genera, _a_14, _q_14, _p_14, will form a small family; _b_14 and _f_14
a closely allied family or sub-family; and _o_14, _e_14, _m_14, a third
family. These 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 life, and
yet retain throughout a vast period the same general characteristics.
This is represented in the diagram by the letter F14.
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
250