In three of these genera (A, F, and I) a species has transmitted
modified descendants to the present day, represented by the fifteen genera
(a14 to z14) on the uppermost horizontal line. Now, all these modified
descendants from a single species are related in blood or descent in 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 parent
I. But the existing genus F14 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 organisms belong to Silurian genera. So that the comparative
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
stage.
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