How to iterate through all geometries using a search cursor?

I would like to use a python add-in which uses a button click to iterate through each selected polygon one by one.

The following code manages to zoom only to the first polygon. How can I adapt it to zoom to each selected polygon?

mxd = arcpy.mapping.MapDocument('CURRENT')
df = arcpy.mapping.ListDataFrames(mxd, "Layers") [0]
Envelopes = [] # store extents here

# find the selection set
SelLayer = arcpy.mapping.ListLayers(mxd,data_frame=df)[0] # first layer
fidSet   = arcpy.Describe(SelLayer).FIDSet

if len(fidSet) == 0:
    arcpy.AddMessage("Nothing selected")
else:
    # now cursor through it an get the geometries
    # storing their extents into a list
    with arcpy.da.SearchCursor(SelLayer,"SHAPE@") as SCur:
        for feat in SCur:
            # I'm going to assume polygon/polyline
            Envelopes.append(feat[0].extent) # grab the envelope 

    df.extent = Envelopes[0] # first extent
    arcpy.RefreshActiveView()