There’s a point that’s been made in the debate over bridge construction in Quebec that needs to be attacked for a logical fallacy.

The point, which was repeated in a letter to the editor this morning, basically goes like this: Bridges constructed centuries ago are still standing because they’re well constructed. Why can’t we build bridges like this today?

The problem with the argument is that it’s based on observing only those bridges which are still standing today. It ignores all those whose weaker construction led to their collapse not long after they were built. It ignores those rope-and-wood-plank bridges we see in the movies. It ignores things like the Tacoma Narrows Bridge, which collapsed spectacularly months after being built in 1940.

Some bridges still stand today, because they were very well built. That’s not because engineers of the time knew more than we do now, or that they were more cautious about introducing safety margins than we are. It’s because natural selection has already weeded out those bridges whose construction wasn’t strong enough, leaving us the mistaken impression that bridges from back then were better constructed than those of today.