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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
→include few
identical species, by the general succession of
forms of life. As species are produced and exterminated by slowly acting and still existing causes, and not by miraculous acts of
and
→by catastrophes; 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
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
→lapse of actual
time. A number of species, however, keeping in a body might remain for a long period unchanged, whilst within
same period, several of these species, by migrating into new countries and coming into competition with foreign associates, might become modified; so that we must not overrate the accuracy of organic change as a measure of time. During early periods of the
history, when the forms of life were probably fewer and simpler, the rate of change was probably slower; and at the first dawn of life, when very few forms of the simplest structure existed, the rate of change may have been slow in an extreme degree. The
history of the world, as at present known, although of
→quite incomprehensible by us,
will hereafter be recognised as
→a mere fragment of time,
compared with the ages which
elapsed since the first
the
of innumerable extinct and living descendants,
→was created.
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