AGE OF MAN. 575 



The formations here enumerated, whether along lakes, rivers, or 

 sea-coasts, are usually underlaid by Post-tertiary beds of similar 

 character, situated at varying depths below, often but a few feet, 

 sometimes hundreds of feet ; and the modern and Post-tertiary de- 

 posits are so closely alike that the limits of the two cannot be easily 

 made out. The difficulty is the greater because the shells of the 

 Post-tertiary were all of species now living. In many cases deposits 

 are proved to belong to the age of Man by containing relics of the 

 peculiar species of the age, as explained beyond. 



The agency of air, fresh, and marine waters, heat and life, in giving origin to 

 these deposits, might be here considered. But these topics are discussed under 

 Dynamical Geology; and to that part of the work the reader is referred. 



II. Life. 



The approximate number of living species of Plants is 100,000. 

 The number of species of Animals of the sub-kingdom of Eadiates 

 is about 10,000 ; of Mollusks, 20,000 ; of Articulates, 300,000 ; of 

 Vertebrates, 21.000 ; making a total in the Animal kingdom of 

 about 350,000. Of existing Vertebrates the number of species of 

 Fishes is about 10.000; of Reptiles, 2000 ; of Birds, 7000; of Mam- 

 mals, 2000 = 21,000. 



The increase during the Tertiary period in the extent of dry 

 land and rivers, the height and number of mountains, and the 

 diversities of the zones of climate, augmented greatly the variety 

 of geographical conditions over the globe to which life could 

 be accommodated. This is especially true of the land; but only 

 in a limited degree for the ocean, which has smaller extremes of 

 temperature than the land, and is less affected by its changes of 

 level. 



The terrestrial life of the globe should therefore, on this principle, 

 have undergone a vast increase in the course of the later Tertiary 

 and the period of the Post-tertiary, especially in the classes of In- 

 sects, Birds, and Mammals, and the tribes of fresh-water Fishes. 

 Reptiles should have undergone less increase, for the species belong 

 mainly to the warmer climates, and this type had already culmi- 

 nated and was on the decline. 



Insects and Birds appear to have had their times of culmination 

 in the age of Man, while Mammals, gigantic and ferocious, especially 

 in their larger species, passed their climax in the period next pre- 

 ceding, and disappeared as the age of Man began. Most species of 

 plants and animals have their parasitic insects ; and an augmenta- 



