lied in the same degree in blood to their common progenitor, may differ
greatly, being due to the different degrees of modification which they have
undergone; and this is expressed by the forms being ranked under different
genera, families, sections, or orders. The reader will best understand what
is meant, if he will take the trouble of referring to the diagram in the
fourth chapter. We will suppose the letters A to L to represent allied
genera, which lived during the Silurian epoch, and these have descended
from a species which existed at an unknown anterior period. Species of
three of these genera (A, F, and I) have transmitted modified descendants
to the present day, represented by the fifteen genera (a^{14} to z^{14}) on
the uppermost horizontal line. Now all these modified descendants from a
single species, are represented as related in blood or descent to the same
{421} 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 organic beings belong to Silurian genera. So that the
amount or value of the differences between organic beings 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 period. If,
however, we choose to suppose that any of the descendants of A or of I have
been so much modified as to have more or less completely lost traces of
their parentage, in this case, their places in a natural classification
will have been more or less completely lost,--as sometimes seems to have
occurred with existing organisms. All the descendants of the genus F, along
its whole line of descent, are supposed to