The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

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 （） and growth of human races.