affinities. Nothing can be easier than to define a number of characters
common to all birds; but in the case of crustaceans, such definition
has hitherto been found impossible. There are crustaceans at the
opposite ends of the series, which have hardly a character in common;
yet the species at both ends, from being plainly allied to others, and
these to others, and so onwards, can be recognised as unequivocally
belonging to this, and to no other class of the Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity
of this practice in certain groups of birds; and it has been followed
by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, sub-orders, families, sub-families, and
genera, they seem to be, at least at present, almost arbitrary. Several
of the best botanists, such as Mr. Bentham and others, have strongly
insisted on their arbitrary value. Instances could be given amongst
plants and insects, of a group of forms, first ranked by practised
naturalists as only a genus, and then raised to the rank of a
sub-family or family; and this has been done, not because further
research has detected important structural differences, at first
overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification are
explained, if I do not greatly deceive myself, on the view that the
natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have
been unconsciously seeking, and not some unknown plan of creation, or
the enunciation of general propositions, and the mere putting together
and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the
_arrangement_ of the groups within each class, in due subordination and
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