418 
CLASSIFICATION. 
Chap. XIII. 
not trouble tliemselves about the physiological yalue 
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- 
