CHAP. X.] IN ANY SINGLE FORMATION. 259 



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 imme- 

 diately surrounding sea ; or, conversely, that some are now abun- 

 dant 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 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 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 

 geographical changes. And in the distant future, a geologist, 

 examining these beds, would 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 continuously accumulating during a long period, 

 sufficient for the slow process of modification ; hence the deposit 

 must be a very thick one; and the species undergoing change 

 must have lived in the same district throughout the whole time. 

 But we have seen that a thick formation, fossiliferous throughout 

 its entire thickness, can accumulate only during a period of sub- 

 sidence; and to keep the depth approximately the same, which 

 is necessary that the same marine species may live on the same 

 space, the supply of sediment must nearly counterbalance the 

 amount of subsidence. But this same movement of subsidence 

 will tend to submerge the area whence the sediment is derived, 

 and thus diminish the supply, whilst the downward movement 

 continues. In fact, this nearly exact balancing between the 

 supply of sediment and the amount of subsidence is probably 



