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 certain pachyderms.
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