118 
SYSTEM OF NATURE. 
nicates to its contents a volitant character; and this cha¬ 
racter, it must be observed, is due to the propensity or 
economy, and in no degree to structure, which seems to 
suffer any modification consistent with the retention of its 
own distinctness, in order to meet the requirings of this 
aerial influence. With regard to the intensity of the influ¬ 
ence of this radius, it has four well-marked divisions or 
degrees; its tendency being always the same, to incite the 
animal to aerial progression, and supply the means requir¬ 
ed for the gratification of this propensity : in fact, to fur¬ 
nish the animal with “ wings,” the metaphorical character 
of which word has been already pointed out, (p. 83). 
In the Pterota or winged insects the structure is articu¬ 
late, the number of legs is six, and throughout the province 
these are in no instance transmuted into organs of flight: 
but there are other parts especially supplied as organs of 
flight; these are developed from the dorsal surface of the 
third and fourth segments, and are universally known by 
the name of wings. The wings of the Pterota are peculiar 
to this province of the animal kingdom, and are unique as 
organs applicable only to the purpose of flight: winged in¬ 
sects must therefore be considered as influenced by the 
aerial radius in the first or highest degree. As it will be 
quite impossible, within the compass of an essay like the 
present, to analyse each group considered as a system in 
miniature, it may not be amiss to remark in passing, that 
this volitant character appears normal in the normal and 
central class Neuroptera, many of the more conspicuous 
insects of the class spending the day-time of their lives 
entirely on the wing. 
In birds the structure is vertebrate; the limbs or organs 
of locomotion being four: the anterior pair are furnished 
