ad widely and supplant their parent-form until they have been
modified and perfected in some considerable degree. According to this
view, the chance of discovering in a formation in any one country all
the early stages of transition between any two forms, is small, for the
successive changes are supposed to have been local or confined to some
one spot. Most marine animals have a wide range; and we have seen that
with plants it is those which have the widest range, that oftenest
present varieties, so that, with shells and other marine animals, it is
probable that those which had the widest range, far exceeding the limits
of the known geological formations in Europe, have oftenest given rise,
first to local varieties and ultimately to new species; and this again
would greatly lessen the chance of our being able to trace the stages of
transition in any one geological formation.

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.

It has been asserted over and over again, by writers who believe in
the immutability of species, that geology yields no linking f