Dick,
I refer you to the FAA AIM section 3-2-5
http://www.faa.gov/air_traffic/publi.../media/AIM.pdf
4(e)Separation for VFR Aircraft. No separation
services are provided to VFR aircraft.
In the USA a VFR flight in Class D is required to maintain their own separation from other traffic. The tower is just there to sequence aircraft so that the flow is orderly and smooth.