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 to refer to
the diagram in the fourth chapter. We will suppose the letters A to L
to represent allied genera existing during the Silurian epoch, and
descended from some still earlier form. 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. If, however, we suppose any
descendant of A or of I to have become so much modified as to have
lost all traces of its parentage in this case, its place in the natural
system 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 the names of
the groups ha