Chap. XL] SAME TYPES IN THE SAME AREAS. 123 



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, permitting much in- 

 termigration, the feebler will yield to the more domi- 

 nant forms, and there will be nothing immutable in the 

 distribution of organic beings. 



It may be asked in ridicule, whether I suppose that 

 the megatherium and other allied huge monsters, which 

 formerly lived in South America, have left behind them 

 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 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 forma- 

 tion there be six other allied or representative genera 

 each with the same number of species, then we may con- 

 clude that generally only one species of each of the older 

 genera has left modified descendants, 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 ease, 

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

 older genera will be the parents of the new genera: the 



