hen only the two families on the left hand (namely, a^{14}, &c, and
b^{14},&c.) would have to be united into one family; and the two other
families (namely, a^{14} to f^{14} now including five genera, and o^{14} to
m^{14}) would yet remain distinct. These two families, however, would be
less distinct from each other than they were before the discovery of the
fossils. If, for instance, we suppose the existing genera of the two
families to differ from each {333} 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 {334} diagram are the modified
offspring of those which lived at the fi