Municipal Darwinism is a philosophy of technological ecosystem by which most of the world of Mortal Engines adheres to and live. Traction Cities consume one another by gathering other, smaller cities in large hydraulic "jaws". The larger metropolises consume smaller cities on a food chain. These smaller cities, often consume towns, which consume villages, hamlets, and stationary settlements.
Captured cities are dragged inside the body of the predator city, where they are then melted for fuel or salvaged for parts. Their citizens either resettle in the city they were just eaten by, or - in less ethical cities - are enslaved, often to work in the predator city's engines. Old Tech and small goods of value are looted as well.
Although the planet eventually resolved major conflicts, Traction Cities have been moving for so long the people have forgotten why they moved in the first place, and it is now considered normal. At the beginning of the series, Municipal Darwinism has been in practice for approximately a thousand years. Thatcher is the god of Unlimited Municipal Darwinism and is worshipped by the Traktionstadt Alliance particularly.
"..But it's madness, we'd be starting some sort of Municipal Darwinism..."
-Dr Crumb to his fellow Engineer when he was told about the idea of setting London moving.
The actual commencement of the practice of Municipal Darwinism - referring to mobile urbanizations consuming one another - is not mentioned in the series, but what it is known is that after some designs for a very powerful motor engine were found inside a hidden Vault in Fever Crumb's London. After this, throughout the rest of the Fever Crumb Trilogy, London was torn down completely and rebuilt as a vertical city with three tiers stacked on top of one another, all on top of the G.U.T. (the Great Under Tier) which was used to house the engines, machines and fuel, as well as storage space which was later to be used as dismantling yards, all set on top of huge wheels.
It is not known when "jaws" were adapted for predator cities, but it is thought that London had the first, as Dr Crumb, near the end of Scrivener's Moon, tells Quercus that the great undertier is "London's gut after all" and that the big doors at the entrance are its 'mouth parts'.
During London's finishing touches, such as the finishing of the second and the top tier, news arrive saying that the Nomad's armies and Stalkers and traction fortresses were mobilizing south to invade London.
Although London had been the first official moving city, the inspiration towards traction metropolises originates from the Nomad Traction Fortresses during the time of Fever Crumb (about 1000 years before Mortal Engines), which had been used by the Nomads - Tribes from the north of Europe which had been waging wars against one another for years using mobile fortresses. Like later traction cities, these too evolved from primary carriages carrying the tribe leaders and chieftains to later war vehicles and fortresses. These were powered by different things such as the man power of slaves, giant mammoths, and later powerful motor engines. Although Auric Godshawk , London's tyrannical Scriven leader 18 years before London started moving, was a brilliant scientist and engineer as well and was the first man to have a vision of an entire city stacked upon itself on wheels; powered by huge, powerful engines - similar but bigger than those of the traction fortresses, which he himself designed and built.
Fourteen years after Godshawk's death, Nickolas Quercus invaded London with his personal traction fortress with the idea in mind to continue Auric's dream. He found the Motor Engine in the hidden vault under Auric's "Nonsuch house". Over the course of the Fever Crumb Trilogy, Quercus dismantles London and re-builds it in the form of a giant vehicle on wheels with three tiers, although London sets off north to fight off a threatening Nomad invasion, abandoning a large proportion of it's "unskilled and irrelevant" population before the town is even completed having only the bottom tier completed and the middle tier only half-done and the top tier unlaid.
Survival of the Fittest
Municipal Darwinism hinges on the cycle of predator and prey; if the bigger town is faster than the smaller, the smaller town will be eaten. The biggest tenet is Survival of the Fittest. But if the smaller town is faster than the bigger town, the bigger town risks running out of fuel. However, the world of Municipal Darwinism is not limited to predator and prey only; parasites and scavengers also have a role. During Mortal Engines, the main characters find an empty and abandoned city being slowly stripped of goods and scrap metal by scavenger crews from small towns and airships, reminiscent of a decaying carcass being decomposed by bacteria and insects. In Predator's Gold, airships are seen hovering around a large city and sifting through its exhaust smoke to recover minerals, similar to flies hovering around a larger animal. Later in the book, a small aquatic vessel secretly attaches itself to the underside of Anchorage like a barnacle or limpet, and the crew sneak into the city at night to pilfer valuables. Nimrod Pennyroyal also mentions vampire towns in Nuevo Maya that stick to the bottom of Ziggurat Cities and drain them of everything inside, leaving them as empty shells (though this may not be true).
Municipal Darwinism is the centre of life for the people of Traction Cities, bordering on a religion. It is considered dirty and wrong to set foot on bare earth, and "unnatural" for cities to be stationary.
Municipal Darwinism is opposed by such groups as the Anti-Traction League, which sees Traction Cities as obstacles that hinder the recovery of the Earth to its natural state, and view their citizens as barbaric. Likewise, the citizens of Traction Cities also view Anti-Tractionists as barbaric, often using the derogatory term "mossie" to describe them.
"In the better days, London wouldn't have bothered with such small prey." -Tom Natsworthy.
From the start of the Mortal Engines series, which takes place roughly five to six hundred years into the practice of Municipal Darwinism, it is mentioned that 'prey' is scarce and that larger cities often chase very small towns and villages that wouldn't otherwise have bothered with in the past when a catch was much more common and much more bigger, suggesting that Municipal Darwinism is not a sustainable way of life.
At the epilogue of A Darkling Plain, anything from a few decades to thousands of years into the future after the war, Municipal Darwinism has become obsolete and traction cities are known as ancient legends.