348 ORIGIN OF SPECIES 



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 dur- 

 ing 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 

 those 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 under- 

 going 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 ap- 

 proxi'mately 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 subsi- 

 dence. 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 con- 

 tinues. 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 palaeontologist, 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 inter- 

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



