Chap. XIV.] CLASSIFICATION. 211 



est 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 de- 

 fine a number of characters common to all birds; but 

 with crustaceans, any such definition has hitherto been 

 found impossible. There are crustaceans at the oppo- 

 site ends of the series, which have hardly a character in 

 common; yet the species at both ends, from being plain- 

 ly 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 neces- 

 sity 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 strong- 

 ly 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 this has 

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