408 CLASSIFICATION. 



urged by those great naturalists, Milne Edwards and Agas- 

 siz, that embryological characters are the most important 

 of all; and this doctrine has very generally been admitted 

 as true. Nevertheless, their importance has sometimes been 

 exaggerated, owing to the adaptive characters of larvae not 

 having been excluded ; in order to show this, Fritz Mtiller 

 arranged, by the aid of such characters alone, the great class 

 of crustaceans, and the arrangement did not prove a natural 

 one. But there can be no doubt that embryonic, excluding 

 larval, characters, are of the 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 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 in- 

 sists on the utility or even necessity of this practice in cer- 

 tain groups of birds ; and it has been followed by several 

 entomologists and botanists. 



Finally, with respect to the comparative value of the yari- 

 rious 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 among 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 been done, not because further 

 research has detected important structural differences, ^ at 

 first overlooked, but because numerous allied species, with 

 slightly different grades of difference, have been subsequently 

 (iiscovered. 



