PLEASE tell me you didn't just compare smoking to slicing your lungs with a razorblade!

Granted, inhaling particulate carbon isn't great for your lungs, but it's nothing your lungs can't handle. Your lungs were designed to filter out airborne particulates-- thus coughing up phlegm. It's a natural response to the smoke, and it's only bad if you smoke so much that you no longer have phlegm to coat your lungs and throat.

That said, heavy smokers DO have an increased chance of lung infection--the article confirmed that. However, lung cancer is a whole different thing, and marijuana has been shown to be protective against lung cancer.

Finally, you're confusing evolution and Darwinian natural selection. Natural selection occurs over many generations through genetic mutation, but evolution can occur on a cellular level within a human. For instance, every time you get a virus, the body develops new antibodies to fight it off, and after that first sickness, you never get sick from that virus again--hence people only getting Chicken Pox once. That's evolution, plain and simple. As to whether your lungs actually get more resistant to smoke over time, I don't know, but don't rule it out as a flaw in evolutionary theory, because it's not.

To summarize, marijuana doesn't scrub your lungs clean, and it doesn't give you cancer. Certain effects are detrimental, and others are beneficial. To get the best of both worlds, don't smoke in excess, and only smoke good weed to avoid inhaling more smoke than necessary. Most of all, just enjoy being high!