290 



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 first 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, haunch-bone or ilium ; p, p', pubis ; is, ischium ; a, cup for head 

 of thigh-bone. — After Marsh. 



DS1 



