To both of you.

"Someone must have children or the human race will wither away."

Did I ever say we should stop having children entirely? No, I don't think I did. What I'm getting at is that our current rate of population growth is unsustainable. Our numbers have tripled since the 1940s and you're suggesting that we're in danger of extinction? The unrestrained increase in our numbers is a far greater threat than couples having one or two children as opposed to, say, fifteen. Zero population growth doesn't mean zero children, it simply means that the numbers are level, which os something that many societies have tried to do when they recognized that they were too numerous to survive in a world with limited resources.

"We're roughly the same size as China, but we have something like 23% of its population -- and the Chinese are heavily concentrated in cities too. Even China still has lots of room, lots of potential farmland, and so on."

Bad example. China is headed for the worst environmental and economic crash in human history. They have pollution that makes U.S. pollution look like organic tofu. They're starting to scout for resources in Africa and elsewhere because they know they'll soon use up all of theirs. As for farmland, you're ignoring the issue of soil fertility. A patch of dirt is not enough to grow food, and soil nutrients can be drained very quickly. And even then you're not taking forests into account. We still need wood, and trees require far longer to grow than crops. Don't be fooled; countries like China and India are already feeling the crunch, and it is only going to get worse.

"Let's all pray for a larger heart, first."

This might be a good idea. After all, human hearts might become a staple food if things keep going like they are. Bigger ones would naturally feed more people.

I would recommend the book Collapse to all of you. :-)