What a neat run exchange-traded funds have had.

From the germ of an idea at the American Stock Exchange in 1993, to a business holding $2 trillion in assets worldwide and $1.4 billion in the United States in 2013.

But the hole in the dike that was started with the first State Street SPDR fund two decades has not exactly produced a flood of assets, even so.

Mutual funds still command assets ten times as great, at $13.5 trillion of assets in the United States.

There's been a fairly clear split, so far.

Mutual funds are where mainstream investors go to find managers who can get them better than average returns on their money.

And still sleep at night.

Active management of their funds, for the better, they hope.

Exchange-traded funds, by contrast, have been the embodiment of couch potato investing.

Pick a benchmark, pick a sector, sit back, relax. The manager only has to rebalance the fund to make sure it's in keeping with the index being followed. Don't get out of line. John Bogle should be made proud.

But ETFs are hot.

The ability to buy or sell shares whenever you want appeals to large institutions, more than mom-and-pop investors.

But who doesn't like flexibility?

In a day and age when anything can be bought online at an instant's notice, it's only on stock exchanges where the ability to make a transaction in a microsecond gets questioned.

Here are a couple proof points:

Eaton Vance will try to bridge the gap between passively managed ETFs and actively managed mutual funds with a new category of product, called exchange-traded managed funds.

Where mutual funds' managers get to keep their cards hidden for 90 days at a time, the ETMF managers match them with quarterly disclosures of holdings.

Should suffice, to avoid giving away the store.

And, with that, the fundamentals of ETFs and mutual funds become pretty much the same. Only ETFs have lower costs and more tradeability.

Guess where this goes.

There's never been a national stock exchange or exchange of any type focused on mutual funds. How could there be?

Net asset values for those funds are set only once a day. ETF prices are set every 15 seconds.

Now there will be two exchanges focused on exchange-traded funds: NYSE Arca, which has taken over the American Stock Exchange's pioneering ETF business, and Nasdaq OMX Group's PSX exchange, which will be re-launched on April 24.

Why?

ETFs soared from 4.91% of all trading on national stock exchanges in 2006, to 15.41% in 2011. In 2012, that dropped to 14.3%, according to Rosenblatt Securities.

But is pretty much the only significant source of fresh volume on exchanges since the credit crisis in 2008.

Mutual funds are not going away. But the momentum-and the business model that matters-is in the midst of a long-term, momentous and, most likely, irrevocable shift.