27C ABSENCE OF INTERMEDIATE VARIETIES [Chap. X. 



Woodward, have concluded that the average duration of each for- 

 mation is twice or thrice as long as the average duration of specific 

 forms. But insuperahle 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 he 

 rash in the extreme to infer that it had not elsewhere previously 

 existed. So again when we find a species disappearing hefore the 

 last layers have been deposited, it would he equally rash to suppose 

 that it then became extinct. 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 cor- 

 related 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 appearing in any forma- 

 tion, the probability is that it only then first immigrated into that 

 area. It is well known, for instance, that several species appeared 

 somewhat earlier in the palaeozoic beds of North America than in 

 those cf Europe ; time having 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 every- 

 where been noted, that some few still existing species are common 

 in the deposit, but have become extinct in the immediately sur- 

 rounding sea ; or, conversely, that some are now abundant in the 

 neighbouring sea, but are rare or absent in this particular deposit. 

 It is an excellent lesson to reflect on the ascertained amount of 

 migration of the inhabitants of Europe during the glacial epoch, 

 which forms only a part of one whole geological period ; and like- 

 wise 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 America during this space of time. When 

 such beds as were deposited in shallow water near the mouth of the 

 Mississippi during some part of the glacial period shall have been 

 upraised, organic remains will probably first appear and disappear 

 at different levels, owing to the migrations of species and to geo- 

 graphical changes. And in the distant future, a geologist, examining 



