Chap. XIII. 
CLASSIFICATION. 
419 
bryonic leaves or cotyledons, and on the mode of deve¬ 
lopment 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 cha¬ 
racter 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 especi¬ 
ally in very large groups of closely allied forms. Tem- 
minck 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. 
