434 CLASSIFICATION. 



As in most groups of animals, important organs, such as 

 those for propelling the blood, or for aerating it, or those 

 for propagating the race, are found nearly uniform, they 

 are considered as highly serviceable in classification; but in 

 some groups all these, the most important vital organs, are 

 found to offer characters of quite subordinate value. Thus, 

 as Fritz Miiller has lately remarked, in the same group of 

 crustaceans, Cypridina is furnished with a heart, while in 

 two closely allied genera, namely Cypris and Cytherea, 

 there is no such organ; one species of Cypridina has well- 

 developed branchise, while another species is destitute of 

 them. 



We can see why characters derived from the embryo 

 should be of equal importance with those derived from the 

 adult, for a natural classification of course includes all 

 ages. But it is by no means obvious, on the ordinary 

 view, why the structure of the embryo should be more im- 

 portant for this purpose than that of the adult, which 

 alone plays its full part in the economy of nature. Yet it 

 has been strongly urged by those great naturalists, Milne 

 Edwards and Agassiz, that embryological characters are 

 the most importMit of all; and this doctrine has very gen- 

 erally been admitted as true. Nevertheless, their impor- 

 tance has sometimes been exaggerated, owing to the adap- 

 tive characters of larvse not having been excluded; in order 

 to show this, Fritz Miiller arranged, by the aid of such 

 characters alone, the great class of crustaceans, and the ar- 

 rangement 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 im- 

 mediately 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 



