from each other by a less number of characters;
for they would at this early stage of descent have diverged in a less
degree from their common progenitor. Thus it comes that ancient and
extinct genera are often in a greater or less degree intermediate
in character between their modified descendants, or between their
collateral relations.

Under nature the process 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 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. 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 an