Georgia's ethnic composition varied from one historical epoch to another and this happened as the result of certain economic, political or social factors. Georgian academic Vakhtang Jaoshvili identified four major stages in the history of Georgia that influenced the ethnic composition of Georgia: from medieval times to the late 18th century; from the 19th century to the 1921 Soviet invasion of Georgia; from 1921 to the collapse of the USSR; and present days, starting with the Georgian declaration of independence.