Read this: In 1860, Montevideo had 57,913 inhabitants including a number of people of African origin who had been brought as slaves and had gained their freedom around the middle of the century. By 1880, the population had quadrupled, mainly because of the great European immigration. In 1908, its population had grown massively to 309,331 inhabitants. In the course of the 20th century the city continued to receive large numbers of European immigrants, especially Spanish and Italian, followed by French, Germans or Dutch, English or Irish, Polish, Greek, Hungarians, Russians, Croats, Lebanese, Armenians, and Jews of various origins. The last wave of immigrants occurred between 1945 and 1955.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): The population quadrupled by 1880 due mainly to what?
the great European immigration