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How did geography have an effect here? I don't see what you mean. And China hasn't really had too much stability. It's always been full of revolutions, competing empires, dynastic changes, warring states, shifts in power, changes in ethnic ruling class, and so on. Go through a list of deadliest wars and revolutions and a good chunk of them are in what is today China. |
It's been invaded at least in part a few times (Mongols, 13th c., English, 18th c., and Japanese, 20th c., most especially), but retained its overall identity and either assimilated (Mongols) or repelled (English, Japanese) the invaders, eventually.
Contrast Europe which has seen vast shifts in control, and utter replacements or eradication of local culture or tribes multiple times, going to prehistoric times, to the present (past century certainly, past few decades quite arguably). There have been very few constant borders or identities, certainly not on the scale of China.
Even written language in China is still largely intelligible to moderns, from a thousand years ago or more. English, more than about 400 years ago is almost wholly foreign:
Hwæt,ic swefna cyst secgan wylle, hwæt me gemætte to midre nihte siþþan reordberend reste wunedon.
("Behold, I wish to tell the best of dreams which I dreamt at the middle of the night, after speakers remained in rest." https://www.arts.gla.ac.uk/stella/readings/OE/ROOD.HTM)