great northern land mass into the wide seas of the southern hemisphere. They are: 3. The Ethiopian region, comprising the whole of Africa south of the Sahara, with Madagascar and the Mascarine islands.
4. The Oriental region, comprising India and Ceylon, Siam and southern China, and the Malay archipelago. Bearing a somewhat similar relation to the Nearctic region is 5. The Neotropical region, comprising the whole of America from Mexico southward, and the Antilles. Finally more isolated than any is 6. The Australasian region, including Australia with Tasmania and New Guinea, New Zealand and the islands of the Pacific.