number and position of the embryonic leaves or cotyledons, and on
the mode of development of the plumule and radicle. In our discussion
on embryology, we shall see why such characters are so valuable, on the
view of classification tacitly including the idea of descent.

Our classifications are often plainly influenced by chains of
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 proposit