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 palaeontologists, 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.