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
group. We can clearly see how it is that all living and extinct forms can
be grouped together within a few great classes; and how the several members
of each class are connected together by the most complex and radiating
lines of affinities.
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