Boston in 1838 - Boston-Roxbury border shown in red. Note Roxbury's Gravelly Point on the left. The border along today's Public Garden sat between what is now Arlington and Berkeley streets (BPL).
As plans were made to extend Boston into the Charles river estuary south and west of the Common, there was a problem to be dealt with. The problem was that the border through the back bay had been set to follow a channel that cut through shallow tidal flats. And that channel ran quite close to the Boston shore. When the Mill Dam was built from Boston to Brookline to enclose the bay and run tidal mills, Beacon street was extended across it, and Boston was given possession of the new-made land. The above Boston-Roxbury border was a straight-line adjustment of the original meandering border that had been formed by the path of the channel that ran out from the Stony brook outflow to the deeper bed of the Charles river.
So what was to be done, when the whole point of the land-making operation was to create a residential for well to do Boston residents? Between the state, which would claim a portion of the new lands for itself, to be sold at a profit, and the city, seeking an extension of its own area to prevent what might be called Brahmin flight, the two simply stole the Back Bay flats from Roxbury by fiat. Just as in the case of Dorchester Heights/South Boston at the start of the century (discussed in an earlier entry), there was money to be made, and the state and city joined hands in a profitable land grab from a bordering town.
So imagine if the Back Bay south of Arlington street had been made part of the town of Roxbury. The entire upscale residential district, plus everything over to Columbus avenue, would have put money into the Roxbury tax coffers. Would it have made sense for Roxbury to allow itself to be annexed by Boston in the 1868s? And if Roxbury had not been annexed, then the town of West Roxbury, including Jamaica Plain, Roslindale and West Roxbury would not have had a direct border with Boston, making annexation of those districts far less likely. So the Boston we know today - stretching from East Boston south to the Dedham line, depended on a political land grab of mud flats. Evidently, city building is like sausage making - it's best left unobserved.