1462 



BIRDS 



the universal enthusiasm for what have been most appropriately designated ' ' our 

 feathered friends." As the result of this widespread popularity, the literature de- 

 voted to Birds is far more extensive than that relating to any other group of ani- 

 mals 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 representa- 

 tives presenting no approach to the differences distinguishing nearly-allied Mam- 

 malian 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. 

 D - . . Birds form a class in the Vertebrates ranking on the same level as 



Character- the Mammalia, and technically known as Aves; and from the afore- 

 istics of said structural uniformity of all its members, there is no difficulty in 

 Birds 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 hair, and being similarly developed 



from pits sunk in the super- 

 ficial 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 Mam- 

 mals 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 walking 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 condi- 

 tion. The power of true flight, which is such an essential characteristic of the ma- 

 jority 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 



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.) 



