GENERAL REMARKS. 677 



used for rough purposes, the smoothed examples being 

 saved for fighting or the chase ; and, even up to the 

 present day, stone implements are more or less employed 

 by certain savage tribes." Fifty years ago, war knives 

 of bone and hard wood were in common use among the 

 natives of New Zealand. Professor S. Schonland has 

 pointed out that the Palaeolithic Age has persisted in South 

 Africa, until comparatively recent times, which is proved 

 by the fact that the stone implements of the natives of that 

 country are not polished in any way. 



In tracing the evolution of the horse, we need not go 

 further back than the Tertiary ; because, during it, hoofed 

 animals became for the first time abundant on the earth. 

 "If the Secondary periods could appropriately be grouped 

 together under the name of the ' Age of Reptiles,' Tertiary 

 time may not less fitly be called the ' Age of Mammals.' 

 As the manifold reptilian types died out, the mammals, in 

 ever-increasing complexity of organisation, took their 

 place in the animal world " (Geikie). 



According to the calculations made by many palae- 

 ontologists, the respective durations of the following 

 periods may be estimated approximately as follows : — 



Primary 17,000,000 years 



Secondary . 7,000,000 ,, 



( Eocene . ■ . . 2,100,000 years j 

 Tertiary < Miocene . . . 600,000 ,, > 3,000,000 ,, 



( Pliocene . . . 300,000 ,, ) 

 Quaternary ....... 300,000 ,, 



Total duration of life on the earth = 27,300,000 ,, 



The time since a solid crust was formed on the earth 

 is variously estimated at from 25,000,000 to 100,000,000 

 years. 



Calculations as to the duration of the Primary, Secondary 

 and Tertiary periods are based chiefly on the time taken 

 in the formation of sedimentary strata (which amount in 

 thickness to about 20 miles), " denudations of the mountains, 



