292 SUCCESSION OF THE [CHAP. XL 



in 1839 and 1845, on this "law of the succession of types," on 

 " this wonderful relationship in the same continent between the 

 dead and the living." Professor Owen has subsequently extended 

 the same generalisation to the 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 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 condi- 

 tions, for the dissimilarity of the inhabitants of these two con- 

 tinents ; 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 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 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 inhabi- 

 tants 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 inhabi- 

 tants 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, per- 



