METHODS — GEOLOGICAL 13 



ment in handwriting, the palaeographer can hardly do more 

 than determine the decade in which a manuscript was written ; 

 no one would expect him to fix upon the exact year, still less the 

 month, from the study of handwriting alone. As is the month 

 in recorded human history, so is the millennium in the long 

 course of the earth's development. 



In the second place, there are great differences in the con- 

 temporary life of separate regions and such geographical differ- 

 ences there have always been, so far as we can trace back the 

 history of animals and plants. A new organism does not 

 originate simultaneously all over the world but, normally 

 at least, in a single area and spreads from that centre until it 

 encounters insuperable obstacles. Such spreading is a slow 

 process and hence it is that new forms often appear in one re- 

 gion much earlier than in others and in the very process of ex- 

 tending their range, the advancing species may themselves 

 be considerably modified and reach their new and distant homes 

 as different species from those which originated the movement. 

 Extinction, likewise, seldom occurs simultaneously over the 

 range of a group, but now here and now there in a way that 

 to our ignorance appears to be arbitrary and capricious. The 

 process may go on until extinction is total, or may merely 

 result in a great restriction of the range of a given group, or 

 may break up that range into two or more distinct areas. 



Of such incomplete extinctions many instances might be 

 given, but one must suffice. The camel-tribe, strange as it 

 may appear, originated in North America and was long con- 

 fined to that continent, while at the present day it is repre- 

 sented only by the llamas of South America and the true 

 camels of Asia, having completely vanished from its early 

 home. These facts and a host of similar ones make plain how 

 necessary it is to take geographical considerations into 

 account in all problems that deal with the synchronizing of the 

 rocks of separate areas and continents. 



Properly to estimate the significance of a difference in the 



