INTRODUCTION 9 



either physical or pakeontological. If then we have 

 determined the order of succession of the formations in 

 any one area by means of their relative positions, the 

 newer resting on the older, it is fairly easy in any other 

 district, merely by examining the fossils, to refer any set 

 of beds to its proper position in the geological record. 

 But although this law of the identification of strata by 

 the fossils which they contain is of great value, it must not 

 be applied without some caution, for even if two formations 

 were deposited at exactly the same time, it does not 

 necessarily follow that all the genera and species found 

 in the two will be identical. Thus for instance in the seas 

 at the present day the same forms of life do not occur in 

 all parts ; animals which live in water of moderate depth 

 are distributed in provinces which depend largely on 

 climatic conditions, each province possessing some forms 

 peculiar to itself. The organisms now being entombed 

 in deposits formed, say, off the British coasts, will as a 

 whole be different from those off the Canary Islands ; but 

 still, some of the species and many of the genera will be 

 common to both areas, and would enable us to identify 

 the two deposits as having been formed within the same 

 general period, though perhaps not to prove them abso- 

 lutely synchronous. Then again there is a distribution 

 of organisms according to the depth of the sea, and the 

 nature of the sea-bottom; so that the fauna of a deep- 

 water formation will necessarily be different from that of a 

 shallow-water one, and that of a sandy deposit different 

 from that of a mud. But in addition to the animals living 

 on the sea-bottom there are others which live near the 

 surface of the ocean, far from land ; such pelagic forms 

 have a wider geographical range than those which live 

 on the sea-floor in shallow water, and are consequently of 



