Complex sentence: It is possible Mennonite farmers in the Vistula delta of Poland grew the wheat that was shipped out of the port of Danzig, then shipped from Danzig to Glasgow, where a friend of David Fife sent a sample to Canada.
Split into simpler sentences: It is possible Mennonite farmers in the Vistula delta of Poland grew the wheat that was shipped out of the port of Danzig. Perhaps it originated in the Vistula delta of what is now Poland, then shipped from Danzig to Glasgow, where a friend of David Fife sent a sample to Canada.