RESPIRATORY EXCHANGE IN DIFFERENT ANIMALS 139 



certain extent mutually interdependent, and it involves a serious diffi- 

 culty when it must be admitted that the resting metabolism of a warm- 

 blooded animal is not directly governed by the conditions for loss of 

 heat. The objection has been met by Rubner by the assumption 

 that the standard metabolism cannot undergo rapid changes, that the 

 oxidative energy of the cells is adapted to the usual conditions re- 

 garding loss of heat and is altered very gradually with those conditions. 



The limitation, " other things being equal," represents a very for- 

 midable further objection, because the other things in question are 

 practically never even approximately equal. Between different dogs, 

 for instance, the differences in the insulating properties of the fur are 

 very large. If the metabolism was correlated with the heat loss dogs 

 with a thick fur ought to have a lower metabolism per unit surface, 

 but this is certainly not so. Hoesslin [1888] made the experiment 

 to keep two exactly similar young dogs for a long time at very differ- 

 ent temperatures. According to Rubner's theory differences in 

 standard metabolism ought to develop, but they did not, whereas 

 after some weeks the fur of each animal was changed in evident 

 correlation with the different temperature conditions. 



Hoesslin [1888], and more recently Zuntz [1906], have attempted 

 to correlate the standard metabolism with the functional activity. 

 Hoesslin especially has treated the whole problem theoretically at 

 great length. He assumes that, when different animals are to com- 

 pete with one another, their velocity of locomotion must be nearly the 

 same. Natural selection will exterminate those which are slower. 

 He attempts to show then by an elaborate calculation, the details 

 of which I must confess myself unable to follow, that in animals of 

 different size, if they be taken to move with the same maximum 

 velocity, the corresponding maximum metabolism must be proportional 

 to the cross section (or surface) of each, and he takes it for granted 

 further that the standard (or minimum) metabolism is a constant frac- 

 tion of the maximum attainable. This latter proposition is certainly 

 arbitrary. 



Following another line of argument Hoesslin points out that the 

 bodily functions are essentially surface functions (the absorption of 

 food for instance a function of the surface of the small intestine) and 

 that they must therefore be proportional to the active surfaces of 

 organs in animals of different size. When similarity of structure is 

 to be preserved the active surfaces will, according to Hoesslin, be 

 approximately proportional to the body surface of each animal. The 

 truth of this proposition has been demonstrated in recent years for 



