Take the Family of Owls as distinguished from the Falcons, Kites, etc.  Here the difference of form is in the position of the eyes.  In the Owl, the sides of the head are prominent and the eye-socket is brought forward.  In the Falcons and Kites, on the contrary, the sides of the head are flattened and the eyes are set back.  The difference in the appearance of the birds is evident to the most superficial observer; but to call the one Strigidae and the other Falconidae tells us nothing of the anatomical peculiarities on which this difference is founded.

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 zooelogists 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.

The questions proposed to themselves by all students who would characterize Families should be these:  What are, throughout the Animal Kingdom, the peculiar patterns of form by which Families are distinguished? and on what structural features are these patterns based?  Only the most patient investigations can give us the answer, and it will be very long before we can write out the formulae of these patterns with mathematical precision, as I believe we shall be able to do in a more advanced stage of our science.  But while the work is in progress, it ought to be remembered that a mere general similarity of outline is not yet in itself evidence of identity of form or pattern, and that, while seemingly very different forms may be derived from the same formula, the most similar forms may belong to entirely different systems, when their derivation is properly traced.  Our