260 | STUDY OF NATURAL HISTORY. 
of rapacious birds may be used as a subordinate 
mark of discrimination, if accompanied by deviations 
in structure, yet not otherwise. The nature of the 
ordinary food of an animal is almost always indicated 
by its external organisation. Weknow by experience 
that certain habits of life are indicated by certain 
peculiarities of form, so that by studying the con- 
formation of an animal which we have never seen 
alive, we can arrive, with a degree of certainty 
almost incredible, at a general knowledge of its 
habits and economy. This is more particularly the 
case among birds; because, perhaps, they have 
been more especially studied with reference to these 
circumstances than any other class of animals. In 
these the form of the bill is of as much importance 
indetermining the food, as are the teeth of quadrupeds 
or the jaws of insects. Purely insectivorous birds 
must always be considered as of a different type to 
such as partake more or less of vegetable food, even 
should such deviations be found in the same sub- 
family or even genus. A singular instance of this 
occurs in the sub-family of the Titmice (Pariane). 
All these birds live entirely upon insects excepting 
one genus, that of Accentor, to which belongs our 
common hedge-sparrow: this bird, as every one 
knows, feeds as much upon small seeds as upon 
minute insects; yet it is so intimately and un- 
questionably related to the group it has been placed 
in, that the perfection and unity of the whole divi- 
sion would be destroyed, if, because it was not 
purely insectivorous, we took it out of the circle, and 
endeavoured to find a place for it elsewhere : never- 
theless, we still make use of this peculiarity of habit 
