GEOLOGICAL SUCCESSION 305 



chiefly or solely produced in Australia ; or that Eden- 

 tata 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 dis- 

 coveries, that northern India was formerly more closely 

 related in its mammals to Africa than it is at the pre- 

 sent 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, permit- 

 ting much inter -migration, the feebler will yield 

 to the more dominant forms, and there will be no- 

 thing immutable in the laws of past and present dis- 

 tribution. 



It may be asked in ridicule, whether I suppose that 

 the megatherium and other allied huge monsters have 

 left behind them in South America, the sloth, armadillo, 

 and anteater, as their degenerate descendants. This 

 cannot for an instant be admitted. These huge animals 

 have become wholly extinct, and have left no progeny. 

 But in the caves of Brazil, there are many extinct 

 species which are closely allied in size and in other 



