306 ON THE ORIGIN OF SPECIES 



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 forgotten 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 forma- 

 tion, and in the next succeeding formation there be six 

 other allied or representative genera with the same 

 number of species, then we may conclude 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 old 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 

 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 incalculable 

 number of generations which must have passed away 

 even during a single formation ; that, owing to sub- 

 sidence being necessary for the accumulation of 

 fossiliferous deposits thick enough to resist future 

 degradation, enormous intervals of time have elapsed 

 between the successive formations ; that there has prob- 

 ably been more extinction 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 



