Would it ? This method would mean that more air flows 'through the lungs allowing for more exchange. After all the air for it to enter into the sac would flow through the lung and similarly on the way out. Double exchange of gasses. So the idea that there would be 'more effort' and 'no advantage to the animal' is strongly debatable here. If the airsacs are surrounded my muscle tissue or perhaps ribs, the breathing could be made easier by providing for a 'pump' like action.
Donald Howes: <<Unless the airsac could itself draw in air then it would be a hassle for the lung, and even if it could, it would be much better off with a bigger lung.>>
Perhaps but there is no ultimate design in evolution. Evolution is not forward looking. A lung with airsacs is better than a lung with no air sacs.
Donald Howes: If it then seperated and had a pipe joining it to the lung, then it would serve no purpose, take up more room, and could only store stale air, or fresh air, neither of which is of benifit! >>
Again this is not necessarily true. First of all there would be no 'dead air' in the lung space and more air would flow through the lungs allowing for more efficient exchange.
Donald Howes: <<Much better to simply have a bigger lung.>>
Perhaps but that is not the argument. THe argument is that the lung with the airsacs is better than the lung by itself.
Donald Howes: << If it did then have a valve system, there may have been some benifit, but here we see that it must be the whole system changing at once, or it won't help.>>
On the contrary, without valves it might be less efficient but certainly not less efficient than merely a lung. Adding valves later would increase efficiency, again supporting my argument that it can be achieved in small steps leading to increased efficiency.
<<P.S. Surviving is not the issue in evolution, passing on your genes is. Therefore for something to be an advantage, means you have to be able to get the ladies to like it.>>
Without Surviving, there remains little for the ladies to be interested in.