378 CLASSIFICATION. Chap. XIII. 



naturalists, iMiliic Edwards and Ag'assiz, tliat embryoloijifal 

 characters ai o the most important of all ; and this doctrhie has 

 very g'encrally been admitted as true. Nevertheless, their im- 

 portaTicc has sometimes been exaggerated; in order to show 

 tlii.s, Fritz Miillcr arranged by the aid of such characters the 

 great class of crustaceans, and the arrangement did not prove 

 a natural one. But there can be no douljt that characters de- 

 rived from tlie embryo are generally of the highest value, not 

 only Avith animals but Avith plants. Thus the two main divis- 

 ions of flowering plants are founded on differences in the em- 

 bryo — on the number and position of the cotyledons, and on 

 the mode of development of the plumule and radicle. Wc 

 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 

 aflinities. Nothing can be easier than to define a number of 

 chanicters 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 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 per- 

 haps not quite logically, in classification, more especially in 

 very largo 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 entomol- 

 ogists and botanists. 



Finally, wutli respect to the comparative value of the vari- 

 ous 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. 

 IJontham and others, have strongly insisted on their arbitrary 

 value. Instances could be given among plants and insects, of 

 :i group of forms, first ranked by ])ractised 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 de- 

 tected important structural differences, at first overlooked, but 

 because nmnerous allied species, with slightly-different grades 

 of difference, have been subsequently discovered. 



All the foregoing rules and aids and difBcultics in classifi- 



