UNIFORMITY OF CHANGE 431 



reproduced. The submergence, then, of land must be often 

 attended by the commencement of a new class of sedimentary 

 deposits, characterized by a new set of fossil animals and 

 plants, while the reconversion of the bed of the sea into land 

 may arrest at once and for an indefinite time the formation 

 of geological monuments. Should the land again sink, strata 

 will again be formed; but one or many entire revolutions 

 in animal or vegetable life may have been completed in the 

 interval. 



As to the want of completeness in the fossiliferous series, 

 which may be said to be almost universal, we have only to 

 reflect on what has been already said of the laws governing 

 sedimentary deposition, and those which give rise to fluctua- 

 tions in the animate world, to be convinced that a very rare 

 combination of circumstances can alone give rise to such a 

 superposition and preservation of strata as will bear testi- 

 mony to the gradual passage from one state of organic life 

 to another. To produce such strata nothing less will be req- 

 uisite than the fortunate coincidence of the following con- 

 ditions: first, a never- failing supply of sediment in the same 

 region throughout a period of vast duration ; secondly, the 

 fitness of the deposit in every part for the permanent preser- 

 vation of imbedded fossils ; and, thirdly, a gradual subsidence 

 to prevent the sea or lake from being filled up and converted 

 into land. 



It will appear in the chapter on coral reefs, that, in cer- 

 tain parts of the Pacific and Indian Oceans, most of these 

 conditions, if not all, are complied with, and the constant 

 growth of coral, keeping pace with the sinking of the bottom 

 of the sea, seems to have gone on so slowly, for such indefi- 

 nite periods, that the signs of a gradual change in organic 

 life might probably be detected in that quarter of the globe 

 if we could explore its submarine geology. Instead of the 

 growth of coralline limestone, let us suppose, in some other 

 place, the continuous deposition of fluviatile mud and sand, 

 such as the Ganges and Brahmapootra have poured for 

 thousands of years into the Bay of Bengal. Part of this bay, 

 although of considerable depth, might at length be filled up 

 before an appreciable amount of change was effected in the 

 . fish, mollusca, and other inhabitants of the sea and neigh- 



