Endosymbiosis in biology refers to the idea that organelles of eukaryotic cells (like mitochondria and chloroplasts) were originally free-living micro-organisms that combined symbiotically (to mutual benefit). In Chapter 7: Good Roads of The Transportation Experience: Second Edition, we identify an example of what we might call techno-endosymbiosis, when we write:
Charles Kettering developed the electric starter, which temporarily over- loaded the motor. Interestingly Kettering modeled his innovation on the self-starter with the his work on motorizing the cash register when he was an engineer at National Cash Register in Dayton, Ohio. Kettering later founded Dayton Engineering Laboratories Company (DELCO) soon acquired by Gen- eral Motors. The self-starter eliminated the disease of “Ford’s Fracture” a broken arm resulting from cranking accidents 160. After Kettering, the automobile become an electric system in miniature: Its generator (with the battery) was the central station, which distributed current through a network to uses like starting the car, but also for headlights, and later radios and other purposes. Surprisingly, battery makers boomed not from selling batteries to makers of EVs but from selling to makers of gasoline-powered cars containing an electric self-starter 161. ... The internal combustion engine adopted the battery as a self-starter, and is a technological version of this biological process [endosymbiosis]. Hybrid vehicles, which ramp up the battery so that the vehicle can travel on either electric or gasoline power, are another version of this.
Technologies are analogous to species in many ways. Cities are not, rather they are analogous to colonies in the insect world. Where the complex of technologies called cities, with their people, can reproduce by spawning what are appropriately called colonies. Rome famously did this two millennia ago, with its colonia. Cities, or somewhat more precisely, metropolitan areas, are not simply legal jurisdictions, but have an economic definition. Wikipedia writes:
The U.S. Office of Management and Budget defines a set of core based statistical areas (CBSAs) throughout the country. CBSAs are delineated on the basis of a central urban area or urban cluster – in other words: a contiguous area of relatively high population density. CBSAs are composed of counties and county equivalents. The counties containing the core urban area are known as the central counties of the CBSA. Additional surrounding counties, known as outlying counties, can be included in the CBSA if these counties have strong social and economic ties to the central counties as measured by commuting and employment. Outlying counties are included in the CBSA if the employment interchange measure (total of in- and out-commuting) is 25% or more, although these numbers are estimates and exceptions are made.
One can certainly imagine the whole country, or large swaths of it, becoming large megaregions with overlapping flows, particularly as individual central cities decline in importance and networks of subcenters arise. Nevertheless, we are looking historically here: Cities (commuting regions) grow spatially and incorporate formerly independent cites and towns that become subcenters. They also spawn their own colonia, often local, that are called suburbs. To illustrate the concept locally (though the same process happened everywhere I have lived*), consider the Twin Cities. St. Paul and Minneapolis were once somewhat independent market areas, connected by the river and trails, but for which transport costs were expensive to go between them. With the advent of the horsecar, then the streetcar, and finally the motorcar, interaction costs declined and the cities were bound together as a single economic unit, even if governance remains divided. But these are not the only two cities in the region ultimately forged into a single unit by urban-endosymbiosis. There are places that existed before the Twin Cites, or before the Twin Cities became "The Twin Cities"**, and were later incorporated, and those that were spawned by the Twin Cities. Just among the counties in the 7 county Metropolitan Council region.
County, County Seat (year county seat founded as continuous European settlement) [1860 Population]
Anoka, Anoka (1844) [*1880 population 2,766*] Carver, Chaska (1851) [*1880 population 1,068*] Dakota, Hastings (1833) [1,642] Hennepin, Minneapolis (1838) [5,809] Ramsey, Saint Paul (1838) [10,401] [[1850 population 1,112]] Scott, Shakopee (1851) [1,138] Washington, Stillwater (1837) [2,380]
In general, the county seats are on one of the major rivers (Mississippi, Saint Croix, Minnesota), and were founded from downstream to upstream as places continued to develop and settlers moved farther inland in search of unclaimed land and resources. Which of these County Seats (or any other early town) was to be the eventual winner (the title of which is now held by the primary city: Minneapolis) is contingent both on geography and history. Hastings, for instance, was promoted as a "New Chicago" by Ignatius Donnelly until 1857. Minneapolis grew because of the power of the St. Anthony Falls waterfall, which was important only because electric grids were not yet developed. Had history been a little different, Minneapolis might be a suburb of Hastings or farther afield, Red Wing (1853) [1,250].
St. Paul - Minneapolis, Minnesota Twin City Map. By 1891, Minneapolis and St. Paul had grown together into a single unit, the opposite of Mitosis. At some point after the construction of intercity railroads (beginning in the 1850s) streetcars (beginning in the late 1880s) and paved state highways (from the 1920s), these semi-independent outposts, firmly attached to their location in the ground, became more and more mutually inter-dependent. Today development is contiguous and people are as likely to identify with the metropolitan area (or state) as they are with their most local level of government. There is no exact date for which an independent town becomes more part of the metropolitan system and less an isolated entity. The change is a process that develops over time, not an instantaneous phase shift. There is also no obvious threshold (25% out-commuters, 50%?) for in- vs. inter-dependent. Yet at some point a town is so enmeshed in a larger urban web it can no longer simply stand alone if its links were cut off, or returned to earlier (say 1860s) levels. The urban area as a whole may not existentially need Shakopee, which, among other things, notably exports entertainment like Racing and Amusement Parks to the pleasure-seekers in the rest of the region, but Shakopee does need the rest of the region. The body can cut off the hand and survive, if diminished. The hand cannot cut off the body.
* Some examples from other cities I have lived: London and Westminster (among dozensn of towns) now form a fully integrated unit; Atlanta economically absorbed Decatur; Baltimore connected to once far-more separate Ellicott City with the B&O railroad in 1830, but is slowly being drawn into the faster-growing Washington, DC system; San Francisco, Oakland, and San Jose have grown together.
**Why are the Twin Cities called the Twin Cities. InfoPlease says:
According to the Minnesota Historical Society, the nickname "Twin Cities" originally had nothing to do with St. Paul, the state capital of Minnesota. The term was first applied to two settlements on either side of the Mississippi River—St. Anthony's Falls on the east and Minneapolis on the west—in the 1840s. The two towns were later linked by a suspension bridge. Minneapolis was chartered as a city in 1867 and in 1872 it and St. Anthony's Falls were united to form one city. As a result, nearby St. Paul assumed the nickname while it and the new Minneapolis grew during the 20th century to become Minnesota's two biggest cities.