The well-known meaning of the words generic and specific may serve, in the absence of a more precise definition, to express the relative importance of those groups of animals called Genera and Species in our scientific systems.  The Genus is the more comprehensive of the two kinds of groups, while the Species is the most precisely defined, or at least the most easily recognized, of all the divisions of the Animal Kingdom.  But neither the term Genus nor Species has always been taken in the same sense.  Genus especially has varied in its acceptation, from the time when Aristotle applied it indiscriminately to any kind of comprehensive group, from the Classes down to what we commonly call Genera, till the present day.  But we have already seen, that, instead of calling all the various kinds of more comprehensive divisions by the name of Genera, modern science has applied special names to each of them, and we have now Families, Orders, Classes, and Branches above Genera proper.  If the foregoing discussion upon the nature of these groups is based upon trustworthy principles, we must admit that they are all founded upon distinct categories of characters,—­the primary divisions, or the Branches, on plan of structure, the Classes upon the manner of its execution, the Orders upon the greater or less complication of a given mode of execution, the Families upon form; and it now remains to be ascertained whether Genera also exist in Nature, and by what kind of characteristics they may be distinguished.  Taking the practice of the ablest naturalists in discriminating Genera as a guide in our estimation of their true nature, we must, nevertheless, remember that even now, while their classifications of the more comprehensive groups usually 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 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