Paragraph Eight All types of men now on earth have a common origin. All men belong to one great family; though long, long ago there was separation into groups, and the groups moved outward from their center or centers, and a gradual development of different types （which we now call races） took place.The s______ and growth of human races.
【正确答案】：source
【题目解析】：根据文章：“All types of men now on earth have a common origin.”可以得出答案。