GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 131 



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 forma- 

 tion there be six other allied or representative genera 

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

 clude 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 

 yn\j 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, 

 s absolutely as nothing compared with the number of 

 j^enerations which must have passed away even during a 



