212 A BOOK-LOVER'S HOLIDAYS 



of the western hemisphere. Again, in North 

 America, it is as yet quite impossible to be sure 

 as to the exact succession, or contemporaneity 

 of all of the many extinct species of horse and 

 elephant. It is with our present knowledge 

 equally impossible to be sure of the exact time 

 relations between any given North American 

 fauna and the Eurasiatic fauna most closely re- 

 sembling it. Moreover, as yet we have only the 

 vaguest idea of the duration of even modern 

 geologic time ; good observers vary as to whether 

 a given period covers hundreds of thousands or 

 only tens of thousands of years. 



This does not impair the value of the general 

 picture which we can make in our minds. It 

 is not essentially different from what is the 

 case in history. If we speak of the Grseco- 

 Roman world from the days of Aristides to 

 those of Marcus Aurelius, we outline a his- 

 torical period which has a real unity, and of 

 which all the parts are bound together by real 

 ties and real resemblances. Nevertheless, there 

 were sharp differences in the successive cultures 

 of this period; even the two centuries which 

 intervened, say, between Miltiades and Deme- 

 trius Poliorketes, or between Marius and Trajan, 

 showed such differences. Dealing roughly with 

 the period as a whole, it would not be necessary 



