Survival of the Towniest
There's a huge image in Mortal Engines that's next to impossible to get out of your head: the image of the massive half-mile-tall rolling traction cities that rule the planet. Our first glimpse of London is "a moving mountain of metal that rose in seven tiers like the layers of a wedding cake, the lower levels wreathed in engine smoke [...] and above it all the cross on top of St. Paul's Cathedral glinting gold, two thousand feet above the ruined earth" (1.4). You don't want to meet one of those in a dark alley.
The whole socioeconomic structure of the world as depicted in Mortal Engines is based on something called Municipal Darwinism. This is like Darwinism or Social Darwinism on a much larger scale. Municipal Darwinism imagines what Darwinism's ultimate endgame might be. After all, when does "survival of the fittest" stop? When only the fittest are left, how will they survive?
Town Eat Town World
When Magnus Crome goes a little batty... okay, battier at the end, he asks himself this same question: "How long will a new hunting ground support us? A thousand years? Two thousand? One day there will be no more prey left anywhere, and London will have to stop moving" (34.9).
Since this man has Municipal Darwinism hammered into his very soul, his answer isn't, hmm, you know, maybe we should look at this another way? It's out-of-this-world insane: "We will build great engines, powered by the heat of the earth's core, and steer our planet from its orbit. We will devour Mars, Venus, and the asteroids. We shall devour the sun itself, and then sail on across the gulf of space. A million years from now our city will still be traveling, no longer hunting towns to eat, but whole new worlds" (34.10).
A Different World
Seeing the extremes to which Municipal Darwinism can go forces Tom to reevaluate his worldview. At the beginning, Tom thinks Municipal Darwinism is just the way things are. "It was natural that cities ate towns, just as the towns at smaller towns, and smaller towns snapped up the miserable static settlements" (1.29).
Once he sees the pirate town of Tunbridge Wheels, though, Tom's mental wheels start turning. He sees that the town behaves in the exact same way London does; the pirate town just has more of a crass attitude about it. Has London been slyly pulling the wool over Tom's eyes this whole time? By the end of the book, Tom doesn't know what to believe in. All he knows is this: just because you were brought up a certain way doesn't mean that way is noble, beautiful, or for the good of society as a whole.