74 THE ORIGIN OF SPECIES 



whether, in any quarter of the world, sedimentary depos- 

 its, 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 ma- 

 rine 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 de- 

 posited in shallow water near the mouth of the Missis- 

 sippi 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 dis- 

 tant future, a geologist, examining these beds, would be 

 tempted to conclude that the average duration of life 

 of the imbedded 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 subsidence; 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 



