The two primary divisions NOTOGAEA and ARCTOGAEA are fundamental. The four secondary divisions, namely the _Australian_ and _Neotropical_, _Periarctic_ and _Palaeotropical regions_, also stand the test of application to the various classes and main groups of Vertebrata; but naturally, under the present configuration of the world, the Palaeotropical region is nothing but the Southern continuation of the Eastern half of the Periarctic mass of land. This is especially obvious so far as India is concerned. There is, however, that broad belt of desert, sand, and salt-steppes, which extends from North-West Africa to Manchuria, and this belt is one of the most important physical features of the Old World. It is complicated by the system of mountain-chains which, broadly speaking, centre at the Pamirs, and radiate westwards through the Caucasus and Alps into Spain, eastwards through the Himalayas into China, and north-eastwards to Kamtschatka; interrupted by Bering's Sea, it is continued as the backbone of both Americas to Patagonia. The tertiary divisions, the subregions, have no real existence. They depend upon the class, or even order, of animals, which we happen to study. The faunistic distribution of the Urodela is not that of the Anura, and both follow separate lines of dispersal, different from those of the various orders of Reptiles, Birds, and Mammals. This must be so. There is no doubt that the distribution of land and water was totally different in the Coal Age from what it is now. The face of the globe at the Jurassic Age can scarcely be compared with the aspect which the world has assumed in the Miocene period. This leads to another consideration, often neglected. We {71}know that the various classes, orders, families, etc., of animals have appeared successively upon the stage. A group which arose in the Coal Age followed lines of dispersal different from one which was not evolved until Jurassic times, and post-cretaceous creatures could not avail themselves of what assisted their ancestors, and _vice versâ_. The Amphibia are bound absolutely to the land and to fresh water; transportation across salt water is not excluded, but must be accidental, and is not a case of regular "spreading." Speaking generally, the older a group, the more likely is it to be widely distributed. If it appears scattered, this may be due to extinction in intermediate countries or to submergence of former land-connexions.