CHIEF CHARACTERISTICS 5 



by a dilatation of the superficial blood-vessels, by increasing 

 the evaporation of water in sweating and breathing, and to 

 a slight extent, perhaps, by decreasing the internal production 

 of heat, e.g. by keeping very quiet. As birds have no sweat 

 glands, the regulation of a high internal temperature, 

 consequent on intense metabolism, is effected to some 

 extent by the evaporation of water from the internal surfaces 

 of the lungs and air-sacs. This may be aided, perhaps, by 

 a reduction of activities in very hot weather or in the hottest 

 part of the day. 



In any case, in a cold country and during a cold night, 

 the adult bird is able to keep its normal body-temperature 

 constant ; and similarly in a warm country and during the 

 heat of the day. This is eifected by an intricate nervous 

 mechanism which regulates production and loss of heat. 

 And just as this regulatory arrangement is gradually estab- 

 lished in the individual bird, so it must have been in the 

 race. It is plain that as it was established it would make 

 life an easier problem for the bird. It is also plain that 

 the coat of non-conducting feathers would greatly assist the 

 conservation of the internal heat in cold surroundings. 



§ 5. Feathers 



There is no doubt that the bird's solution of the problem 

 of flight is bound up with the evolution of feathers, forming 

 a sail that strikes the air. But on the evolution of feathers, 

 which may have taken a million years, we have no light. 

 We wish to suggest, however, that there may be an advantage 

 in a flank attack, namely, in recognising the import of 

 plausible pre-conditions of the power of flight. Other 

 things equal, activity and a good circulation will tend to 

 favour the development of integumentary structures, and 

 birds are by hypothesis creatures of intense metabolism, 

 fine vascular system, and great activity. There is also a 

 marked abundance of blood-vessels in the bird's dermis. 



No one knows how feathers evolved. All that is clear 

 is that there is a general resemblance in the development of 



