368 SUCCESSION OF THE 



bold mau 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 continents ; and, on the 

 other hand through similarity of conditions, for the uni- 

 formity 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 Amer- 

 ica the law of distribution of terrestrial mammals was for- 

 merly different from what it now is. North America for- 

 merly partook strongly of the present 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 Fal- 

 coner and Cautley's discoveries, that Northern 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 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 modi- 

 fied descendants. If the inhabitants of one continent for- 

 merly 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 intermigration, the feebler will yield to the more 

 dominant forms, and there will be nothing immutable in 

 the distribution of organic beings. 



It may be asked in ridicule whether I suppose that the 

 megatherium and other allied huge monsters, which for- 

 merely lived in South America, have left behind them the 

 sloth, armadillo, and ant-eater, as their degenerate descend. 



