210 CLASSIFICATION. [Chap. XIV. 



characters are always found in combination, though no 

 apparent bond of connection can be discovered between 

 them, especial value is set on them. As in most groups 

 of animals, important organs, such as those for pro- 

 pelling the blood, or for aerating it, or those for pro- 

 pagating 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 Muller has lately remarked, in the same 

 group of crustaceans, Cypridina is furnished with a 

 heart, whilst in two closely allied genera, namely 

 Cypris and Cytherea, there is no such organ ; one 

 species of C)^ridina has well-developed branchiae, 

 whilst 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 ordi- 

 nary view, why the structure of the embryo should be 

 more important 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 natural- 

 ists, Milne Edwards and Agassiz, 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 

 Muller 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 



