470 
ZOOLOGY OF 
of latitude in South Africa and South America. In all 
these cases there is a wide extent of sea separating the 
countries, which few animals can pass over ; so that, sup- 
posing the animal productions to have been originally 
distinct, they could not well have become intermixed. 
In each of these countries we find well-marked smaller 
districts, appearing to depend upon climate. The tro- 
pical and temperate parts of America and Africa have, 
generally speaking, distinct animals in each of them. 
On a more minute acquaintance with the animals of 
any country, we shall find that they are broken up into 
yet smaller local groups, and that almost every district 
has peculiar animals found nowhere else. Great moun- 
tain-chains are found to separate countries possessing 
very distinct sets of animals. Those of the east and 
west of the Andes differ very remarkably. The Rocky 
Mountains also separate two distinct zoological districts ; 
California and Oregon on the one side, possessing plants, 
birds, and insects, not found in any part of North Ame- 
rica east of that range. 
But there must be many other kinds of boundaries 
besides these, which, independently of climate, limit the 
range of animals. Places not more than fifty or a hun- 
dred miles apart, often have species of insects and birds 
at the one, which are not found at the other. There 
must be some boundary which determines the range of 
each species ; some external peculiarity to mark the line 
which each one does not pass. 
These boundaries do not always form a barrier to the 
progress of the animal, for many birds have a limited 
range, in a country where there is nothing to prevent 
