Ancient Arctogaea

This fauna is clearly quite different from that of any other part of the world and gives a sound basis for the establishment of Madagascar as an independent region. We know that Hippo potamus only came into existence about at the end of the Miocene times and that it first reached Africa in the Pliocene : it is there fore clear that it must be a relatively recent immigrant. If it had crossed over a land-bridge it would necessarily have been accom panied by those other elements of the African fauna which live in close association with it. We should in fact expect to find in Madagascar representatives of those antelopes which live in the forests or swamps which border rivers, the Cape buffaloes and ele phants, true monkeys, and an assemblage of carnivores which prey upon them. The complete absence of such forms implies that the hippopotamus did not cross on dry land and it is probable that it reached Madagascar by swimming. The pig is also a water loving form and may also have swum across. It is clear from their many peculiarities that the viverrids have lived longer in Madagascar: they cannot have crossed on land because of the absence of any associates, and it is most improbable that they swam. The lemurs represent a group well established in Eocene times and their wide radiation again implies a residence. Finally the centetids are known to have existed in North America in the Basal Eocene. Neither group could have crossed land bridges without bringing with them other forms, so that it seems clear that they were transported by some other method, the only plausible suggestion is that they crossed on rafts of tangled vegetation washed down from the great rivers, fore-runners of the Limpopo and Zambezi, which drained Africa throughout Tertiary times.

Madagascar, in the absence from its fauna of most ungulates and carnivores, is representative of all those islands which have never been connected with the great northern land-masses during Tertiary times. Of such "oceanic islands" the next largest is New Zealand, which possesses no mammals whatsoever except two bats, each the sole representative of a peculiar genus. New Caledonia and most of the Pacific islands are of the same type, whilst the islands of the Malay archipelago have a fauna derived from that of Asia, modified by the occasional influx of a few marsupials from the Australian region.

The foregoing discussion will have shown that only when the history of the animals involved is known in detail, is it possible to interpret an existing geographical distribution, and as this condition is only fulfilled in the case of the mammals and there imperfectly, it is unnecessary to discuss the distribution of other groups in detail. There are, however, certain features of general interest presented by individual members of other groups.

Birds.

Amongst the birds, the large cursorial and flightless forms belonging to the Ratitae, are represented by the emus and cassowaries of Notogaea, the rheas of South America and the ostrich of Africa and Arabia. Extinct ostriches were abundant

in Pleistocene times in China and Mongolia, and have been found in the Upper Tertiary of India and the Lower Pliocene of Samos, in association with that great fauna which we have seen to include the ancestors of the living African animals. In addition, in the Pleistocene of Madagascar, there is a series of birds (Aepyornis) one of which is the largest known bird, standing some 12 ft. high, with an egg with a capacity of 2 gal. A similar group of forms, very variable as to size and proportions, lived in New Zealand long enough to be hunted by the Maoris, and filled in that island the role more normally assumed by ungulates. The occurrence of these giant flightless birds in continental areas is interesting because flightless representatives of other groups are generally restricted to islands where they were not exposed to the attacks of carnivorous mammals : of such forms the most familiar is the dodo, probably allied to the pigeons, which formerly lived in Mauritius, whilst a similar form occurred in Rodriguez. The Chatham islands were inhabited by a whole series of flight less rails.

Reptiles.

The reptiles present an example of a group of animals whose distribution is limited by temperature. The body temperature is not uniform, but varies with the surroundings and is kept slightly above them by muscular activity. As the rate of the heart-beat alters with the temperature, being greatest when warm and becoming very slow as freezing point is approached, it is clear that reptiles can only exhibit much activity in warm climates. In most reptiles an egg, similar to that of a bird, is laid in the ground and incubated not by the warmth of the mother's body but by the sun or by decaying vegetation. As development proceeds at a negligible rate at low temperatures, reptiles are only capable of reproducing their kind within a belt of the earth's surface where the summer temperature is fairly high. The British Isles are very nearly at the extreme northern limit of reptile existence, and are inhabited by very few forms; the mud tortoise, Emys, which lives in Belgium, being incapable of main taining itself in England. Even within this belt the distribution of certain forms is restricted to the warmer regions. The crocodiles, for example, are in the main tropical, although they reach as far north as the Mississippi in North America and the Yellow river in China.

Amphibia.

The distribution of Amphibia presents some interesting and puzzling features. The Gymnophiona, probably the most archaic group, are strictly confined to the Tropics and occur in the Neotropical, Ethiopian and Oriental regions. The Urodela, a very ancient group, are not now represented in Notogaea, an area which it seems certain they must once have reached. They are at present most abundant and varied in the temperate zone of eastern Asia and North America. The distribu tion of frogs cannot be intelligently discussed owing to our lack of knowledge of the history of the group.