These few examples, selected purposely among closely allied and universally known animals, may be sufficient to show, that, beyond the general complication of the structure which characterizes the Orders, there is a more limited element in the organization of animals, bearing chiefly upon their form, which, if it have any general application as a principle of classification, may well be considered as essentially characteristic of the Families. There are certainly closely allied natural groups of animals, belonging to the same Order, but including many Genera, which differ from each other chiefly in their form, while that form is determined by peculiarities of structure which do not influence the general structural complication upon which Orders are based, or relate to the minor details of structure on which Genera are founded. I am therefore convinced that form is the criterion by which Families may be determined. The great facility with which animals may be combined together in natural groups of this kind without any special investigation of their structure, a superficial method of classification in which zoölogists have lately indulged to a most unjustifiable degree, convinces me that it is the similarity of form which has unconsciously led such shallow investigators to correct results, since upon close examination it is found that a large number of the Families so determined, and to which no characters at all are assigned, nevertheless bear the severest criticism founded upon anatomical investigation.