294 IMPEEFECTION OF THE Chap. IX. 



only then first immigrated into that area. It is well 

 known, for instance, that several species appeared some- 

 what earlier in the palaeozoic beds of North America 

 than in those of Europe ; time having apparently been 

 required for their migration from the American to the 

 European seas. In examining the latest deposits of 

 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 immedi- 

 ately surrounding 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 period, 

 which forms only a part of one whole geological period ; 

 and likewise to reflect on the great changes of level, 

 on the inordinately great change of climate, on the 

 prodigious 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 dur- 

 ing the whole of the glacial period near the mouth of 

 the Mississippi, within that limit of depth at which ma- 

 rine animals can flourish ; for we know what vast geo- 

 graphical 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 Mis- 

 sissippi 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 migration 

 of species and to geographical changes. And in the 

 distant future, a geologist examining these beds, might 

 be tempted to conclude that the average duration of life 



