130 SCIENCE PROGRESS. 



has shown that when a fish is asphyxiated it quickly uses 

 up the oxygen in its air-bladder. Evidently therefore the 

 oxygen may pass outwards into the blood, and considering 

 the physical properties of the air-bladder it is difficult to 

 believe that diffusion outwards is not constantly occurring 

 to a certain extent. 



If the gas is liberated from combination within the cells, 

 then we have in the animal kingdom a process which may 

 be compared to the liberation of oxygen from the green 

 parts of plants, and the fixation of free nitrogen by the 

 parasitic organisms in Leguminosae. Life is commonly re- 

 garded as being essentially an oxidation process, the activity 

 of the green parts of plants in presence of light being 

 looked upon as something exceptional, directly due in some 

 way to the presence of a specialised pigment. The fact 

 that free oxygen may be liberated even in the case of 

 animals must tend to shake this belief, which in any case 

 is rendered difficult by the fact that the only plausible 

 theory to account for the presence of free oxygen in our 

 atmosphere seems to be the presence of living organisms 

 capable of liberating oxygen. Organisms capable of 

 liberating oxygen must thus apparently have preceded 

 in order of development those living on free oxygen. 

 Physiologists in recent times have been apt to regard 

 life too much from a physical and chemical, and too 

 little from a biological, point of view, and it seems possible 

 that just as morphological identity is concealed under the 

 greatest diversities of physical form, so physiological 

 identity is concealed under great diversities of physical 

 and chemical process, and that comparative physiology, 

 working by means of biological and not merely physical 

 and chemical conceptions, will fill up the apparent gap 

 between physiological processes which in a chemical sense 

 are as widely separated from one another as those of 

 oxidation and liberation of free oxygen. 



J. S. Haldane. 



