taceans 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, suborders, families, subfamilies, 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 among plants and
insects, of a group first ranked by practised naturalists as only a
genus, and then raised to the rank of a subfamily 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 may
be 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, all true classification being 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
each other, must be strictly genealogical in order to be natural; but
that the AMOUNT of difference in the several branches or groups, though
allied in the same degree in blood to their common progenitor, may
differ greatly, being due to the different degrees of modification which
they have undergone; and this is expressed