In endeavouring to present at a glance in the most convenient manner, the distribution of the families in the several regions and sub-regions, it was necessary to arrange them, so that those whose relations to each other were closest should stand side by side; the first and last being those between which the relations were least numerous and least important. Influenced by the usual opinions as to the relations between Australia and South America, the series was at first begun with the Nearctic, and terminated with the Australian and Neotropical regions; and it was not till the whole of the vertebrate families had been gone through, and their distribution carefully studied, that these last two regions were seen to be really wider apart than any others of the series. It was therefore decided to alter the arrangement, beginning with the Neotropical, and ending with the Australian {175}regions; and a careful inspection of the diagrams themselves, taken in their entirety, will, it is believed, show that this is the most natural plan, and most truly exhibits the relations of the several regions. In the portion of our work now commencing, we are not, however, by any means bound to begin at either end of this series. Each region is studied by itself, but reference will often have to be made to all the other regions; and wherever we begin, we must occasionally refer to facts which will be given further on. As, however, the great northern continents form the central mass from which the southern regions, as it were, diverge, and as the Palæarctic region is both more extensive and much better known than any other, it undoubtedly forms the most convenient starting-point for our proposed survey of the zoological history of the earth. We thus pass from the better known to the less known--from Europe to Africa and tropical Asia, and thence to Australia, completing the series of regions of the Eastern Hemisphere. Beginning again with the Neotropical region, we pass to the Nearctic, which has such striking relations with the preceding and with the Palæarctic region, that it can only be properly understood by constant reference to both. We thus keep separate the Eastern and Western hemispheres, which form, from our point of view, the most radical and most suggestive division of terrestrial faunas; and as we are able to make this also the dividing point of our two volumes, reference to the work will be thereby facilitated. _Cosmopolitan Groups._--Before proceeding to sketch the zoological features of the several Regions it will be well to notice those family groups which belong to the earth as a whole, and which are so widely and universally distributed over it that it will be unnecessary, in some cases, to do more than refer to them under the separate geographical divisions.