282 THE ORIGIN OF SPECIES 



period; and likewise to reflect on the changes of level, on the ex- 

 treme change of climate, and on the great lapse of time, all in- 

 cluded within this same glacial period. Yet it may be doubted 

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

 cluding 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 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 de- 

 posited 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, extend- 

 ing 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 sub- 

 sidence; 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 subsidence 

 will tend to submerge the area whence the sediment is derived, 

 and thus diminish the supply, while the downward movement 

 continues. In fact, this nearly exact balancing between the supply 

 of sediment and the amount of subsidence is probably a rare con- 

 tingency; for it has been observed by more than one palaeontolo- 

 gist that very thick deposits are usually barren of organic re- 

 mains, except near their upper or lower limits. 



It would seem that each separate formation, like the whole pile 

 of formation in any country, has generally been intermittent in 

 its accumulation. When we see, as is so often the case, a forma- 

 tion composed of beds of widely different mineralogical compo- 



