CHAPTER IX. 



THE RESPIRATORY EXCHANGE IN DIFFERENT ANIMALS. 



THE task of instituting a valid comparison between the oxidative 

 energy of the tissues of different animals is extremely difficult, and, 

 when types belonging to different zoological classes or even larger 

 systematic groups are concerned, at present almost hopeless. It 

 is not surprising, therefore, that so far very few conclusions of a 

 general character have been reached and that opinions are divided 

 even with regard to fundamental problems. 



Though the number of determinations of respiratory exchange 

 made is very large, and representatives of all larger groups have been 

 studied, the material available for comparison is scanty, because com- 

 parable experimental conditions have only rarely been maintained. 



It is necessary to approach the problem with great caution by 

 comparing first the standard metabolism in individuals belonging to 

 the same species and of approximately the same size (weight), next 

 in individuals belonging to the same species but of different weight. 

 These comparisons will furnish the necessary, though insufficient, basis 

 for the further comparison between different species, genera and wider 

 systematic groups. 



Comparison between Individuals belonging to the Same Species 

 and of (nearly) the Same Size. 



A sufficient material for this comparison exists only in the case of 

 man, and even with man the results have to be reduced to a common 

 standard by division with the body weight. It is obvious that even if 

 the metabolism should not be proportional to the weight the errors 

 introduced cannot be large, when the individuals to be compared differ 

 only slightly. 



Numerous experiments by Magnus-Levy and Falk [1899], Loewy 

 [1889, Op.], and especially Benedict and his collaborators [1914, 1915], 

 have shown that men of the same size and weight may differ con- 

 siderably with regard to their standard metabolism. This follows also 



131 9* 



