nguished from their more immediate parents and descendants. Yet the
arrangement in the diagram would still hold good and would be natural;
for, on the principle of inheritance, all the forms descended, for
instance from A, would have something in common. In a tree we can
distinguish 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 one class which have lived throughout all time and space.
Assuredly we shall never succeed in making so perfect a collection:
nevertheless, in certain classes, we are tending toward this end;
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 follows from the
struggle for existence, and which almost inevitably leads to
extinction and divergence of character in the descendants from any
one 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 under one species, although they may have
but few characters in common; we use descent in classing acknowledged
varieties, however different they may be from their parents; and I
believe that this element of descent is the hidden bond of connexion
which naturalists have sought under the term of the Natural System.
On this idea of the natural system being, in so far as it has been
perfected, genealogical in its arrangement, with the grades of
difference expressed by the terms genera, families, orders, etc., we
can understand the rules which we are compelled to follow in our
classification. We can understand why we value certain resemblances far
more than others; why we use rudimentary and useless organs, or others
of trifling physiological importance; why, in finding the relations
between one group and another, we summarily reject analogical or
adaptive characters, and yet use these same characters within the limits
of the same