THE FORERUNNERS OF THE HORSE 241 



period to the horse of the present day in other 

 words, what was the length of the Tertiary 

 period, or age of mammals is a question which 

 must occur to all. To answer that question with 

 any approach to correctness in terms of years or 

 centuries is a practical impossibility, although it 

 has been frequently attempted ; and all that can be 

 done is to endeavour to convey by means of the 

 stupendous events which have taken place during 

 the Tertiary period some faint idea of the enormous 

 length of time represented by that latest stage in 

 the geological history of our globe. 



This mode of gaining some idea of the immense 

 lapse of time which has taken place during the slow 

 evolution of the Eocene Hyracotherium into the 

 modern Equus, or, in other words the birth of 

 mountain-chains during the Age of Mammals, has 

 been well expressed by Professor H. F. Osborn, 1 

 who writes as follows : 



"The Rocky Mountains, it is true, began their 

 elevation during the close of the Age of Reptiles 

 [that is to say, during the Secondary period, which 

 immediately preceded the Tertiary, and includes 

 the Chalk and Oolites] ; they had only attained a 

 height of four or five thousand feet when the Age 

 of Mammals commenced ; they continued to rise 

 during the entire period. But consider the map of 

 Europe and Asia at the beginning of Eocene time 



1 The Age of Mammals, New York, 1910, p. 58. 



Q 



