418 CLASSIFICATION. Chap. XIII. 



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. St. Hilaire. If certain 

 characters are always found correlated with others, 

 though no apparent bond of connexion can be dis- 

 covered 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 propagating the race, are found nearly uniform, they 

 are considered as highly serviceable in classification ; 

 but in some groups of animals all these, the most im- 

 portant vital organs, are found to offer characters of quite 

 subordinate value. 



We can see why characters derived from the embryo 

 should be of equal importance with those derived from 

 the adult, for our classifications of course include all 

 ages of each species. 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 em- 

 bryonic characters are the most important of any in the 

 classification of animals ; and this doctrine has very 

 generally been admitted as true. The same fact holds 

 good with flowering plants, of which the two main divi- 

 sions have been founded on characters derived from 

 the embryo, — on the number and position of the em- 



