SIR CHARLES LYELL 217 



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 fluctuations in the animate world, to be con- 

 vinced that a very rare combination of circumstances can alone give 

 rise to such a superposition and preservation of strata as will bear 

 testimony to the gradual passage from one state of organic life to an- 

 other. To produce such strata nothing less will be requisite than the 

 fortunate coincidence of the following conditions : 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 preservation of imbedded fossils ; and, thirdly, a gradual 

 subsidence to prevent the sea or lake from being filled up ^nd con- 

 verted into land. 



It will appear in the chapter on coral reefs, that, in certain 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 indefinite 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 neighbouring land. But if the bottom be lowered by sinking at 

 the same rate that it is raised by fluviatile mud, the bay can never 

 be turned into dry land. In that case one new layer of matter may 

 be superimposed upon another for a thickness of many thousand feet, 

 and the fossils of the inferior beds may differ greatly from those 

 entombed in the uppermost, yet every intermediate gradation may be 

 indicated in the passage from an older to a newer assemblage of 

 species. Granting, however, that such an unbroken sequence of 

 monuments may thus be elaborated in certain parts of the sea, and 



