BIRDS 



BY W. P. PYCRAFT, F.Z.S. 



Assistant in Charge of the Osteological Collections, British Museum 



(Natural History) 



CHAPTER X 



THE ESSENTIAL CHARACTERS OF BIRDS, AND A GENERAL OUTLINE 



OF THEIR STRUCTURE 



To properly appreciate the position which birds hold in the 

 animal kingdom, and their relation to other animals, a clear 

 idea of their essential characters is necessary ; but it is not pro- 

 posed here to enter into details of anatomy which concern the 

 specialist. 



Birds, then, hold a unique position in the animal kingdom ; 

 and this because of the peculiar structure of the outer covering 

 of the body, which is composed, invariably, of the peculiar horny 

 outgrowths known as feathers. By this character alone they 

 may be at once distinguished from all other animals. 



While they, in common with the animals of the class Mam- 

 malia (p. 19), have a four-chambered heart and warm blood, 

 they are yet, as a matter of fact, very closely related to the 

 reptiles, creatures of a lower grade in the scale of life. 



The many characters which birds share in common with 

 reptiles happily are of a kind which demand no deep anatomical 

 knowledge to appreciate. They may indeed be very well used 

 to serve as a demonstration of the way in which animals are 

 classified, or placed together in the order of their relationship 

 as nearly as this can be found. 



Birds and reptiles agree, then, in having but a single spherical 

 " boss " (condyle) to the skull whereby this articulates with, or 

 is hinged on to, the neck ; and in the structure of the shoulder 



