Undisputed, i`ve been watching this thread and their are some good replys, and from what i can gather from your replys you seem to be coming down on the side of, Nurture....
Well heres my two penneth for what its worth...
For Lurchers i reckon its 70% Nature 30% nurture, because to get a decent days/nights work from a lurcher it needs to be reasonably obedient, but it also has to have the phyisical and mental attributes to want to work which can only be passed on by breeding worker to worker, and best to best..
With Terriers i.m.o. i reckon its 100% nature, the dog Has To Want to B