10 LAND MAMMALS IN THE WESTERN HEMISPHERE 



was ascertained from the comparative study of manuscripts, 

 the date of which could be ascertained from other evidence, 

 but, when once estabhshed, the changes in handwriting are 

 used to fix the period of undated manuscripts. Just so, the 

 succession of fossils, when learned from a series of superposed 

 beds, may then be employed to fix the geological date of 

 strata in another region. Similarly, the archaeologist has 

 observed that there is an evolution or development in ever}^ 

 sort of the work of men's hands and therefore makes use of 

 coins, inscriptions, objects of art, building materials and methods, 

 etc., to date ancient structures. In the German town of Trier 

 (or Treves) on the Moselle, the cathedral has as a nucleus a 

 Roman structure, the date and purpose of which had long been 

 matters of dispute, though the general belief was that the 

 building had been erected under Constantine the Great. In 

 the course of some repairs made not very long ago, it became 

 necessary to cut deep into the Roman brickwork, and there, 

 embedded in the undisturbed mortar, was a coin of the emperor 

 Valentinian II, evidently dropped from the pocket of some 

 Roman bricklayer. That coin fixed a date older than which the 

 building cannot be, though it may be slightly later, and it well 

 illustrates the service rendered by fossils in determining geo- 

 logical chronology. 



Other methods of making out the chronology of the earth's 

 history have been proposed from time to time and all of them 

 have their value, though none of them renders us independent 

 of the use of fossils, which have the pre-eminent advantage of 

 not recurring or repeating themselves at widely separated inter- 

 vals of time, as all physical processes and changes do. An 

 organism, animal or plant, that has become extinct never 

 returns and is not reproduced in the evolutionary process. 



Great and well founded as is our confidence in fossils as 

 fixing the geological date of the rocks in which they occur, it 

 must not be forgotten that the succession of the different kinds 

 of fossils in time was first determined from the superposition of 



