258 ABSENCE OF INTERMEDIATE VARIETIES [CHAP. X. 



new varieties or species will be formed; and it is during these 

 very periods of subsidence, that the deposits which are richest in 

 fossils have been accumulated. 



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 Ammonites; 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 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 proportional weight to the following con- 

 siderations. 



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 palaeonto- 

 logists, 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 on 

 this head. When we see a species first appearing in the middle 

 of any formation, it would be rash in the extreme to infer that it 

 had not elsewhere previously existed. So again when we find a 

 species disappearing before the last layers have been deposited, 

 it would be equally rash to suppose that it then became extinct. 

 We forget how small the area of Europe is compared with the 

 rest of the world ; nor have the several stages of the same forma- 

 tion throughout Europe been correlated with perfect accuracy. 



We may safely infer that with marine animals of all kinds 

 there has been a large amount of migration due to climatal and 

 other changes ; and when we see a species first appearing in any 

 formation, the probability is that it only then first immigrated 

 into that area. It is well known, for instance, that several species 

 appear somewhat earlier in the palaeozoic beds of North America 

 than in those of Europe ; time having apparently been required 



