Local Adaptive Branching

It is restricted to the differences arising within a group of closely related forms whose life habits are similar. For instance, Africa supports two very distinct types of rhinoceros today: The square mouth orwhite rhinoceros with the square lip and hypsodont or long crowned teeth and is largely grazing. It inhibits the open country, especially the broad grassy valleys between the tracts of bush south of the Zambesi River. The other is pointed-lip or black rhinoceros with the brachydont or short crowned teeth. It is mainly browsing in habit and is, therefore, found in waded and watered districts from Abyssinia to Cape Colony.

Continental Adaptive Radiation

It is on a broad scale and has been said, embraces the entire fauna of a given class (i.e. Mammalia). Such radiations have appeared more than once.

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Contemporaneous Radiations

The world’s surface may be divided into three zoo-geographical realms: Arctogaea, Neogaea and Notogaea.

Arctogaea includes all the northern hemisphere and Africa. The Neogaea includes South America and Notogaea includes Australia.

These three great realms have been the centres respectively of three remarkable adaptive radiations of mammals during Tertiary Period, when all there were isolated each from the other in such a way as to prohibit absolutely for a long period all found interchange.

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This isolation is true of Australia to this time. Hence while the S. America mammals have practically ceased to exist, Australian Fauna composed of mammals of Mesozor rather than of Tertiary Type. Between Australia and Arctogaea there are some very remarkabe le parallelisms. A comparision of the two radiations may be made.

In the Arctogaea realm, the cursorial radius is represented by dogs; hoofed runners or unguligrade by deer, antelope, ghorkhar (Equus onager) etc. Arboreal forms are squirrels some of which are flying- squirrels. Truly aerial or volent forms are represented by bats. Aquatic- amphibious forms are represented by the Potomogale, muskrats and beavers, ottors and seals. Truly aquatic forms are porpoises, whales and the sea-cows. The subterranean forms are the shrews, moles, mole-rats etc.

In the Notogaea realm the mammals are represented by Prototherians and Metatherians. True placentals are absent. Cursorial adaptation in Marsupials is shown by Tasmanian Wolf (Thylacynus) which is comparable to the dog and wolf of Arctogaea.

Arboreal marsupials are numberus the “Sugar squirrels” or phalangers being very similar to our squirrel in appearance, while the Arctogaean squinels are represented with great fidelity by the Notogean flying phalanger [Petaurus sciureus). No truly volant forms comparable to the bats exist in the Australian marsupials fauna. Amphibious fauna is represented by prototherian mammals like duck-bill or duck-mole (Ornithorhynchus). Fossorial fauna is represented by the marsupial mole (Notoryctes typhols).

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Thus it is seen that the Australian adaptive radiation practically concides with that of Arctogaea, almost every animal in the former assemblages having some approximate equivalent in the latter. The reverse, however, is not true with Arctogaean realm is relatively very large and its fauna is proportionately more extensive and varied.