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 the 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. 
Recent 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, 
