122 SUCCESSION OF THE [Chap. XI. 



Now what does this remarkable law of the succession 

 of the same types within the same areas mean ? He 

 would be a bold man who, after comparing the present 

 climate of Australia and of parts of South America, 

 under the same latitude, would attempt to account, on 

 the one hand through dissimilar physical conditions, for 

 the dissimilarity of the inhabitants of these two con- 

 tinents; and, on the other hand through similarity of 

 conditions, for the uniformity of the same types in each 

 continent during the later tertiary periods. Nor can it 

 be pretended that it is an immutable law that marsupials 

 should have been chiefly or solely produced in Australia ; 

 or that Edentata and other American types should have 

 been solely produced in South America. For we know 

 that Europe in ancient times was peopled by numerous 

 marsupials ; and I have shown in the publications above 

 alluded to, that in America the law of distribution of 

 terrestrial mammals was formerly different from what it 

 now is. Xorth America formerly partook strongly of 

 the present character of the southern half of the con- 

 tinent ; and the southern half was formerly more closely 

 allied, than it is at present, to the northern half. In a 

 similar manner we know, from Falconer and Cautley's 

 discoveries, that Xorthern India was formerly more 

 closely related in its mammals to Africa than it is at 

 the present time. Analogous facts could be given in 

 relation to the distribution of marine animals. 



On the theory of descent with modification, the great 

 law of the long enduring, but not immutable, succession 

 of the same types within the same areas, is at once ex- 

 plained ; for the inhabitants of each quarter of the world 

 will obviously tend to leave in that quarter, during the 

 next succeeding period of time, closely allied though in 



