SYSTEM OF NATURE. 
119 
with feathers, so constructed as to form efficient imple¬ 
ments of flight: birds therefore are influenced by the aerial 
radius in a less degree than insects, being indebted for the 
power of flight to the transmutation of an organ normally 
provided for a very different employ. The swallows among 
birds, like the dragonflies among insects, spend their lives, 
except during repose, entirely on the wing : the acts of 
eating, drinking, and even of copulation, are performed in 
the air. 
In bats the structure is not only vertebrate, but the ge¬ 
neration placental; and the limbs retain very nearly their 
normal character, the component bones of the foot and 
hand being almost identical with our own; but the fingers 
are lengthened, and connected by a delicate membrane, 
which extends to the hind legs and tail, forming a most 
complete and powerful organ of flight. These animals 
must be considered as less under the influence of the aerial 
radius than birds, because their limbs exhibit less devia¬ 
tion from the normal structure. The normal bats seek 
their food entirely on the wing. 
In the flying lemur (Galeopithecus) it is most interest¬ 
ing to trace the influence of the aerial radius, carried, al¬ 
though in its last or weakest degree, into the very central 
group of the animal kingdom. Every consideration de¬ 
rived from structure or economy must place this strange 
creature at that point of the group Primates which most 
closely approximates to the bats, and therefore at the very 
place traversed by the aerial radius in its direct progress 
towards the centre of the system. I believe this singular 
genus stands perfectly isolated, as far as our present know¬ 
ledge extends : it is characterized, like the flying squirrels 
and phalangers, by a quantity of loose skin appended to 
