Continental and Island Life [pp. 1-29]

CONTINENTAL AND ISLAND LIFE.
inclined to fix on forty-eight millions of years as the probable
limit of available time; but new facts and suggestions recently
obtained induce me to think that the modern rates of denudation and deposit must be taken as far below the average, and
that perhaps the estimate stated by Wallace as resulting from
one mode of calculation, namely, twenty-eight millions, may be
not far from the truth, tho perhaps admitting of considerable
abatement.
This reduced estimate of geological time would still give
scope enough for the distribution of animals and plants, but
it will scarcely give that required by certain prevalent theories
of evolution. When Darwin says, "If the theory (of natural
selection) be true, it is indisputable that before the lowest Cambrian stratum was deposited long periods elapsed, as long as or
probably far longer than the whole interval from the Cambrian
to the present day," he makes a demand which geology cannot
supply; for independently of our ignorance of any formations
or fossils, except the Laurentian and its Eozoon, to represent
this vast succession of life, thie time required would push us
back into a molten state of the planet. This difficulty is akin
to that which meets us with reference to the introduction of
many and highly specialized mammals in the Eocene, or of the
forests of modern type in the Cretaceous. To account for the
origin of these by slow and gradual evolution requires us to
push these forms of life so far back into formations which afford
no trace of them, but, on the contrary, contain other creatures
that appear to be exclusive of them, that our faith in the theory
fails. The only theory of evolution which seems to meet this
difficulty is that, advanced by Mivart, Leconte, and Saporta, of
"critical periods," or periods of rapid introduction of new species alternating with others of comparative inaction. This
would much better accord with the apparently rapid introduc.
tion of many new forms of life over wide regions at the same
period. It would also approach somewhat near, in its manner
of stating the problem to be solved, to the theory of "creation by law" as held by the Duke of Argyll, or to what may
be regarded as "mediate creation," proceeding in a regular and
definite manner, but under laws and forces as yet very imperfectly known, throughout geological time.
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