The geographical distribution of animals, with a study of the relations ...

38 DISTRIBUTION OF ANIMALS. [part i.we can hardly consider them to form more than a nominalconnection. The Isthmus of Suez indeed, being* itself a desert,and connecting districts which for a great distance are more orless desert also, does not effect any real union between the luxuriantforest-clad regions of intertropical Asia and Africa. TheIsthmus of Panama is a more effectual lineof union, since it ishilly, well watered, and covered with luxuriant vegetation ;andwe accordingly find that the main features of South Americanzoology are continued into Central America and Mexico. InAsia a great transverse barrier exists, dividing that continentinto a northern and southern portion ; and as the lowlands occuron the south and the highlands on the north of the great mountainrange, which ischange of climate is produced ;situated not far beyond the tropic, an abruptso that a belt of about a hundredmiles wide, is all that intervenes between a luxuriant tropicalregion and an almost arctic waste.Between the northern partof Asia, and Europe, there is no barrier of importance ;and it isimpossible to separate these regions as regards the main featuresof animal life. Africa, like Asia, has a great transverse barrier,but it is a desert instead of a mountain chain ; and it is foundthat this desert is a more effectual barrier to the diffusion ofanimals than the Mediterranean Sea; partly because it coincideswith the natural division of a tropical from a temperate climate,but also on account of recent geological changes which we shallpresently allude to. It results then from this outline sketch ofthe earth's surface, that the primary divisions of the geographercorrespond approximately with those of the zoologist. Somelarge portionof each of the popular divisions forms the nucleusof a zoological region ; but the boundaries are so changed thatthe geographer would hardly recognise them : it has, therefore,been found necessary to give them those distinct "nameswill be fully explained in our next chapter.Recent Changes in thewhichContinental Areas.—The important facthas been now ascertained, that a considerable portion of theSahara south of Algeria and Morocco was under water at a veryrecent epoch. Over much of this area sea-shells, identical withthose now living in the Mediterranean, are abundantly scattered,