SAME TYPES IN TEE SAME AREAS. 369 



ants. This cannot for an instant l)e 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 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 PBECEDIH-G AN"D PEESEKT 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 

 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 for- 

 mations ; that there has probabljy been more extinction 

 during the periods of subsidence, and more variation 

 during the periods of elevation, and during the latter 



