Sir, - Mr Quinn, the Labour Party leader, simply asserts the economic benefit of bringing industrial and economic development in each of the Irish economies under a single agency, which in turn would be responsible to the proposed North-South executive body ("South yet to grasp the effects of Northern accord", The Irish Times, January 28th). Where is his evidence?If economies of scale are really so strong, one would have expected Northern Ireland to have given up its own Industrial Development Board and thrown in its lot with London a long time ago. Why, indeed, do Scotland, Wales and parts of England all retain separate industrial location agencies?It is true that a single "all-Ireland" agency might appear to be a way of avoiding damaging subsidy wars between the IDA and IDB, but the US and Canada have been able to co-operate in this area through using the joint dispute resolution mechanisms of the Canada-USA Free Trade Agreement without recourse to common industrial development authorities or an executive body straddling the 49th Parallel.In one thing, at least, Mr Quinn is right. Any cross-Border sharing of executive authority would cut both ways. Would Southern politicians and voters be willing to make the sacrifices which "interference" from Northern Ireland could involve? For example, since it is almost inconceivable that current rates of corporate profit taxation in Northern Ireland would be reduced to the very low level in the Republic of Ireland, would the 10 per cent profit tax incentive be relinquished for the sake of harmonisation? - Yours, etc.,Dr Esmond Burnie,Queen's University,Belfast 7.