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All of London's bollards can be traced back to one of three common ancestors.
Leapfrogus lithos gave rise to the simple, smooth-topped bollards beloved of gymnastic schoolchildren and drunken Londonist writers.
The ancestral species known as Cannonus cannonus were first seen on London's streets in the early 19th century and are themselves evolved from captured French cannon. Their descendants, beginning with Cannonus archetypus, adapted to display a cannonball in the muzzle. Most bollards to be seen in the capital today are direct descendants of this cannon-and-ball bodyplan.
Finally, Squatus gunnus is another form of captured weapon. The type specimen can be viewed towards the rear of St Helen's church, Bishopsgate. It gave rise to a notable family of dwarf species. The most common is Squatus campanulus, whose territory extends all over the city. The more specialised Squatus bermondsia is a protected species found only in Bermondsey Square.
The phylogenetic tree of London's bollards is incomplete, and lacks many intermediate and rare species. Please feel free to suggest additions in the comments, or email the author on email@example.com.
Finally, those with an interest in the history of bollards should check out the Bollards of London blog.
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