37° PRINCIPLES OF ANIMAL NUTRITION. 
species will in the main apply to a comparison of different species. 
It is true that what data we have indicate that there may be more 
or less difference between the critical temperatures for different 
species, but in view of the enormous range in the size of the animals 
experimented on this cannot largely modify the results. Any 
reasonable assumptions as to critical temperatures and as to rates 
of variation per degree in heat production would still leave the 
corrected results substantially proportional to the surface. Appar- 
ently we must conclude that in all these different species, as well 
.as in larger and smaller animals of the same species, the internal 
work, as measured by the total metabolism at the critical tem- 
perature, is substantially proportional to the surface. 
How generally this may be true we have at present no means 
of judging. It is clear, however, that in the process of organic 
evolution one of the very important factors has been the demand 
for heat exerted by the environment upon the animal. This has 
been met to some extent by modifications in the coat of the animal, 
but to a very large degree by changes in the rate of heat produc- 
tion, with the result that, other things being equal, those forms have 
survived whose normal heat production, resulting from internal 
work alone, was sufficient to maintain their temperature under the 
average conditions surrounding them without, on the one hand, 
calling largely into play the processes of “chemical” regulation, 
nor, on the other hand, producing so much heat as to render it 
difficult for the body to get rid of it. 
Revation of Heat Propuction to Mass or TissuE.—As 
already indicated, E. Voit. in his article cited above, has shown 
that while the heat production is in general proportional to the sur- 
face, there is also another determining factor, viz., the mass of the 
active cells in the organism, a rough measure of which is the total 
nitrogen of the body exclusive of that of the bones and the skin. 
This conclusion is based chiefly on experiments with fasting animals. 
As the weight of such an animal decreases, its relative surface must 
increase, and, as was shown on p. 364, probably more rapidly than 
in proportion to the two-thirds power of the weight. Under these 
circumstances we should naturally expect that the relative heat 
production would increase, but as a matter of fact it rather shows 
atendency to decrease. E. Voit, in discussing the results of Rubner 
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