306 ABSENCE OF INTERMEDIATE .VARIETIES 



deference, namely Bronn and Woodward, have concluded 

 that the average iuration of each formation is twice 01 

 thrice as long as the average duration of specific forms. 

 But insuperable difficulties, as it seems to me, prevent us 

 from coming to any just conclusion on this head. When we 

 see a species first appearing in the middle of any formation, 

 it would be rash in the extreme to infer that it had not else- 

 where previously existed. So again, when we find a species 

 disappearing before the last layers have been deposited, it 

 would be equally rash to suppose that it then became ex- 

 tinct. We forget how small the area of Europe is, compared 

 with the rest of the world ; nor have the several stages of 

 the same formation throughout Europe been correlated with 

 perfect accuracy. 



We may safely infer that with marine animals of all kinds 

 there has been a large amount of migration due to climatal 

 and other changes ; and when we see a species first appear- 

 ing in any formation, the probability is that it only then first 

 immigrated into that area. It is well known, for instance, 

 that several species appear somewhat earlier in the palaeozic 

 beds of North America than in those of Europe ; time hav- 

 ing apparently been required for their migration from the 

 American to the European seas. In examining the latest 

 deposits, in various quarters of the world, it has everywhere 

 been noted, that some few still existing species are common 

 in the deposit, but have become extinct in the immediately 

 surrounding sea ; or, conversely, that some are now abundant 

 in the neighboring sea, but are rare or absent in this partic- 

 ular deposit. It is an excellent lesson to reflect on the ascer- 

 tained amount of migration of the inhabitants of Europe 

 during the glacial epoch, whi^h forms only a part of one 

 whole geological period ; and likewise to reflect on the 

 changes of level, on the extreme change of climate, and on 

 the great lapse of time, all included within this same glacial 

 period. Yet it may be doubted whether, in any quarter of 

 the world, sedimentary deposits, including fossil remains, 

 have gone on accumulating within the same area during the 

 whole of this period. It is not, for instance, probable that 

 sediment was deposited during the whole of the glacial period 

 near the mouth of the Mississippi, within that limit of depth 

 at which marine animals can best flourish : for we know that 

 great geographical changes occurred in other parts of Amer- 

 ica during this space of time. When such beds as were de- 

 posited in shallow water near the mouth of the Mississippi 



