R help - Jul 2011 - hold position of vertices constant in network {statnet}?

I am a novice with network fuctions! I have been exploring the network
function in the statnet package, but haven't been able to figure out
how to hold vertices in position while varying edge features. Can
anyone advise on whether this is possible, and if so, how to do it?
Thanks!
-- 
Matthew Bakker, Ph.D.
Department of Plant Pathology
University of Minnesota
495 Borlaug Hall
1991 Upper Buford Circle
Saint Paul, MN? 55108 USA