482 RECAPITULATION. 



ical formation, they appear as if suddenly created there, 

 and will \e simply classed as new species. Most forma- 

 tions 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 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 be- 

 neath 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 cceans. With respect to 

 the lapse of time not having been sufficient 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 philoso- 

 phers are not as yet willing to admit that we know enough 

 of the constitution of the universe and of the interior of 

 our globe to speculate with safety on its past duration. 



That the geological record is imperfect all will admit; 

 but that it is imperfect to the degree required by our 

 theory, few will be inclined to admit. If we look to long 

 enough intervals of time, geology plainly declares that 

 species have all changed; and they have changed in the 

 manner recjuired by the theory, for they have changed 

 slowly and m a graduated manner. We clearly see this in 

 the fossil remains from consecutive formations invariably 

 being much more closely related to each other than are the 

 fossils from widely separated formations. 



Such is the sum of the several chief objections and 



