As a fairly totalitarian and isolationist country surrounded on most sides by geographical hindrances, China really didn't have many immigrants begging to get in.
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china's early leaders limited contact with outsiders
Mountain ranges & deserts
The lesson is that the decline of a civilization is not simply the result of attack by outside invaders.
Canton
The history of civilization does not include everybody. No hunting or nomadic peoples could generate a civilization -- they lacked the stability and resources, and, with the exception of a limited numbers of signs and symbols, they never developed writing, unless it came from the outside.