Susanne Freidberg, a professor of geography at Dartmouth, has been guest blogging here about the food supply. This is her final post; we thank her very much. You can thank her too by picking up a copy of her just-released book, “Fresh: A Perishable History.” Photo: Stephen Ausmus A White Leghorn hen. Last week’s post talked about early-20th-century “egg gamblers” . . .