THE CHARACTERS OF GENERA. 127 
Genera proper. If the foregoing discussion upon 
the nature of these groups is based upon trust- 
worthy principles, we must admit that they are 
all founded upon distinct categories of characters, 
—the primary divisions, or the Branches, upon 
plan of structure, the Classes upon the manner 
of its execution, the Orders upon the greater 
or less complication of a given mode of execu- 
tion, the Families upon form; and it now re- 
mains to be ascertained whether Genera also 
exist in Nature, and by what kind of character- 
istics they may be distinguished. 
Taking the practice of the ablest naturalists 
in discriminating Genera as a guide in our esti- 
mation of their true nature, we must, neverthe- 
less, remember that even now, while their classi-. 
fications of the more comprehensive groups usu-— 
ally agree, they differ greatly in their limitation 
of Genera, so that the Genera of some authors 
correspond to the Families of others, and vice 
versa. This undoubtedly arises from the absence 
of a definite standard for the estimation of these 
divisions. But the different categories of struct- 
ure forming the distinctive criteria of the more 
comprehensive divisions once established, the 
question is narrowed down to an inquiry into the 
special category upon which Genera may be de- 
termined ; and if this can be accurately defined, 
no difference of opinion need interfere hereaf 
ter with their uniform limitation. 
