Chap. X. SAME TYPES IN SAME AREAS. 341 



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 ani- 

 mals have become wholly extinct, and have left no pro- 

 geny. But in the caves of Brazil, there are many 

 extinct species which are closely allied in size and in 

 other characters to the species still living in South 

 America ; and some of these fossils may be the actual 

 progenitors of living species. It must not be for- 

 gotten that, on my theory, all the species of the same 

 genus have descended from some one species ; so that 

 if six genera, each having eight species, be found in one 

 geological formation, and in the next succeeding forma- 

 tion there be six other allied or representative genera 

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

 clude that only one species of each of the six older 

 genera has left modified descendants, constituting the 

 six new genera. The other seven species of the old 

 genera have all died out and have left no progeny. Or, 

 which would probably be a far commoner case, two or 

 three species of two or three alone of the six older 

 genera will have been the parents of the six new 

 genera ; the other old species and the other whole 

 genera having become utterly extinct. In failing 

 orders, with the genera and species decreasing in 

 numbers, as apparently is the case of the Edentata of 

 South America, still fewer genera and species will have 

 left modified blood-descendants. 



Summary of the 'preceding and present Chapters. — I 

 have attempted to show that the geological record is 

 extremely imperfect ; that only a small portion of the 

 globe has been geologically explored with care ; that 



