68 ABSENCE OF INTEKBIEDIATE VARIETIES [Chap. X. 



including fossil remains, have gone on accumulating 

 witliin 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 geograpMcal 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 accumiilate 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 must nearly counterbalance the 

 amount of subsidence. But this same movement of 



