Number VI. but none from beneath this line, then only the two families
on the left hand (namely, _a_14, etc., and _b_14, etc.) would have to
be united into one family; and the two other families (namely, _a_14 to
_f_14 now including five genera, and _o_14 to _m_14) 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 pal ontologists seems frequently to
be the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory
manner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period
in the earth s history will be intermediate
in general character between that which preceded and that which
succeeded it. Thus, the species which lived at the sixth great stage of
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