IMPERFECTION OF GEOLOGICAL RECORD 263 



cases are on record of the same species presenting 

 distinct varieties in the upper and lower parts of the 

 same formation, but, as they are rare, they may be here 

 passed over. Although each formation has in- 

 disputably required a vast number of years for its 

 deposition, I can see several reasons why each should 

 not include a graduated series of links between the 

 species which then lived ; but I can by no means pre- 

 tend to assign due proportional weight to the following 

 considerations. 



Although each formation may mark a very long lapse 

 of years, each perhaps is short compared with the period 

 requisite to change one species into another. I am 

 aware that two palaeontologists, 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 

 duration of specific forms. But insuperable difficulties, 

 as it seems to me, prevent us 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 uppermost layers have been 

 deposited, it would be equally rash to suppose that it 

 then became wholly 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. 



With marine animals of all kinds, we may safely 

 infer a large amount of migration during 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 appeared 

 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 of 



