ese three
families, together with the many extinct genera on the several lines of
descent diverging from the parent-form (A), will form an order; for all
will have inherited something in common from their ancient and common
progenitor. On the principle of the continued tendency to divergence of
character, which was formerly illustrated by this diagram, the more recent
any form is, the more it will generally differ from its ancient progenitor.
Hence we can understand the rule that the most ancient fossils differ most
from existing forms. We must not, however, assume that divergence of
character is a necessary contingency; it depends solely on the descendants
from a species being thus enabled to seize on many and different places in
the economy of nature. Therefore it is quite possible, as we have seen in
the case of some Silurian forms, that a species might go on being slightly
modified in relation to its slightly altered conditions of {332} life, and
yet retain throughout a vast period the same general characteristics. This
is represented in the diagram by the letter F^{14}.

All the many forms, extinct and recent, descended from (A), make, as before
remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.

By looking at the diagram we can see that if many of the extinct forms,
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a^1, a^5, a^{10}, f^8, m^3, m^6, m^9, were
disinterred, these three families would be so closely linked together that
they probably would have to be united into one great family, in nearly the
same manner as has occurred with ruminants and pachyderms. Yet he who
objected to call the extinct genera, which thus linked the living genera of
three families together, intermediate in character, would be justified, as
they are intermediate, not directly, but only by a long and circuitous
course through many widely different forms. If many extinct forms were to
be discovered above one of the middle horizontal lines or geological
formations --for instance, above No. VI.--but none from beneath this line,
t