228 THE DOCTRINE OF DESCENT. 
—an expression which we borrow from Riitimeyer,— 
with the encampments of their former kindred, whence 
are derived general points of view as to the causes of 
the present geographical apportionment of organisms. 
If in the preliminary establishment of facts we there- 
fore confine ourselves to the Mammalia, exclusive of 
whales and bats, a superficial survey is enough to show 
that not only single species, but families also, have each 
a certain region of greatest density of occurrence, a 
focus of distribution, and that from thence radiations 
-have taken place according to the convenience and fit-- 
ness of the territory. Lion and tiger, elephant and . 
camel, range over a definite area; the monkeys of the 
‘New World differ from those of the Old World not only 
geographically, but also in family characteristics. Mar- 
supials are chiefly concentrated in Australia ; sloths and 
armadilloes in South America. And these examples, 
easy to multiply, indicate how individuals of widely 
dispersed species, and the species themselves, emanated 
from single points of the earth’s surface and flowed over 
the territory of distribution now occupied. When to 
this observation is added the other, that in past eras 
also the same groups had the same centres of distri- 
bution,—for instance, Brazil not only harbours sloths 
and armadilloes now, but was once peopled by more 
numerous and partly colossal species of these families, 
and Australia has furnished the most numerous and 
important fossil remains of Marsupials,—the cogniz- 
ance of this persistent localization becomes very signi- 
ficant, and we account for the “repetition” of these 
forms by derivation. 
Now if the centres of distribution, at the first glance 
