264 ON THE ORIGIN OF SPECIES 



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 during 

 the whole of the glacial period near the mouth of 

 the Mississippi, within that limit of depth at which 

 marine animals can flourish ; for we know what vast 

 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 

 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 of the embedded fossils had been less than that 

 of the glacial period, instead of having been really far 

 greater, that is extending from before the glacial epoch 

 to the present day. 



In order to get a perfect gradation between two forms 

 in the upper and lower parts of the same formation, the 

 deposit must have gone on accumulating for a very long 

 period, in order to have given sufficient time for the 

 slow process of variation ; hence the deposit will gener- 

 ally have to be a very thick one ; and the species 

 undergoing modification will have had to live on the 



