IN A SHOW-STOPPER veto of his very own bill, Virginia Gov. John N. Dalton has struck a blow for ecological uncertainty. Dead of unnatural causes is a former pride and job of the governor -- a measure that would have put new restrictions on development in the state's vast and fragile wetlands areas. Equally fragile, apparently, was Mr. Dalton's cherished relationship with the state's big manufacturers and leaders of the Chamber of Commerce, who met with the governor Monday to tell him how bad his bill was.

His own secretary of commerce and resources, Maurice B. Rowe, pleaded with Mr. Dalton to resist the business pressure for a veto, but the governor stood his not-so-firm ground on wetlands and proceeded to do in the measure. The effect, if you're still holding out for one, was to leave protection of these large areas in the hands of the federal government -- the same federal government that Mr. Dalton has always derided.

Maybe that's why some of the conservationists who supported the bill weren't all that devastated by Gov. Dalton's reversal. At least one lobbyist observed with a degree of satisfaction that the extraordinary effort by the business groups was a sign of how close the conservationists had come to an untraditional victory. As for the bill that isn't, it would have brought an estimated 75,000 to 100,000 acres under stricter protections than now cover certain other wetlands areas in the state.

At least Mr. Dalton's ultimate rationale, as expressed by his press secretary, is on about the same sound footing as his decision. It centers on a certain additional provision that the governor originally neither supported nor opposed. He wound up requesting it, but it was crushed in the House. Now, says press secretary Paul G. Edwards, "Without the . . . language, it was not an administration bill. He feels there aren't safeguards for private owners and developers." If this is a legislative pattern of things to come, the lawmakers may never have to pigeonhole a Dalton bill again -- if they just wait long enough, every proposal will self-destruct only weeks after adjournment.