376 THE ORIGIN OF SPECIES 



sagaciously saw, as Jussieu observes, that this genus should still 

 be retained among the Malpighiaceae. This case well illustrates 

 the spirit of our classifications. 



Practically, when naturalists are at work, they do not trouble 

 themselves about the physiological value of the characters which 

 they use in defining a group or in allocating any particular species. 

 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 use 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. Saint-Hilaire. If several 

 trifling 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, im- 

 portant 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 classifica- 

 tion; but in some groups all these, the most important vital or- 

 gans, 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, while 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 characters are 

 the most important of all; and this doctrine has very generally 

 been admitted as true. Nevertheless, their importance has some 

 times 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, excluding larval, 

 characters, are of the highest value for classification, not only 

 with animals but with plants. Thus the main divisions of flower- 

 ing plants are founded on differences in the embryo — on the 



