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 palaeontologists 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 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