66 ABSENCE OF IKTEUMEDIATE VARIETIES [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 gradu- 

 ated 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 Swit- 

 zerland. Although each formation has indisputably 

 required a vast number of years for its deposition, sev- 

 eral 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 reqiiisite to change one species into another. I 

 am aware that two palsontologists, whose opinions are 

 worthy of much deference, namely Bronn and Wood- 

 ward, have concluded that the average duration of each 

 formation is twice or thrice as long as the average dura- 

 tion 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 



