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 rare cases, to fill up the wide intervals in the natural system,
and thus to 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 is frequently 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, are explained in a satisfactory manner. And they are
wholly inexplicable on any other view.
On this same theory, it is evident that the fauna during any one 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 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.
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