These causes, taken
conjointly, will to a large extent explain why--though we do find many
links--we do not find interminable varieties, connecting together all
extinct and existing forms by the finest graduated steps. It should also
be constantly borne in mind that any linking variety between two forms,
which might be found, would be ranked, unless the whole chain could be
perfectly restored, as a new and distinct species; for it is not pretended
that we have any sure criterion by which species and varieties can be
discriminated.
He who rejects this view of the imperfection of the geological record, will
rightly reject the whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the successive stages of
the same great formation? He may disbelieve in the immense intervals of
time which must have elapsed between our consecutive formations; he may
overlook how important a part migration has played, when the formations of
any one great region, as those of Europe, are considered; he may urge the
apparent, but often falsely apparent, sudden coming in of whole groups of
species. He may ask where are the remains of those infinitely numerous
organisms which must have existed long before the Cambrian system was
deposited? We now know that at least one animal did then exist; but I can
answer this last question only by supposing that where our oceans now
extend they have extended for an enormous period, and where our oscillating
continents now stand they have stood since the commencement of the Cambrian
system; but that, long before that epoch, the world presented a widely
different aspect; and that the older continents, formed of formations older
than any known to us, exist now only as remnants in a metamorphosed
condition, or lie still buried under the ocean.
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