THE ORGANISM AND ITS ENVIRONMENT 349 



hand, the various types of beaks in birds, and so on. It is unneces- 

 sary to enumerate more. It is profitable, however, to discuss in de- 

 tail the structural adaptations of certain animals. The bird body 

 will serve to illustrate the principle of the unity of adaptive features 

 within the individual; other examples will illustrate special cases. 



The Bird As an Example of Fitness for Habitat. The 

 exoskeleton of the bird (Fig. 93) consists essentially of a set of 

 greatly modified scales, the feathers. The lightness of the feather 

 coat with its entrapped air makes for buoyancy, as well as provid- 

 ing effective insulation against loss of heat and a protection against 

 weather. The beak, together with the bones of the head, forms a 

 stream-lined unit with a minimum of resistance against the air. The 

 vertebrae of the neck have saddle-shaped joints, allowing wide free- 

 dom of movement and when in flight the neck forms a straight 

 column back of the extended head. The skeleton of the body is a 

 compact light frame forming the shape of a projectile. Buoyancy is 

 added by the fact that the shafts of many of the bones contain air 

 sacs continuous with the lungs. Teeth have been dispensed with, 

 which decreases the weight of the head. Their function is performed 

 by a modified stomach, commonly called the gizzard (Fig. 212). 

 The voice-producing apparatus has been shifted out of the head 

 back to the region where the trachea divides to form the bronchial 

 tubes. Here the trachea is modified to form a voice box known as 

 the SYRINX. It contains a vibratory member (pessulus) suitably con- 

 trolled by muscles. Sounds are produced by the passage of air over 

 this structure. 



All unnecessary weight is dispensed with. The large intestine and 

 rectum are much reduced in size; there is no urinary bladder. 

 Wastes are expelled promptly. The shifting of the voice apparatus 

 from the head into the body, the elimination of teeth, the shortening 

 of the tail, the habit of drawing the feet and legs against the body 

 when in flight, all centralize the weight into the body proper. The 

 centre of gravity is low and the vertebral column rigid in the body 



