Nothing can be easier than to define a number of characters common to all
birds; but with crustaceans, any 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, 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.
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