346 U. S. NATIONAL MUSEUM BULLETIN 217 



Several other judges to whom I explained the idea placed the feathers 

 in about the same order, so that it appears to have some significance. 

 It should be noticed that these comparisons were made with the feathers 

 of birds which had reached their summer condition and when the 

 arctic weather was warm enough to have the migratory examples pres- 

 ent for comparison, so that if this is an insulating characteristic of 

 feathers, it is retained in summer. The dissipation of heat, which 

 must be important for birds in summer is not facilitated by as much 

 thinning of the feathers as would anywhere near compensate for the 

 heat of the arctic smnmer. The heat is dissipated by some other pos- 

 tural disposition of the feathers or through other physiological chan- 

 nels. 



Neither geographical nor seasonal variation in feather thickness 

 clearly adapts the insulation of birds to their climates, although dif- 

 ferences in structure exist which would permit the feathers to func- 

 tion by erection when at rest, with various effects useful in different 

 temperatures. 



Fur can be regarded as freely modifiable for insulation, and that 

 seems to be its common function, but feathers serve primarily for 

 fiight. Birds fly alike in all climates, and the requirements of aero- 

 dynamics which they encounter are unaffected by temperature. It is 

 to the specific mechanical and power characteristics of internal 

 muscular and bony mechanisms, and probably to the metabolic pro- 

 vision of power, characteristics not related to climate, that the feathers 

 must conform. Probably little modification of the thickness of a 

 given bird's feathers is possible during flight ; only when at rest is the 

 bird free to modify the insulation value, utilizing the finer structure 

 and regulating the position of the feathers to vary the thickness. As I 

 have said, however, this adaptive use for insulation is obscured and 

 perhaps repressed by the primary integration of the feathers into 

 mechanisms for flight that operate unaffected by climate or weather. 



I suspect, too, that the necessity for the contour of diving and 

 swimming birds to conform to the requirements of the aquatic medium 

 as well as to flight may limit the climatic modifiability of the thickness 

 of their feathers for insulation. The combination of these various 

 functions in the feathers of birds precludes any simplified view that 

 their main use is for insulation. 



Variability of Insulation in Animals 



The fur clothing made by Eskimos preserves comfortable waraith 

 in arctic cold when the wearer is inactive or even at rest, but in a warm 

 house, or during the sunny part of a warm winter day, adequate 

 arctic clothing is unbearably hot, and even Eskimos, with all their 

 skill in the preparation and use of clothing, have not succeeded in 



