62 ORGANISM AND ENVIRONMENT 
suddenly reminds us that it is alive: for it begins to do 
something which at once recalls living things when it 
delivers oxygen at a higher pressure than that at 
which it receives it. The passage of oxygen molecules 
is accelerated in the inward direction, and this accel- 
eration applies to them alone, and not to other mole- 
cules, so it is selective. It does not occur in a non- 
living membrane, and its presence is evidently depend- 
ent, firstly upon the peculiarities of the living mem- 
brane, and secondly upon the presence of a special 
stimulus acting on the membrane. We know, also, 
that the specific peculiarities of living tissues depend 
upon the maintenance of their external environment. 
Hence we can say that the acceleration depends, not 
only upon the factors just mentioned, but upon the 
integrity of the general environment of the mem- 
brane—in more familiar words, upon its nutrition, 
temperature, etc., and upon the regulated removal of 
so-called waste products. 
Active secretion of oxygen is not a new phenomenon 
in physiology. It is now over a century since the 
famous physicist Biot made the discovery that the gas 
in the swim bladder of deep sea fishes is nearly pure 
oxygen. The pressure of oxygen in sea water is only 
about a fifth of an atmosphere, and is doubtless less 
than a tenth of an atmosphere in the blood circulating 
outside the walls of the swim bladder. Yet inside the 
swim bladder the oxygen pressure in the case of deep 
sea fishes may be 100 atmospheres or more. It was 
shown in 187% by Moreau that fishes secrete just 
sufficient oxygen into their swim bladders to bring 
