350 tr. S. NATIONAL MUSEUM BULLETIN 217 



needed plasticity at the cold temperature often observed in the pe- 

 ripheral tissues (L. Irving, Schmidt-Melsen, and Abrahamsen, 1957). 



A similar gradient of melting points was observed in the fats from 

 the legs of herbivorous reindeer and porcupines, as well as in the 

 carnivorous fox, dog, and wolf from arctic Alaska. Among the large 

 arctic manmials it appears common that fats of low melting point 

 are selectively deposited in the parts of extremities where the tissues 

 are known to be cool. But a similar distribution of low melting fats 

 has also been found in the legs of cattle from temperate regions, and 

 the distribution of fats in the leg of a Panamanian brocket deer 

 {Mazama americana) was found to be identical with that observed 

 in homologous bones of arctic herbivores and carnivores. Selective 

 distribution of fats appears useful or even essential for arctic life, 

 but since it also occurs in temperate and tropical regions, it cannot 

 be an adaptation recently evoked for arctic life nor can it be a bad 

 condition for life in a warmer climate. 



In the deposition of fats, selection, according to melting point, 

 takes place differentially in different anatomical parts. Since the 

 difference between a high and low melting fat is commonly based 

 upon the proportion of saturated to unsaturated fatty acid in the 

 natural mixture of fats, and since change in hardness is often ef- 

 fected by the relative amounts of oleic and stearic acids, which differ 

 only in hydrogenation, the difference in properties of fats is ascrib- 

 able to a biochemical step in fat synthesis. 



Biological Significance of Tissue Modification 



The composite function of insulation, described in terms of metab- 

 olism and temperature, shows, in proper physiological terms, that 

 the common metabolic economy of warm-blooded life in all climates 

 can be maintained in arctic cold. By comparison with tropical forms 

 adaptation to cold shows very clearly among arctic animals. But 

 this kind of adaptation does not provide explicit characters, by the 

 selective inheritance of which arctic races might become naturally 

 differentiated. The composite physiological functions are useful to 

 the animal, and the view of them is useful for the physiologist, but 

 they are not expressed in terms useful for the natural philosopher 

 speculating upon the methods which have established the adaptive 

 differentiation of animals. Hair, fur, and feather structures, on the 

 other hand, are visible characters produced by tissues, and inheritance 

 can be observed in them. 



