SAME TYPES IN SAME AREAS. 345 



mammals of the Old World. We see the same law in this 

 author's restorations of the extinct and gigantic birds of 

 New Zealand. We 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 mollusks, 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 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 t} r pes 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 par- 

 took 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 rela- 

 tion 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 th© 



