The noble science of geology loses glory from the extreme imperfection of
the record. The crust of the earth, with its embedded remains, must not be
looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual occurrence of
favourable circumstances, and the blank intervals between the successive
stages as having been of vast duration. But we shall be able to gauge with
some security the duration of these intervals by a comparison of the
preceding and succeeding organic forms. We must be cautious in attempting
to correlate as strictly contemporaneous two formations, which do not
include many identical species, by the general succession of the forms of
life. As species are produced and exterminated by slowly acting and still
existing causes, and not by miraculous acts of creation; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism--the improvement of one
organism entailing the improvement or the extermination of others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the relative, though not
actual lapse of time.
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