ast a natural arrangement, would be
possible. We shall see this by turning to the diagram: the letters, A
to L, may represent eleven Silurian genera, some of which have produced
large groups of modified descendants. Every intermediate link between
these eleven genera and their primordial parent, and every intermediate
link in each branch and sub-branch of their descendants, may be supposed
to be still alive; and the links to be as fine as those between the
finest varieties. In this case it would be quite impossible to give any
definition by which the several members of the several groups could be
distinguished from their more immediate parents; or these parents from
their ancient and unknown progenitor. Yet the natural arrangement in the
diagram would still hold good; and, on the principle of inheritance, all
the forms descended from A, or from I, would have something in common.
In a tree we can specify this or that branch, though at the actual fork
the two unite and blend together. We could not, as I have said, define
the several groups; but we could pick out types, or forms, representing
most of the characters of each group, whether large or small, and thus
give a general idea of the value of the differences between them. This
is what we should be driven to, if we were ever to succeed in collecting
all the forms in any class which have lived throughout all time
and space. We shall certainly never succeed in making so perfect a
collection: nevertheless, in certain classes, we are tending in this
direction; and Milne Edwards has lately insisted, in an able paper, on
the high importance of looking to types, whether or not we can separate
and define the groups to which such types belong.

Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction
and divergence of character in the many descendants from one dominant
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages, although having few characters in common,
under one species; we use descent in classing acknowledged varieties,
however different they may be from their parent; and I believe this
element of descent is the hidden bond of connexion which naturalists
have sought under the term of the Natural System. O