FAMILY CHAKACTERISTICS. 115 



ence of form consists 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 Strigidse and the other Falconidse 

 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 character- 

 izes 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 re- 

 late to the minor details of structure on which 

 Genera are founded. I am, therefore, convinced 

 that form is the criterion by which Families may 



