FAMILY CHARACTERISTICS. 114 
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 Strigide and the other Falconida 
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. Iam, therefore, convinced 
that form is the criterion by which Families mav 
