72 THE ORIGIN OF SPECIES 



On the Absence of Numerous Intermediate Varieties in any 



Single Formation 



From these several considerations it cannot be doubted 

 that the geological record, viewed as a whole, is ex- 

 tremely imperfect; but if we confine our attention to any 

 one formation, it becomes much more difficult to under- 

 stand why we do not therein find closely graduated vari- 

 eties between the allied species which lived at its com- 

 mencement and at its close. Several cases are on record 

 of the same species presenting varieties in the upper and 

 lower parts of the same formation; thus, Trautschold 

 gives a number of instances with Ammonites; and Hil- 

 gendorf has described a most curious case of ten grad- 

 uated forms of Planorbis multiformis in the successive 

 beds of a fresh-water formation in Switzerland. Although 

 each formation has indisputably required a vast number 

 of years for its deposition, several reasons can be given 

 why each should not commonly include a graduated 

 series of links between the species which lived at its 

 commencement and close; but I cannot assign due pro- 

 portional weight to the following considerations. 



Although each formation may mark a very long lapse 

 of years, each probably is short compared with the period 

 requisite to change one species into another. I am aware 

 that two paleontologists, whose opinions are worthy of 

 much deference, namely Bronn and Woodward, have 

 concluded that the average duration of each formation 

 is twice or thrice as long as the average duration of 

 specific forms. But insuperable difficulties, as it seems 

 to me, prevent us from coming to any just conclusion 



