fth 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 in any one region for the immigration of new
forms from other regions, 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 palaeontologists 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 {335} 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 hav