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. {420}

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 relation to the
other groups, must be strictly genealogical in order to be natural; but
that the _amount_ of difference in the several branches or groups, though
al