| →allied genera existing
during the Silurian
→and
descended from
→some still earlier form.
three of these genera (A, F, and
→I), a species has
transmitted modified descendants to the present day, represented by the fifteen genera
(
a
14 z
14
)
on the uppermost horizontal line. Now all these modified descendants from a single species, are
related in blood or descent
the same degree; they may metaphorically be called cousins to the same millionth degree; yet they differ widely and in different degrees from each other. The forms descended from A, now broken up into two or three families, constitute a distinct order from those descended from I, also broken up into two families. Nor can the existing species, descended from A, be ranked in the same genus with the parent A; or those from I, with the parent I. But the existing genus
F
14
may be supposed to have been but slightly modified; and it will then rank with the parent-genus F; just as some few still living
belong to Silurian genera. So that the
value of the differences between
→these organic beings, which are
all related to each other in the same degree in blood, has come to be widely different. Nevertheless their genealogical
arrangement
remains strictly true, not only at the present time, but at each successive period of descent. All the modified descendants from A will have inherited something in common from their common parent, as will all the descendants from I; so will it be with each subordinate branch of descendants, at each successive
If, however, we
→suppose any
of
or of
→I, to have become
so much modified as to have
→lost all
traces of
parentage, in this case,
in
natural
will
→be lost, as
seems to have occurred with
→some few existing
organisms. All the descendants of the genus F, along its whole line of descent, are supposed to have been but little modified, and they
form a single genus. But this genus, though much isolated, will still occupy its proper intermediate position. The representation of the groups, as here given in the diagram on a flat surface, is much too simple. The branches ought to have diverged in all directions. ↑ If
→OMIT
the names of the groups had been
→simply written down
in a linear series,
would have been still less
→natural;
and it is notoriously not possible to represent in a series, on a flat surface, the affinities which we discover in nature amongst the beings of the same group. Thus,
the
→OMIT
natural system is genealogical in its
like a
but the
of modification which the different groups have
to be expressed by ranking them under different so-called genera, sub-families, families, sections, orders, and classes. |
