Chap. XL] SAME TYPES IN THE SAME AREAS. 311 



see it also in the birds of the caves of Brazil. Mr. Woodward has 

 shown that the same law holds good with sea-shells, but, from the 

 wide distribution of most molluscs, it is not well displayed by 

 them. Other cases could be added, as the relation between the 

 extinct and living land-shells of Madeira ; and between the extinct 

 and living brackish water-shells of the Aralo-Caspian Sea. ■ 



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 continents ; 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 publi- 

 cations 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 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 Falconer 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 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- 

 migration, the feebler will yield to the more dominant forms, and 

 thei'e will be nothing immutable in the distribution of organic 

 beings. 



It may be asked in ridicule, whether I suppose that the megathe- 

 rium and other allied huge monsters, which formerly lived in 



