roups; 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, {433} 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. 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 between the descendants from a common parent,
expressed by the terms genera, families, orders, &c., 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
are permitted to use rudimentary and useless organs, or others of trifling
physiological importance; why, in comparing one group with a distinct
group, we summarily reject analogical or adaptive characters, and yet use
these same characters within the limits of the same group. We can clearly
see how it is that all living and extinct forms can be grouped together in
one great system; and how the several members of each class are connected
together by the most complex and radiating lines of affinities. We shall
never, probably, disentangle the inextricable web of affinities between the
members of any one class; but when we have a di