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 ovei looked, but because numerous allied species, 

 with slightly different grades of difference, have been 

 subsequently discovered. 



