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 



