340 GEOLOGICAL SUCCESSION. Chap. X. 



and, on the other hand, by similarity of conditions, for 

 the uniformity of the same types in each 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 Eden- 

 tata 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. 

 North America formerly partook strongly of the pre- 

 sent character of the southern half of the continent ; 

 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 dis- 

 coveries, that northern India was formerly more closely 

 related in its mammals to Africa than it is at the pre- 

 sent 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 

 explained ; 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 some degree modified descendants. If 

 the inhabitants of one continent formerly differed 

 greatly from those of another continent, so will their 

 modified descendants still differ in nearly the same 

 manner and degree. But after very long intervals of 

 time and after great geographical changes, permitting 

 much inter-hiigration, the feebler will yield to the 

 more dominant forms, and there will be nothing im- 

 mutable in the laws of past and present distribution. 



