104 ORGANISM AND ENVIRONMENT 
organic activity. The oxygen which passes into the 
lungs is just ordinary oxygen, driven inwards to the 
alveoli by an ordinary atmospheric pressure difference. 
The process is organically regulated, but the regulation 
appears to be something external to the oxygen, which 
still retains its usual properties. We can then trace 
its diffusion into the blood, its combination with haemo- 
globin, its carriage onwards by the pumping action 
of the heart, and its dissociation from the haemoglobin 
in the systemic capillaries. It has come under more 
intimate organic control in the blood, but we can still 
trace it as molecules of ordinary oxygen. When it 
reaches and is absorbed by the tissues in cell metabol- 
ism the organic control becomes far more intimate. 
It is caught up in a whirl in which its behaviour is 
from the physical and chemical standpoint utterly mys- 
terious. We can imagine no form of chemical com- 
bination which will now explain the behaviour of the 
oxygen. The mental picture of oxygen atoms or mole- 
cules seems to fade away, and to be replaced by an- 
other picture in which organisation is not something 
external to organised material, but is absolutely iden- 
tical with the material, so that both the material 
and its movements are nothing but manifestations of 
the organisation. It is life and not matter which we 
have before us. 
We can endeavour to hold on to the physical and 
chemical picture, and to seek for substances in the 
living structure which combine with, or enter into 
other physical or chemical relations with the oxygen. 
But a little consideration shows that even if we find 
