Evidence has been cropping up here in the Rock River Valley that an advanced form of intelligent life exists on earth.

The first sightings go back about 20 years, when William A. Hewitt, the chairman of Deere & Co., the farm equipment firm, asked Eero Saarinen, the great Finnish-born architect, to design his headquarters.

This was completed in 1964, three years after Saarinen's death. The building is an architectural composition without parallel in American business.

The building is set in a wooded ravine in the company's 1,044-acre preserve. Sheets of glass, sheltered by deep sun screens, enclose and liberate the interior space. Two lakes frame this setting. Fountains create a gridlike pattern of mist.

Good-size goldfish swim lazily in the lakes, and ducks, geese, and swans promenade along the terraces to the water.

Out on a little island, with a bridge leading to it, is a bronze sculpture, Hill Arches, by Henry Moore.

Saarinen's building is a mixture of agrarianism and urbanity. It is expressed in the building material, acorrosion-resistant, self-maintaining steel called Corten (its use was pioneered here). Over the years, the steel has taken on a deep reddish-brown hue.

To the east, across the ravine to the hillside, a glass-enclosed bridge connects with a high, airy, display hall where farming equipment of every age and application has been placed.

Last year another glass-enclosed bridge was built to an office building, called Deere West, designed by Kevin Roche, John Dinkeloo & Associates. They were closely associated with Saarinen on the original building.