relation to the other groups, must be strictly genealogical in order to
be natural; but that the _amount_ of difference in the several branches
or groups, though allied 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
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 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 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 have been but little
modified, and they yet form a single genus. But this genus, though much
315