Chap. XIV. RECAPITULATION. 465 



proved ; and when they do spread, if discovered in a 

 geological formation, they will appear as if suddenly 

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

 Most formations have been intermittent in their ac- 

 cumulation ; and their duration, I am inclined to believe, 

 has been shorter than the average duration of specific 

 forms. Successive formations are separated from each 

 other by enormous blank intervals of time ; for fossili- 

 ferous formations, thick enough to resist future de- 

 gradation, can be accumulated only where much sedi- 

 ment is deposited on the subsiding bed of the sea. 

 During the alternate periods of elevation and of station- 

 ary level the record will be blank. During these latter 

 periods there will probably be more variability in the 

 forms of life ; during periods of subsidence, more ex- 

 tinction. 



With respect to the absence of fossiliferous formations 

 beneath the lowest Silurian strata, I can only recur to 

 the hypothesis given in the ninth chapter. That the 

 geological record is imperfect all will admit ; but that 

 it is imperfect to the degree which I require, few will 

 be inclined to admit. If we look to long enough in- 

 tervals of time, geology plainly declares that all species 

 have changed ; and they have changed in the manner 

 which my theory requires, for they have changed slowly 

 and in 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 formations distant from each other in 

 time. 



Such is the sum of the several chief objections and 

 difficulties which may justly be urged against my theory ; 

 and I have now briefly recapitulated the answers and 

 explanations which can be given to them. I have felt 

 these difficulties far too heavily during many years to 



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