yet are far more closely allied to each other than are the species
found in more widely separated formations; but to this subject I shall
have to return in the following chapter.
One other consideration is worth notice: with animals and plants that
can propagate rapidly and are not highly locomotive, there is reason to
suspect, as we have formerly seen, that their varieties are generally
at first local; and that such local varieties do not spread widely and
supplant their parent-forms 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 probably those
which have had the widest range, far exceeding the limits of the known
geological formations of Europe, which 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 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 have been collected from many places; and in the case of
fossil species this could rarely be effected by pal ontologists. 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 have 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 only by the future
geologist discovering in a fossil state numerous intermediate
gradations; and such success seems to me improbable in the highest
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