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 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 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 Number VI.--but
none from beneath this line, then only the two families on the left
hand (namely, a14, etc., and b14, etc.) would have to be united into
one family; and the two other families (namely, a14 to f14 now including
five genera, and o14 to m14) 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