The isthmuses which connect Africa with Asia, and North with South America, are, however, so small and insignificant compared with the vast extent of the countries they unite that {38}we can hardly consider them to form more than a nominal connection. The Isthmus of Suez indeed, being itself a desert, and connecting districts which for a great distance are more or less desert also, does not effect any real union between the luxuriant forest-clad regions of intertropical Asia and Africa. The Isthmus of Panama is a more effectual line of union, since it is hilly, well watered, and covered with luxuriant vegetation; and we accordingly find that the main features of South American zoology are continued into Central America and Mexico. In Asia a great transverse barrier exists, dividing that continent into a northern and southern portion; and as the lowlands occur on the south and the highlands on the north of the great mountain range, which is situated not far beyond the tropic, an abrupt change of climate is produced; so that a belt of about a hundred miles wide, is all that intervenes between a luxuriant tropical region and an almost arctic waste. Between the northern part of Asia, and Europe, there is no barrier of importance; and it is impossible to separate these regions as regards the main features of animal life. Africa, like Asia, has a great transverse barrier, but it is a desert instead of a mountain chain; and it is found that this desert is a more effectual barrier to the diffusion of animals than the Mediterranean Sea; partly because it coincides with the natural division of a tropical from a temperate climate, but also on account of recent geological changes which we shall presently allude to. It results then from this outline sketch of the earth's surface, that the primary divisions of the geographer correspond approximately with those of the zoologist. Some large portion of each of the popular divisions forms the nucleus of a zoological region; but the boundaries are so changed that the geographer would hardly recognise them: it has, therefore, been found necessary to give them those distinct names which will be fully explained in our next chapter.