312 Summary of the Chap. XL 



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 species and the other whole 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, still fewer genera and species will leave 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 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 necessary 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 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 forma- 

 tion has not been continuously deposited ; that the duration of 

 each formation is, probably, short compared with the average dura- 

 tion of specific forms ; that migration has played an important part 

 in the first appearance of new forms in any one area and formation ; 



