346 SUMMARY OF THE 



same manner and degree. But after very long intervals 

 of time, and after great geographical changes, permitting 

 much inter-migration, 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 for- 

 merly lived in South America, have left behind them the 

 sloth, armadillo, and ant-eater, as their degenerate descend- 

 ants. 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 ex- 

 tinct 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 progeni- 

 tors 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 for- 

 mation, 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 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, 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 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 number of generations which must have 

 psssed away even during a single formation ; that, owing 



