Chap. XIV. RECAPITULATION. 417 



less likely. LckmI varieties will not spread into other and dis- 

 tant reg^ions until they are considerably modilied and improved ; 

 and when they have spread, and are discovered in a ji^eological 

 formation, they will appear as if silddenly 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 spe- 

 cific 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 degrada- 

 tion 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 

 recortl will generally be blank. During these latter periods 

 tliere will i^robably be more variability in the forms of life ; 

 during periods of subsidence, more extinction. 



Witii respect to the absence of strata rich in fossils beneath 

 the Caml)rian formation, I can recur only to the hypotliesis 

 given in the ninth chajiter. That the geological record is im- 

 perfect all will admit ; but that it is imperfect to the degree re- 

 quired l)y 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, for they have changed slowly and in a grad- 

 uated manner. We clearly see this in the fossil remains from 

 consecutive formations invariably being much more closely re- 

 lated to each other than are the fossils from widely-separated 

 formations. 



Such is the sum of the several chief objections and dilhcul- 

 ties which may justly be urged against the theory ; and 1 have 

 now briefly recapitulated the answers and explanations which 

 can be given. I have felt these diiliculties far too heavily dur- 

 ing many years to doubt their weight. But it deserves espe- 

 cial notice that the more important objections relate to ques- 

 tions on which we are confessedly ignorant ; nor do we know 

 how ignorant we are. AVe do not know all the possible tran- 

 sitional gradations between the simplest and the most perfect 

 organs; it cannot be pretendecl that we know all the varied 

 means of Distril)uti()n during the long lapse of years, or that 

 we know how imperfect the Geological Kecord is. ^ferious as 

 these several objections are, in \uy judgment they are not 

 suflicient to overthrow the theory of descent with subsequent 

 modification. 



