With regard to the number and limits of the zoölogical regions into which the world may be divided, Huxley raised a number of problems which have not yet reached a full solution. Mr. Sclater had divided the world into six great regions: the Nearctic, including the continent of North America, with an overlap into what is called South America by geographers; the Palæarctic, comprising Europe and the greater part of Asia; the Oriental, containing certain southern portions of Asia, such as India south of the Himalayas and many of the adjacent islands; the Ethiopian, including Africa, except north of the Sahara, and Madagascar; the Australian, containing Australia and New Zealand and some of the more southeastern of the islands of Malay; the Neotropical, including South America. Huxley first called attention to certain noteworthy resemblances between the Neotropical and the Australian regions of Sclater, and held that a primary division of the world was into _Arctogæa_, comprising the great land masses of the Northern Hemisphere with a part of their extension across the equator, and _Notogæa_, which contained Australia but not New Zealand and South America. Although this acute suggestion has not been generally accepted as a modification of Mr. Sclater's scheme, it called attention in a striking fashion to some very remarkable features in the distribution of animals. Subsequent writers have considerably extended Huxley's conception of the similarities to be found among the more southern land areas. They have pointed out that the most striking idea of the distribution of land and water on the surface of the globe is to be got by considering the globe alternately from one pole and from the other. In the south, a clump of ice-bound land, well within the Antarctic Circle, surrounds the pole. All else is a wide domain of ocean broken only where tapering and isolated tongues of land, South America, the Cape, Australia, lean down from the great land masses of the north. On the other hand, all the great land masses expand in the Northern Hemisphere, and shoulder one another round the North Pole. America is separated from Asia only by the shallowest and narrowest of straits; an elevation of a few fathoms would unite Greenland with Europe. Science points definitely to some part of the great northern land area as the centre of life for at least the larger terrestrial forms of life. We know that these arose successively, primitive birds like the ostriches being older than higher forms like the parrots and singing birds; the pouched marsupials preceding the antelopes and the lion; the lemurs coming before the man-like apes. Each wave of life spread over the whole area producing after its kind; then, pressing round the northern land area, it met a thousand different conditions of environment, different foods, enemies, and climates, and broke up into different genera and species. But there was never a wave of life that was not followed by another wave.