38 DISTRIBUTION OF ANIMALS. [part i. 



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 luxu- 

 riant 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 tlie north of .the great moun- 

 tain 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. 



Becent Changes in the Continental Areas. — The important fact 

 has been now ascertained, that a considerable portion of the 

 Sahara south of Algeria and Morocco was under water at a very 

 recent epoch. Over much of this area sea-shells, identical with 

 those now living in the Mediterranean, are abundantly scattered, 



