RECAPITULATION AND CONCLUSION 505 



intermediate links in any one formation less likely. Local 

 varieties will not spread into other and distant regions until 

 they are considerably modified and improved; and when they 

 have spread, and are discovered in a geological formation, 

 they appear as if suddenly created there, and will be simply 

 classed as new species. Most formations have been inter- 

 mittent in their accumulation; and their duration has prob- 

 ably 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 fos- 

 siliferous formations thick enough to resist future degrada- 

 tion can as a general rule be accumulated only where much 

 sediment is deposited on the subsiding bed of the sea. Dur- 

 ing 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 conti- 

 nents 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 for- 

 mations much older than any now known may lie buried be- 

 neath the great oceans. 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 philosophers are not as yet willing 

 to admit that we know enough of the constitution of the uni- 

 verse 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 required by 

 the theory, for they have changed slowly and in a graduated 



