376 GENERAL ZOOLOGY 



inspired air distributed all over the body, so that the aera- 

 tion of the blood is not confined to the limited area of an 

 ordinary respiratory organ." Other important structural 

 peculiarities are the light, toothless beak and the small 

 number of digits in the anterior extremities. Still other 

 characters have been referred to in the discussion of the in- 

 ternal anatomy. 



The many varieties of the domestic pigeon afforded Dar- 

 win much material for his book The Variation of Animals 

 and Plants under Domestication. From this study he ob- 

 tained many of the conclusions which led to the statement 

 of the principle of natural selection. These domestic varie- 

 ties differ among themselves in appearance far more than 

 many species in nature. Some well-known varieties are the 

 pouters, fantails, tumblers, and carriers. The pouters are 

 large birds with elongate body and legs, and often with in- 

 flated crop and esophagus. The fantails are known by the ex- 

 traordinary development of tail feathers. The tumblers have 

 the remarkable habit of turning somersaults backward in the 

 air from a considerable height nearly to the ground. 



The carriers have the "homing faculty" developed to 

 such an extent that they are useful as messengers. Though 

 shut within a basket and removed long distances from their 

 home, they have been able to find their way back. Carriers 

 have, been used by man for many centuries. During the 

 World War homing pigeons were much used at the front 

 when other communication was entirely cut off. In one in- 

 stance a pigeon carried and delivered a message a distance 

 of almost twenty-five miles in just twenty-five minutes, al- 

 though one leg had been shot off and the breast injured by a 

 machine gun. 



