CLASSIFICATION 219 



nection can be discovered between them, especial value is 

 set on them. As in most groaps 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, thej 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, (^vhilo in two closely allied genera, 

 namely, Cypris and Cytherea, there is no such organ; 

 one species of Cypridina has well-developed branchiae, 

 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 

 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 naturalists, 

 Milne Edwards and Agassiz, that embryological char- 

 acters 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 Miiller 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, ex- 

 cluding larval characters, are of the highest value for 

 classification, not only with animals but with plants. 



