68 ABSENCE OP INTERMEDIATE VARIETIES [Chap. X 



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 de- 

 posited 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 prob- 

 ably first appear and disappear at different levels, ow- 

 ing to the migrations of species and to geographical 

 changes. And in the distant future, a geologist, ex- 

 amining 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 hav- 

 ing been really far greater, that is, extending from be- 

 fore 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 forma- 

 tion, the deposit must have gone on continuously ac- 

 cumulating 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 ac- 

 cumulate only during a period of subsidence; and to 

 keep the depth approximately the same, which is neces- 

 sary that the same marine species may live on the same 

 space, the supply of sediment must nearly counterbal- 

 ance the amount of subsidence. But this same move- 



