66 ABSENCE OF INTEEMEDIATE VAEIETIES [Chap. X. 



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 extremely imperfect ; but if we confine our attention 

 to any one formation, it becomes much more difficult to 

 understand why we do not therein find closely graduated 

 varieties between the allied species which lived at its 

 commencement 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 Am- 

 monites ; and Hilgendorf has described a most curious 

 case of ten graduated forms of Planorbis multiformis in 

 the successive beds of a fresh-water formation in 

 Switzerland. Although each formation has indispu- 

 tably 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 proportional 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 palieontologists, whose opinions are 

 worthy of much deference, namely Bronn and Wood- 

 ward, have concluded that the average duration of each 

 formation is twice or tlu-ice as long as the averao-e 

 duration of specific forms. But insuperable difficulties, 

 as it seems to me, prevent us from coming to any just 

 conclusion on this head. When we see a species first 



