MANY FAUNAS OF ONE AGE. 449 



reached the conception of a continuous succession of life on the earth, 

 modified in successive periods through all past time. Still this did 

 not invalidate the doctrine of the identification of strata lay fossils, 

 though geologists began to find from experience, and taught that some 

 species lost their identity more rapidly than others, and that species 

 have different values in identifying strata. After Edward Forbes had 

 marked out the limits of life in zones of depth, and in his " Natural 

 History of the European Sea " had traced the limits and life of exist- 

 ing natural history provinces, and in his discussion of the geological 

 relations of the British fauna and flora, had traced the geographical 

 and geological antecedents of existing life, 1 only an effort of the 

 imagination was needed to set the existing life provinces in motion 

 with the upheaval and depression of land, and see that the succession 

 of life in time implied migration, and therefore that no flora or fauna 

 was ever universal. 2 This discovery limited the idea of identification 

 of strata by fossils ; for it implied that similar fossils could only 

 prove the continuity of a stratum when the deposit took place in the 

 same zoological province. The moment we travel beyond that central 

 part of the province where its typical species and genera abound, so 

 soon do we find a greater change in life than the geologist would 

 expect in a stratum. It is impossible to run a line along the sea-bed 

 from Britain north to Spitzbergen, or south to the Azores, south- 

 west to the West Indies, or west to Labrador, without passing over 

 groups of organisms which if fossilised would not tend to establish 

 contemporaneous deposition by community of life. Among many 

 groups of animals there are species which are world-wide ; but the 

 wider the range in space the longer is the duration in time, and there- 

 fore the less the value in identifying a stratum ; and in existing seas 

 a geographical interval means a change iii*fauna. So must it have 

 been in all past time ; otherwise there could have been no succession 

 of life in time. It may have been that life provinces in the sea were 

 as numerous in primary or secondary ages as they are now ; in any 

 case we must appeal to them in explanation of the successive changes 

 of life in the geological formations of any one district ; and we cannot 

 imagine such geographical changes as would produce succession of strata, 

 without the elaboration of life provinces at the same time. If we recog- 

 nise this ancient arrangement of life, then it follows that the life of 

 several geological formations existed simultaneously in the same sea, 

 otherwise they could not have been superimposed. That is to say, 

 the life which we term Silurian, Devonian, Carboniferous, must in its 

 main elements have been contemporaneous in horizontal succession in 

 the oceans of the ancient world, otherwise it could not have been 

 accumulated in vertical succession in the strata. But if these faunas 

 were contemporaneous, the fossils cannot identify different limited 

 periods of time in widely separated localities. 



Homotaxis. De la Beche 3 was the first to recognise the difficulties 



1 Mem. Geol. Survey, vol. i. 1846. 



2 Annal. and Mag. Nat. Hist. Dec. 1867. 



3 "Researches in Theoretical Geology," p. 261. 1834. 

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