Subject: Why didn't mammals evolve with lungs and gills?

It would seem that evolution should have produced a large number of 
species that can breath both air and water. The advantages in finding food 
and adapting to changing environments would seem to give a species with 
such a combination major advantages. Yet only a few amphibians have such 
attributes! Is there a good reason why larger more complex creatures did 
not evolve to breathe both air and water? (And I wish I could scuba dive 
without air tanks.)

Re: Why didn't mammals evolve with lungs and gills?