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Chapter XII 



very interesting way; practically the critics were on one side and 

 the experimenters on the other ; those who taught and wrote for the 

 most part refused to accept the secretory theory, whilst with the 

 exception of a few senior physiologists, such as Fredericq and Zuntz, 

 the workers at respiration inclined more and more to the secretory 

 theory. Indeed matters seemed to have reached a deadlock. The 

 diffusion theory was supported entirely by aerotonometer experi- 

 ments, which were held by the supporters of the secretory theory 

 to be fundamentally vicious, on the ground that the blood reduces 

 itself when removed from the vessels; this charge seemed to be 

 unanswerable at all events it was unanswered. Its weight lay in 

 the fact that, at high oxygen pressures, a very slight amount of 

 reduction would produce a very big drop in oxygen pressure in the 

 blood ; therefore, if any appreciable reduction took place in the tono- 

 meter, the blood in the vessels might easily have had twice the 

 oxygen pressure which was observed in the tonometer. This criticism 

 remained unmet; moreover those who refused to accept Haldane's 

 results proved quite unequal to the task of showing where he had 

 gone wrong. 



Experiments on the effects of fall of temperature on the 

 oxygen tension of arterial blood. 



Indeed at the time of which I am writing (six or seven years 

 ago) the supporters of the physical theory had, in their endeavour to 

 throw the burden of proof upon their opponents, frankly taken refuge 

 themselves in metaphysics. They began their arguments with the 

 general statement that we must always believe the most simple of 

 two rival theories unless it can be disproved, because it is the most 

 simple. This statement, like that freely used by the opposite faction, 

 namely, that we must believe the theory which offers the greatest 

 possibility of benefit to the organism because evolution demands it, 

 contains an element of truth. To my mind they break down at 



