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. 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, 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 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 distinct object in view, and do not look to some
unknown plan of creation, we may hope to make sure but slow progress.
_Morphology_. We have seen that the members of the same class,
independently of their habits of life, resemble each other in the
general plan of their organisation. This resemblance is often expressed
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