he 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 a1, a5, a10, f8, m3, m6, m9, 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 certain pachyderms. Yet
he who objected to consider as intermediate the extinct genera, which
thus link together the living genera of three families, would be partly
justified, for 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, then only two of the families (those on the left
hand a14, etc., and b14, etc.) would have to be united into one; and
there would remain two families which would be less distinct from each
other than they were before the discovery of the fossils. So again, if
the three families formed of eight genera (a14 to m14), on the uppermost
line, be supposed to differ from each other by half-a-dozen important
characters, then the families which existed at a period marked VI would
certainly have differed