Chap. XV.] KECAPITULATION. 275 



duration has probably been shorter than th« 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 formations 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 latter 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 always 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, firstly, 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 of the constitu- 

 tion of the universe and of the interior of our globe to 

 speculate with safety on its past duration. 



flThat the geological record is imperfect all will admit ; 

 but" that it is imperfect to the degree required by our 

 theory, feevv will be inclined to admit. If we look to 

 long enough intervals of time, geology plainly declares 



