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



some decree 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 intermigration, 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 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 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 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 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 



