280 ABSENCE OF INTERMEDIATE VARIETIES Chap. IX. 



changes oocurrcd in other parts of America duriiifr this space 

 of time. "\\'h(ui such beds as were deposited in shallow water 

 near the mouth of the Mississippi during some part of the gla- 

 cial period shall have been upraised, organic remains will prob- 

 ably lirst 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 ejioch 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 

 will have to go on continuously accumulating during a very 

 long period, so that there may be time sufficient for the slow 

 procT^ss of modification ; hence the deposit will have to be a 

 very thick one ; and the species undergoing change will have 

 to live in the same district throughout this 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 hve on 

 the same space, the supply of sediment must nearly counter- 

 balance the amount of subsidence. But this same movement 

 of subsidence will tend to submerge the area whence the sedi- 

 ment is derived, and thus diminish the supply while the down- 

 ward movement continues. In fact, this nearly exact balancing 

 between the supply of sediment and the amount of subsidence 

 is probably a rare contingency ; for it has been observed, by 

 more than one paleontologist, that very thick deposits are 

 usually barren of organic remains, except near their upper or 

 lower limits. 



It would seem that each separate formation, like the whole 

 pile of formations in any country, has generally been intermit- 

 tent in its accumulation. When we see, as is so often the case, 

 a formation composed of beds of dilVercnt mineralogical com- 

 position, we may reasonably suspect that the jirocess of deposi- 

 tion has been much intcrnij)t(Ml, as a change in the currents of 

 the sea, and a supply of sediment of a different nature will 

 generally have been due to geographical changes requiring 

 much time. Nor will the closest ins]iection of a formation give 

 any idea of the time which its deposition has consumed. Many 

 instances could be given of beds only a few feet in thickness, 



