ffer 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
structure which form 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 determined; and if
this can be accurately defined, no difference of opinion need interfere
hereafter with their uniform limitation. Considering all these divisions
of the Animal Kingdom from this point of view, it is evident that the
more comprehensive ones must be those which are based on the broadest
characters,--Branches, as united upon plan of structure, standing of
course at the head; next to these the Classes, since the general mode
of executing the plan presents a wider category of characters than
the complication of structure on which Orders rest; after Orders come
Families, or the patterns of form in which these greater or less
complications of structure are clothed; and proceeding in the same way
from more general to more special considerations, we can have no other
category of structure as characteristic of Genera than the details of
structure by which members of the same Family may differ from each
other, and this I consider as the only true basis on which to limit
Genera, while it is at the same time in perfect accordance with the
practice of the most eminent modern zoologists. It is in this way that
Cuvier has distinguished the large number of Genera he has characterized
in his great Natural History of the Fishes, in connection with
Valenciennes. Latreille has done the same for the Crustacea and Insects;
and Milne Edwards, with the cooeperation of Haime, has recently proceeded
upon the same principle in characterizing a great number of Genera among
the Corals. Many others have followed this example, but few have kept
in view the necessity of a uniform mode of proceeding, or, if they have
done their researches have covered too limited a ground, to be taken
into consideration in a discussion of principles. It is, in fact, only
when extending over a whole Class that the study of Genera acquires a
truly scientific importance, as it then shows in a connected manner, in
what way, by what features, and to what extent a large number of animals
are closely linked