348 FOSSILS 



advancing, diversifying, and continually rising to higher and higher 

 planes. We need not stop to inquire how this progression has 

 been effected • for our present purpose it is sufficient to know that 

 progress and change have been unceasing and gradual, though 

 not necessarily occurring at a uniform rate. Accepting, then, the 

 undoubted fact of the universal change in the character of the 

 organic beings which have successively lived on the earth, it follows 

 that rocks which have been formed in widely separated periods of 

 time will contain markedly different fossils, while those which were 

 laid down more or less contemporaneously will have similar fossils. 

 This principle enables us to compare and correlate rocks from all 

 the continents and, in a general way, to arrange the great events 

 of the earth's history in chronological order. 



The general principle that similarity of fossils indicates the ap- 

 proximate contemporaneity of the rocks in which they are found, 

 must not be taken too literally, for it is subject to certain limita- 

 tions and exceptions. 



(a) Exact contemporaneity is not meant, for the progress of 

 life is very slow, and rocks formed thousands of years apart may 

 yet contain precisely similar fossils. 



(b) Animals and plants differ in different parts of the world, so 

 that contemporaneous rocks formed in widely separated regions 

 will always show a certain amount of difference in their contained 

 fossils. In comparing the rocks of two continents, it is often 

 exceedingly difficult to decide just how much of a given difference 

 in the fossils is to be ascribed to a difference in the time of rock 

 formation, and how much to mere geographical separation. 



(c) New forms of animals and plants originate in some particu- 

 lar area and spread in all directions from that area, until stopped 

 by some obstacle of climate or topography which they cannot sur- 

 mount. The diffusion of new forms often occasions the extinction 

 of old ones which were not so well fitted to survive. These pro- 

 cesses take time, and a group of organisms may make its appearance 

 in one part of the world long before it spreads to another, while 

 ancient types may linger in certain localities long after they have 

 elsewhere become extinct. 



