Chap. IX. GEOLOGICAL EECOKD. 293 



stand, why we do not therein find closely graduated 

 varieties between the allied species which lived at its 

 commencement and at its close. Some 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. Al- 

 though each formation has indisputably 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 pretend 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 con- 

 clusion on this head. When we see a species first ap- 

 pearing in the middle of any formation, it would be rash 

 in the extreme to infer that it had not elsewhere pre- 

 viously existed. So again when we find a species disap- 

 pearing 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 Eu- 

 rope is compared with the rest of the world ; nor have 

 the several stages of the same formation 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 



