Some recent literature on "vertical product differentiation" has developed the idea that if the nature of technology and tastes in some industry take a certain form, then the industry must necessarily be "concentrated"; and must remain so, no matter how large the economy becomes. The present paper develops this idea further, and looks at some of its implications. This approach offers a simple unified framework within which to re-explore many issues which arise in considering the relationship between advertising, R & D, and market structure.