Nature may almost be said to have guarded against the
frequent discovery of her transitional or linking forms.
From the foregoing 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 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. 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.
Although 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 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 coming to any
just conclusion on this head.
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