If we
look to rather wider intervals of time, namely, to distinct but consecutive
stages of the same great formation, we find that the embedded fossils,
though universally ranked as specifically different, yet are far more
closely related to each other than are the species found in more widely
separated formations; so that here again we have undoubted evidence of
change in the direction required by the theory; but to this latter subject
I shall return in the following chapter.
With animals and plants that propagate rapidly and do not wander much,
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-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.
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