Chap. X. SAME TYPES IN THE SAME AREAS. 317 



stroni^ly insisted, in 1839 and 1845, on tliis " law of the suc- 

 cession of types," on " tliis -wonderful relationship in the same 

 continent between the dead and the livinrr." Prof. Owen has 

 subsecjuentlv extended the same ^"eneralization to tlie mammals 

 of the Old World. We see the same law in this author's res- 

 torations of the extinct and ^ijj^antic birds of New Zealand. 

 We see it also in the birds of the caves of Brazil. Mr. ^V^)^)d- 

 ward has shown that the same law holds good with sea-shells, 

 but, from the wide distribution of most genera of mollusks, it 

 is not well dis})layed by them. Other cases could be added, as 

 the relation between the extinct and living land-shells of Ma- 

 deira ; and between the extinct and living brackish-water shells 

 of the Aralo-Casjiian 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 Aus- 

 tralia and of parts of South America under the same lati- 

 tude, would attempt to account, on the one hand,bv dissimilar 

 physical conditions, for the dissimilarity of the inhabitants of 

 these two continents, 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 Edentata and otln^r 

 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 

 ])ublications above alluded to, that in America the law of dis- 

 tribution of terrestrial matnmals was foniierly dillV-rent 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 modillcation, the great law 

 of the long-enduring, but not inmiutable, succession of the 

 same t^'pes within the same areas, is at onct; explained; for the 

 inhabitants of each <|uarter of tin; world will obviously tend to 

 leave in that quarter, during the next succeeding period of 

 time closely-allied though in some degree modified descend 



