n 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 by the term "unity of
type;" or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under
the general name of Morphology. This is the most interesting department
of natural history, and may be said to be its very soul. What can be
more curious than that the hand of a man, formed for grasping, that of a
mole for digging, the leg of the horse, the paddle of the porpoise, and
the wing of the bat, should all be constructed on the same pattern, and
should include the same bones, in the same relative positions? Geoffroy
St. Hilaire has insisted strongly on the high importance of relative
connexion in homologous organs: the parts may change to almost any
extent in form and size, and yet they always remain connected together
in the same order. We never find, for instance, the bones of the arm and
forearm, or of the thigh and leg, transposed. Hence the same names can
be given to the homologous bones in widely different animals. We see the
same great law in the construction of the mouths of insects: what can
be more d