456 ORIGIN OF SPECIES 



be the true one ; and by none more clearly than by that ex- 

 cellent botanist, Aug. St. Hilaire. If several trifling char- 

 acters 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, impor- 

 tant 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 impor- 

 tant vital organs, are found to offer characters of quite sub- 

 ordinate values. Thus, as Fritz Miiller has lately remarked, 

 in the same group of crustaceans, Cypridina is furnished with 

 a heart, whilst in too closely allied genera, namely Cypris 

 and Cytherea, there is no such organ ; one species of Cypri- 

 dina 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 exag- 

 gerated, owing to the adaptive characters of larvae not hav- 

 ing been excluded; in order to show this, Fritz MuUer 

 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 di- 

 visions 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. 



