In the first place, it is perfectly clear that if we admit the
conception above mentioned of a continuity of life from the
Laurentian period to the present day, we could never _prove_ our
view to be correct, unless we could produce in evidence fossil
examples of _all_ the kinds of animals and plants that have lived
and died during that period. In order to do this, we should require,
to begin with, to have access to an absolutely unbroken and perfect
succession of all the deposits which have ever been laid down
since the beginning. If, however, we ask the physical geologist
if he is in possession of any such uninterrupted series, he will
at once answer in the negative. So far from the geological series
being a perfect one, it is interrupted by numerous gaps of unknown
length, many of which we can never expect to fill up. Nor are
the proofs of this far to seek. Apart from the facts that we
have hitherto examined only a limited portion of the dry land,
that nearly two-thirds of the entire area of the globe is
inaccessible to geological investigation in consequence of its
being covered by the sea, that many deposits can be shown to
have been more or less completely destroyed subsequent to their
deposition, and that there may be many areas in which living
beings exist where no rock is in process of formation, we have
the broad fact that rock-deposition only goes on to any extent
in water, and that the earth must have always consisted partly of
dry land and partly of water--at any rate, so far as any period
of which we have geological knowledge is concerned.
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