•AHi THE ORIGiy OF SPECIES 



when they have spread, and are discovered in a geo- 

 logical formation, they appear as if suddenly created 

 there, and will be simply classed as new species. Most 

 formations have been intermittent in their accumulation; 

 and their duration has probably been shorter than the 

 average duration of specific forms. Successive formations 

 are in most cases separated from each other by blank 

 intervals of time of great length; for fossiliferous forma- 

 tions thick enough to resist future degradation can as a 

 general rule be accumulated only where much sediment 

 is deposited on the subsiding bed of the sea. During 

 the alternate periods of elevation and of stationary level 

 the record will generally be blank. During these lat- 

 ter periods there will probably be more variability in 

 the forms of life; during periods of subsidence, more 

 extinction. 



With respect to the absence of strata rich in fossils 

 beneath the Cambrian formation, I can recur only to 

 the hypothesis given in the tenth chapter; namely, that 

 though our continents and oceans have endured for an 

 enormous period in nearly their present relative positions, 

 we have no reason to assume that this has alwaj^s been 

 the case; consequently formations much older than any 

 now known may lie buried beneath the great oceans. 

 With respect to the lapse of time not having been suffi- 

 cient since our planet was consolidated for the assumed 

 amount of organic change, and this objection, as urged 

 by Sir William Thompson, is probably one of the 

 gravest as yet advanced, I can only say, first, that we 

 do not know at what rate species change as measured 

 by years, and, secondly, that many philosophers are 

 not as yet willing to admit that we know enough 



