Over half of the country's population is crammed into the narrow, productive strip of land that is America's coastline, and much of our heavy industry is also situated there. But that's surface matter. The coast's real essence, says Simon passionately, is a ""fragile indivisibility,"" dozens of intricate systems of water, sand, fish, and marshes all fine-tuned to the same life metronome. Upset one link and the entire coastal unity goes out of sync, a harbinger of environmental disaster. Yet that is what we're knowingly courting: more ragtag development, dredging, and filling unloved marshlands, and increased dumping of 20th-century detritus--oil on the East Coast, PCBs in northeastern estuaries, nuclear plants in waters off New Jersey, sludge in the New York Bight, and pesticides everywhere. There are limits on how much pounding delicate, flowing systems can take. But after Simon's alarms have gone off, we find we're' learning the sin of our ways. ""We begin to decode the signals that issue from the thin edge."" What to do? ""For this indivisible whole we need an indivisible solution. . . an 'evolutionary leap.'"" It is a lover's call to keep all hands off the coasts. ""Moving inland would be the act of a patriot, serving a national purpose as important as marching off to war."" Nice thought, but even Simon acknowledges that ""New York City is somewhat firmly fixed.