962 STUDY OF NATURAL HISTORY. 
traced downwards, into all the different orders 
which compose a class. In proof of this we cite 
the Hemiptera as the rapacious order of the Ptilota, 
the Libelluline as the analogous group in the 
Neuroptera, and the Predatores ( Chilopodomorpha 
M‘Leay) as the corresponding representation of 
the Fere in the order of Coleoptera. We merely 
intimate these novelties in natural arrangement 
(which will be separately treated of hereafter), to 
show the importance of essential characters founded 
upon the food of animals, or rather on the structure 
by which its nature is indicated. 
(180.) Animals which are omnivorous, or de- 
vourers both of animal and vegetable matter, pre- 
sent a singular union of those characters which 
respectively belong to the groups just mentioned. 
Unless, therefore, we have a previous knowledge of 
the circumstance, it becomes extremely difficult to 
determine, from the simple examination of a dried 
specimen, what was its natural food. The crow 
family (Corvide), which are the most perfectly 
organised of all birds, are of this description. 
They feed upon almost any thing which has life, 
either animal or vegetable, and even upon carrion. 
The toucans and the trogons belong to two different 
tribes ; but as they are the points of union connecting 
the Scansores and the Fissirostres, we find that both 
partake of the same vegetable and animal diet. The 
family of rats are likewise omnivorous, and by this 
peculiarity they make the transition easy from the 
carnivorous to the herbivorous quadrupeds. Such of 
the tyrant fly-catchers of America as show a decided 
affinity to the true shrikes, feed, like them, not only 

