_Order of Succession of the Regions._--We may here explain the reason for taking the several regions in a different succession from that in which they appear in the tabular or diagrammatic headings to each family, in the Fourth, and concluding part of this work. It will have been seen, by our examination of extinct animals (and it will be made still clearer during our study of the several regions) that all the chief types of animal life appear to have originated in the great north temperate or northern continents; while the southern continents--now represented by {174}South America, Australia, and South Africa with Madagascar--have been more or less completely isolated, during long periods, both from the northern continent and from each other. These latter countries have, however, been subject to more or less immigration from the north during rare epochs of approximation to, or partial union with it. In the northern, more extensive, and probably more ancient land, the process of development has been more rapid, and has resulted in more varied and higher types; while the southern lands, for the most part, seem to have produced numerous diverging modifications of the lower grades of organization, the original types of which they derived either from the north, or from some of the ancient continents in Mesozoic or Palæozoic times. Hence those curious resemblances in the fauna of South America, Australia, and, to a less extent, Madagascar, which have led to a somewhat general belief that these distant countries must at one time or other have been united; a belief which, after a careful examination of all the facts, does not seem to the author of this work to be well founded. On the other hand, there is the most satisfactory evidence that each southern region has been more or less closely united (during the tertiary or later secondary epoch) with the great northern continents, leading to numerous resemblances and affinities in their productions.