The world 50 million years after man is one he would still recognize; the climate and vegetation are broadly similar, only the geography has changed. Plate tectonics, the force responsible for continental drift, has driven the continents of Eurasia, Australia and North America together and turned South America into an island. The animals on the other hand, although still falling into the familiar classes offish, mammals, reptiles and so on are remarkably different even though there are, in most cases, underlying similarities between them and the species that man would have known. The most profound differences are in the higher forms of life, the birds and mammals, which, because of their adaptability, respond rapidly to changing environmental conditions by evolving and producing new species quickly. As the earth's major habitats are broadly unchanged they cannot be responsible for the changes that have occurred in the earth's fauna since the time of man. The answer must lie more with the infinite variability of nature and the large number of solutions that exist in terms of form and shape for any one animal in a given environment. Which solution is ultimately adopted bears strongly on the animal's ancestry. If a creature has a particular feature that, in the short term, can be modified to suit a certain role, then that feature will develop in the course of evolution to suit the purpose in preference to one that is hypothetically better but which would have to evolve from scratch. The vacuum created by the demise of man provided the stimulus that led to the creation of this new fauna and it is their solutions to the successful exploitation of the earth's vast variety of habitats that is described on the following pages. Fifty-million years after the Age of Man the world's northern continents of Africa, Eurasia and North America have joined with Australia to form a single unit. South America is separated from North America as it was in the Tertiary. http://www.sivatherium.narod.ru/library/Dixon/main_en.htm