Chap. XIY.] CLASSIFICATION. 211 



highest value for classification, not only with animals 

 but with plants. Thus the main divisions of flowering 

 plants are founded on differences in the embryo, — on 

 the number and position of the cotyledons, and on the 

 mode of development of the plumule and radicle. We 

 shall immediately see why these characters possess so 

 high a value in classification, namely, from the natural 

 system being genealogical in its arrangement. 



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 

 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, 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 first 

 ranked by practised naturalists as only a genus, and 

 then raised to the rank of a sub-family or family ; and 

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