The diagram is much too simple, too few genera and too few
species being given, but this is unimportant for us. The horizontal
lines may represent successive geological formations, and all the
forms beneath the uppermost line may be considered as extinct. The
three existing genera, a14, q14, p14, will form a small family; b14
and f14 a closely allied family or sub-family; and o14, e14, m14, 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.
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