But each
fauna is not necessarily exactly intermediate, as unequal intervals of
time have elapsed between consecutive formations.
It is no real objection to the truth of the statement, that the fauna
of each period as a whole is nearly intermediate in character between
the preceding and succeeding faunas, that certain genera offer
exceptions to the rule. For instance, mastodons and elephants, when
arranged by Dr. Falconer in two series, first according to their
mutual affinities and then according to their periods of existence, do
not accord in arrangement. The species extreme in character are not
the oldest, or the most recent; nor are those which are intermediate
in character, intermediate in age. But supposing for an instant, in
this and other such cases, that the record of the first appearance and
disappearance of the species was perfect, we have no reason to believe
that forms successively produced necessarily endure for corresponding
lengths of time: a very ancient form might occasionally last much
longer than a form elsewhere subsequently produced, especially in the
case of terrestrial productions inhabiting separated districts. To
compare small things with great: if the principal living and extinct
races of the domestic pigeon were arranged as well as they could be in
serial affinity, this arrangement would not closely accord with the
order in time of their production, and still less with the order of
their disappearance; for the parent rock-pigeon now lives; and many
varieties between the rock-pigeon and the carrier have become extinct;
and carriers which are extreme in the important character of length of
beak originated earlier than short-beaked tumblers, which are at the
opposite end of the series in this same respect.
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