318 THE ORIGIN OF SPECIES 



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 man- 

 ner we know, from Falconer and Cautley's discoveries, that North- 

 ern 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 in- 

 habitants oi 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 inter- 

 vals of time, and after great geographical changes, permitting 

 much inter-migration, the feebler will yield to the more dominant 

 forms, and there will be nothing immutable in the distribution 

 of organic beings. 



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

 therium and other allied huge monsters, which formerly lived in 

 South America, have left behind them the sloth, armadillo, and 

 ant-eater, 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 all 

 other characters to the species still living in South America; and 

 some of these fossils may have been the actual progenitors of the 

 living species. It must not be forgotten, that, on our theory, all 

 the species of the same genus are the descendants of some one 

 species; so that, if six genera, each having eight species, be found 

 in one geological formation, and in a succeeding formation there 

 be six other allied or representative genera, each with the same 

 number of species, then we may conclude that generally only 

 one species of each of the older genera has left modified descend- 

 ants, which constitute the new genera containing the several 

 species; the other seven species of each old genus having died out 

 and left no progeny. Or, and this will be a far commoner case, 

 two or three species in two or three alone of the six older genera 

 will be the parents of the new genera: the other species and the 

 other old genera having become utterly extinct. In failing orders, 



