,
and b14, etc.) would have to be united into one family; and the two
other families (namely, a14 to f14 now including five genera, and o14
to m14) would yet remain distinct. These two families, however, would
be less distinct from each other than they were before the discovery
of the fossils. If, for instance, we suppose the existing genera of
the two families to differ from each other by a dozen characters, in
this case the genera, at the early period marked VI., would differ by
a lesser number of characters; for at this early stage of descent they
have not diverged in character from the common progenitor of the
order, nearly so much as they subsequently diverged. Thus it comes
that ancient and extinct genera are often in some slight degree
intermediate in character between their modified descendants, or
between their collateral relations.
In nature the case will be far more complicated than is represented in
the diagram; for the groups will have been more numerous, they will
have endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last volume of the
geological record, and that in a very broken condition, we have no
right to expect, except in very rare cases, to fill up wide intervals
in the natural system, and thus unite distinct families or orders. All
that we have a right to expect, is that those groups, which have
within known geological periods undergone much modification, should in
the older formations make some slight approach to each other; so that
the older members should differ less from each other in some of their
characters than do the existing members of the same groups; and this
by the concurrent evidence of our best palaeontologists seems
frequently to be the case.
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