ARCTIC METABOLIC ECONOMY 351 



The adaptability of the herring gull's nerve to function in cold, 

 for example, is a necessity for the normal operation of the cold feet 

 of the gull, and it was not found in the nerves of hens. This condi- 

 tion is an important character of the composite insulative function, 

 and since it can be localized within a portion of a nerve cell and is 

 measurable by several physical dimensions, the systematic and geo- 

 graphical relations of animals might be nicely definable on this basis. 

 A character of this sort could be viewed as a unit in a system of in- 

 heritance upon which natural selection could operate. 



Similarly, the selective deposition of low melting fats in distal 

 parts of appendages follows a good morphological pattern. It is at- 

 tributable to the biochemical reaction of dehydrogenation, and is ac- 

 cordingly distinguishable in the tissue chemistry. It is not demon- 

 strated as an adaptation to arctic life, but it represents the type of 

 modification which tissue substances of warm-blooded animals must 

 undergo to suit them for operation when cold. It is also the sort of 

 character which might turn out to suit the terms of genetics. 



