CHAP. XL] SAME TYPES IN THE SAME AREAS. 293 



mitting 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 mega- 

 therium 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 descend- 

 ants 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 con- 

 taining 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 gene'iu will be the parents of the new genera : the 

 other species and the other old genera having become utterly 

 extinct. In failing orders, with the genera and species decreasing 

 in numbers as is the case with the Edentata of South America, 

 etill fewer genera and species will leave modified blood-descendants. 



Summary of tlie 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 geo- 

 logically explored with care ; that only certain classes of organic 

 beings have been largely preserved in a fossil state ; that the 

 number both of specimens and of species, preserved in our 

 museums, is absolutely as nothing compared with the number 

 of generations which must have passed away even during a 

 single formation ; that, owing to subsidence being almost neces- 

 sary for the accumulation of deposits rich in fossil species of 

 many kinds, and thick enough to outlast future degradation, 

 great intervals of time must have elapsed between most of our 

 successive formations ; that there has probably been more extinc- 

 tion during the periods of subsidence, and more variation during 

 the periods of elevation, and during the latter the record will have 

 been least perfectly kept ; that each single formation has not 

 'been continuously deoosited that the duration of each formation 



