It is a more important consideration, leading to the same result, as lately
insisted on by Dr. Falconer, namely, that the period during which each
species underwent modification, though long as measured by years, was
probably short in comparison with that during which it remained without
undergoing any change.
It should not be forgotten, that at the present day, with perfect specimens
for examination, two forms can seldom be connected by intermediate
varieties, and thus proved to be the same species, until many specimens are
collected from many places; and with fossil species this can rarely be
done. We shall, perhaps, best perceive the improbability of our being
enabled to connect species by numerous, fine, intermediate, fossil links,
by asking ourselves whether, for instance, geologists at some future period
will be able to prove that our different breeds of cattle, sheep, horses,
and dogs are descended from a single stock or from several aboriginal
stocks; or, again, whether certain sea-shells inhabiting the shores of
North America, which are ranked by some conchologists as distinct species
from their European representatives, and by other conchologists as only
varieties, are really varieties, or are, as it is called, specifically
distinct. This could be effected by the future geologist only by his
discovering in a fossil state numerous intermediate gradations; and such
success is improbable in the highest degree.
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