descent in the diagram are the modified offspring of those which lived
at the fifth stage, and are the parents of those which became still
more modified at the seventh stage; hence they could hardly fail to be
nearly intermediate in character between the forms of life above and
below. We must, however, allow for the entire extinction of some
preceding forms, and for the coming in of quite new forms by
immigration, and for a large amount of modification, during the long
and blank intervals between the successive formations. Subject to these
allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding faunas.
I need give only one instance, namely, the manner in which the fossils
of the Devonian system, when this system was first discovered, were at
once recognised by pal ontologists as intermediate in character between
those of the overlying carboniferous, and underlying Silurian system.
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
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