220 THE ORlLilS OF Sl'FA'lES 



Tliiis the main divisions of flowering j)hints are founded 

 on ditterences in the embryo — on tht; 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 classiiieations are often plainly influenced by 

 chains of affinities. Nothing can be easier than to define 

 n number of characters conimon 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 onward, can be 

 recognized 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 irf certain groups of birds; and it has becH fol 

 lowed by several entomologists and botanists. 



Finally, witli 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, liave strongly 

 insisted on their arbitrary value. Instances could be 

 given, among plants and insects, of a group first ranked 

 by practiced naturalists as only a genus, and then raised 

 to the rank of a sub-family or family; and this has been 



