II] OF BERGMANN'S LAW 35 



birds live on nectar, the richest and most concentrated of foods*. 

 Man consumes a fiftieth part of his own weight of food daily, but 

 a mouse will eat half its own weight in a day; its rate of Hving is 

 faster, it breeds faster, and old age comes to it much sooner than 

 to man. A warm-blooded animal much smaller than a mouse 

 becomes an impossibihty; it could neither obtain nor yet digest the 

 food required to maintain its constant temperature, and hence no 

 mammals and no birds are as small as the smallest frogs or fishes. 

 The disadvantage of small size is all the greater when loss of heat 

 is accelerated by conduction as in the Arctic, or by convection as 

 in the sea. The far north is a home of large birds but not of small ; 

 bears but not mice five through an Arctic winter; the 'least of the 

 dolphins Hve in warm waters, and there are no small mammals in 

 the sea. This principle is sometimes spoken of as Bergmann's Law. 



The whole subject of the conservation of heat and the maintenance 

 of an all but constant temperature in warm-blooded animals interests 

 the physicist and the physiologist ahke. It drew Kelvin's attention 

 many years agof, and led him to shew, in a curious paper, how 

 larger bodies are kept warm by clothing while smaller are only 

 cooled the more. If a current be passed through a thin wire, of 

 which part is covered and part is bare, the thin bare part may glow 

 with heat, while convection-currents streaming round the covered 

 part cool it off and leave it in darkness. The hairy coat of very 

 small animals is apt to look thin and meagre, but it may serve them 

 better than a shaggier covering. 



Leaving aside the question of the supply of energy, and keeping 

 to that of the mechanical efficiency of the machine, we may find 

 endless biological illustrations of the principle of simihtude. All 

 through the physiology of locomotion we meet with it in various 

 ways : as, for instance, when we see a cockchafer carry a plate many 

 times its own weight upon its back, or a flea jump many inches high. 

 ''A dog," says Gahleo, "could probably carry two or three such 

 dogs upon his back; but I believe that a horse could not carry 

 even one of his own size." 



♦ Cf. R. A. Davies and G. Fraenkel, The oxygen- consumption of flies during 

 flight, Jl. Exp. Biol. XVII, pp. 402-407, 1940. 



t W. Thomson, On the efficiency of clothing for maintaining temperature, 

 Nature, xxix, p. 567, 1884. 



