Of the species which do change, only a few within the same country change
at the same time; and all modifications are slowly effected. I have also
shown that the intermediate varieties which probably at first existed in
the intermediate zones, would be liable to be supplanted by the allied
forms on either hand; for the latter, from existing in greater numbers,
would generally be modified and improved at a quicker rate than the
intermediate varieties, which existed in lesser numbers; so that the
intermediate varieties would, in the long run, be supplanted and
exterminated.
On this doctrine of the extermination of an infinitude of connecting links,
between the living and extinct inhabitants of the world, and at each
successive period between the extinct and still older species, why is not
every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? Although geological research has
undoubtedly revealed the former existence of many links, bringing numerous
forms of life much closer together, it does not yield the infinitely many
fine gradations between past and present species required on the theory,
and this is the most obvious of the many objections which may be urged
against it. Why, again, do whole groups of allied species appear, though
this appearance is often false, to have come in suddenly on the successive
geological stages? Although we now know that organic beings appeared on
this globe, at a period incalculably remote, long before the lowest bed of
the Cambrian system was deposited, why do we not find beneath this system
great piles of strata stored with the remains of the progenitors of the
Cambrian fossils? For on the theory, such strata must somewhere have been
deposited at these ancient and utterly unknown epochs of the world's
history.
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