3o8 CLASSIFICATION. [Chap. XIV. 



If they find a character nearly uniform, and common to a great 

 number of forms, and not common to others, they use it as one of 

 high value ; if common to some lesser number, they uue it as of 

 subordinate value. This principle has been broadly confessed by 

 some naturalists to be the true one ; and by none more clearly than 

 by that excellent botanist, Aug. St. Hilaire. If several trifling cha- 

 racters 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 propelling 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 Cypridina 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 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 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 larvse 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 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 affini- 

 ties. Nothing can be easier than to define a number of characters 

 common to all birds ; but with crustaceans, any such definition has 



