THE IMPERFECTION OF THE GEOLOGICAL RECORD 281 



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 multi- 

 formis 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 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 palaeon- 

 tologists, 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 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 appearing in the middle of any forma- 

 tion, it would be rash in the extreme to infer that it had not else- 

 where previously existed. So again, when we find a species dis- 

 appearing 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 formation through- 

 out 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 

 for their migration from the American to the European seas. In 

 examining the latest deposits, in various quarters of the world, 

 it has everywhere been noted, that some few still existing species 

 are common in the deposit, but have become extinct in the im- 

 mediately surrounding sea; or, conversely, that some are now 

 abundant in the neighboring sea, but are rare or absent in this 

 particular deposit. It is an excellent lesson to reflect on the ascer- 

 tained amount of migration of the inhabitants of Europe during 

 the glacial epoch, which forms only a part of one whole geological 



