CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
4. ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION.
>From these several 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 much 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.
Several cases are on record 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 multiformis 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
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. 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 throughout Europe been correlated with perfect accuracy.