From the 58th edition of the CRC handbook of chemistry and physics, I offer the following:
molecular diameter
CO[sub]2[/sub]: 3.34 X 10[sup]-8[/sup] cm
O[sub]2[/sub]: 2.98 X 10[sup]-8[/sup] cm
coefficient of diffusion in air
CO[sub]2[/sub]:0.139 cm[sup]2[/sup]/sec
O[sub]2[/sub]:0.178 cm[sup]2[/sup]/sec
So, Oxygen is a smaller molecule with a higher diffusivity (in air, at least).
I forget…what was the question again?
Except that your river was turned into a lake by someone putting up a dam, which you now need to swim through, and the experts are saying you should cover the dam with wax to keep all the swimmers out.
I love analogies. 
Yeah, but it doesn’t really matter since a given quantity of either would occupy the same space.
I thought the question was whether or not that was true, but I’ve forgotten too.
Okay, let me see if I have this right: If we are able to cap a bottle with a gas mostly consisting of CO2 in the headspace ( it has more CO2 than the outside air, but less O2 than the outside air) even though the combined gas pressure is inside the bottle is greater than the outside air, the O2 in the ouside air will want to enter the bottle and the CO2 inside the bottle will want to exit. Is this correct?
Along with that, some O2 will exit the bottle and some CO2 will enter the bottle because of diffussion?
No, the question was whether one preferentially can permeate a membrane.
I think.
I say “Yes”, because its not absolute pressurepressure, but partial pressure.