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BIRDS. 
appropriately designated “ our feathered friends.” As the result of this widespread 
popularity, the literature devoted to Birds is far more extensive than that relating 
to any other group of animals of equal size. And it may, perhaps, be questioned 
whether, in spite of their many undoubted claims to special interest, Birds have 
not attracted rather more than their fair share of attention; for, after all, the 
whole of the members of the class are wonderfully alike in general structure, even 
its most divergent representatives presenting no approach to the differences dis¬ 
tinguishing nearly allied mammalian orders. It is to a great extent owing to 
this remarkable structural uniformity that such different views still exist as 
to the classification of Birds. 
Distinctive Char- Birds form a class in the Vertebrates ranking on the same level 
acters of Birds. as the Mammalia, and technically known as Aves; and from the 
aforesaid structural uniformity of all its members, there is no difficulty in defining 
a Bird, nor is there any possibility of mistaking any other animal for a Bird. 
All living Birds, and so far as we know all fossil ones likewise, are sharply 
distinguished from every other creature by the possession of feathers; these 
corresponding in essential 
structure to hairs, and being 
similarly developed from pits 
sunk in the superficial layer 
of the skin or epidermis. 
This is the grand and essential 
characteristic of Birds, most 
of their other peculiarities 
being shared by some of the 
other groups of Vertebrates, 
either living or extinct. 
Birds agree with Mam¬ 
mals in having a four- 
chambered heart and hot 
blood, and also in that the 
blood is carried to the body by only a single great artery or aorta; but while 
in Mammals this aorta passes over the left branch of the windpipe or bronchus, 
in Birds it crosses the right. In producing their young from eggs laid by 
the female parent, Birds resemble not only the Egg - laying Mammals, but 
likewise most of the lower Vertebrates. All living members of the class possess 
two pairs of limbs; of which the hinder pair are always adapted either for walk¬ 
ing or swimming, while the front pair are generally specially modified for flight, 
although in the flightless species they are small and more or less rudimentary. 
Except to a small degree in the penguins, they never subserve the purpose of 
walking, at least in the adult condition. The power of true flight, which is such 
an essential characteristic of the majority of Birds, is found elsewhere among 
Vertebrates only in the bats among Mammals, and the extinct pterodactyles among 
Reptiles. An especial peculiarity of Birds is the manner in which their whole 
structure is permeated by atmospheric air taken in through the windpipe. Thus, 
whereas in Mammals the lungs are enclosed in complete sacs (the pleuron), and 
LEFT SIDE OF THE PELVIS OF THE KIWI. 
il, hauncli-bone or ilium ; p, p', pubis ; is, ischium ; a, cup for head 
of thigh-bone.—After Marsh. 
