The four scientists slogged along in the mud on the bank stream, stopping occassionally to look around the valley of brush and rock and mud, hoping for an indication they were in the right place. They had to be close, but their quarry was small and so hidden that none of its kind had been seen for 40 years.

White, blind, the size of newborn baby's little toe, the Hays Spring amphipod is one of the rarest creatures on this earth. It exsists only in one small spring, a habitat roughly the diameter of a manhole cover, filled with debris -- rocks, decaying leaves, dead beetles, dirt. The spring is so small and indistinct it disappears under the brush where the steep bank abruptly becomes the mud of the flood plain by the creek.

Hays Spring doesn't flow; it leaks a stony trickle -- insignificant, a seep in a string of seeps. And the tiny beasts that hide in it would seem to be among the least of living things, so few in number as to suggest they were forgottenby the Maker, or no more than an accidental spill.

Who else should care? What use? What need?

And yet the species is protected by the federal government and the small expedition went to a great deal of trouble to find the critters.

The entire population of the Hays Spring amphipod, every shrimp-like member of the species, could fit in a tea cup with room left over for sauce. Two dozen may live in the muddy bottom of the spring, or twice that, or twice again, but unlikely. They can't be counted without destroying the site, and the species.

There vulnerability is staggereing. A strand of genetic impulse, stretched like a spider web over millions of years, could be broken by a falling tree, a flood, a wheel, a few drops of antifreeze.

The reclusive creatures, hiding even from light, may be unimportant in the scheme of computer chips and chicken hot dogs, but they are an inheritance from a time before most springs were paved and guttered. No more valuable than a fading tintype of your grandmother as a child with her gold glint of a necklace and faint pink blush hand colored.

As yet in the reach of scientific possibilites, as the realm expands to a span outside ordinary imagination, from dead echos on the edge of the universe to ghosts of subatomic particles, man plays apprentice sorcerer. Tap dancing along the spiral staircase of the genetic stage, every living thing carries the probability of importance.

The eyeless, colorless creature living in Hays Spring is a finely tuned adaptation to perpetual, groping darkness, specialized to the edge of extinction.

Its very specialization and vulnerability make it a potential laboratory tool in studies of water purity and the intricacies of the double helix.

Dr. John R. Holsinger has a special interest in blind white subterranean creatures. A professor of biological sciences at Old Dominion University in Norfolk, he is an authority on amphipods.

"I've been working on these things all over the country, all over the world," he says.

Holsinger is the leading expert on Stygobromus hayi, the Hays Spring amphipod, though he had never seen a living specimen until recently. The hayi were first discovered in 1938 by Leslie Hubricht, a biologist who had probed the hundreds of springs then existing in Washington.

Hubricht reported his findings in 1940 and left preserved samples with the Smithsonian Institution, but the spring and hayi were lost soon after, somewhere in Rock Creek Park.

The fugitive beast was, nevertheless, proposed for the endangered species list in 1977.

In April 1978, Hosinger; Dr. Thomas Bowman, of the Smithsonian; Gordaen F. Karaman, a Yugoslavian amphipod specialist; and J. Horsley, of the National Park Service, went looking for the hayi.

The day was pleasant enough for the season in Washington. Though the sky was overcast and the ground wet, it wasn't raining.

"Just about any type of ground water will have some sort of organism living in it," says Holsinger. So every hesitant spring has to be inspected and its residents identified.

Afoot in the park, ignoring the highway and the trails, an explorer soon finds the valley much wider and irregular than those who ride across its bridges. Tangled necks just away from the creek and dig into precipitous banks. Springs multiply in the scrub and underbrush all over.

Holsinger's group soon found one lost spring, its brick casing half hidden in the duff. Amphipods lived in the old litter behnd the brick. Not hayi, but a new species, even smaller and still not named.

The expedition struggled on, had lunch, searched again. Muddy, tired, almost out of ground, Holsinger's party finally spotted a series of six seeps on a rough bank, just above the level of the frequently flooded bottom plain.One seemed to be large enough to last the summer wet.

Holsinger pushed up through the brush to a small rocky basin littered with natural and man-made trash. He squatted and began to gently lift up peices of the substrate -- rocks and decaying leaves.

He found amphipods clinging to the stuff he lifted from the water. Plump, healthy beasts, twitching white in his palm, came up to the light. But, no, they were not blind. They had eyes.

The spring, though, was more established than it had looked at first. The water gurgled from a naveless cathedral of mossy rocks, strong enough to hold the eroding bank away from its opening.

Holsinger gently penetrated deeper into the hollow, below the habitat of the surface-dwelling amphipods. Then, from the lower depths, he pulled out a rock with a different sort of creature -- tiny, unpigmented, lacking even rudimentary eyes.

Using his hunting tools, an artist's camel's-hair brush and a bottle of 70 percent alcohol solution, he took his prey.

He slipped the pale brush between the amphipod and the rock. "It's easy. They're flattened sideways," he says. The crustacean lost its grip and fell to its doom in the preservative solution.

Holsinger could not be positive until he had the eight samples back in his laboratory and had scrutinized them under a dissecting microscope for the distinctive array of appendages that marked the species. But as it turned out hayi had been rediscovered after four decades in the wilderness of Rock Creek Park.

Holsinger first learned of the existence of hayi while he was working for his dissertation at the University of Kentucky. Going through the literature on blind subterranean animals, he found Leslie Hubricht's report on the new species he'd found in Washington.

Hubricht had named his discovery the Hays spring scud, Stygonectes hayi, and given samples to the Smithsonian. Later he recanted, thinking hayi was only a variant of another species. So no one in the exclusive field paid any more attention and hayi was lost.

Holsinger, however, was intrigued with the description. He checked the samples at the Smithsonian on one of his frequent trips to Washington and confirmed the uniqueness of the animal.

"The differences in the appendages are consistent," he says.

Hubricht had placed hayi in the family Gammaridae. Holsinger identified it as a member of the Crangonyctidae and renamed it the Hays Spring amphipod without ever seeing a live specimen.

Originally from Harrisonburg, Va., Holsinger has pursued his interest in subterranean animals in more than 1,000 caves since 1959. When his and Bowman's reports prompted the U.S. Fish and Wildlife Service to begin the process of listing hayi as endangered, the location and status of the amphipod became more important.

Holsinger had attempted to find hayi in 1965 with no success, but then his solitary search had not seemed pressing.

Then, in 1977, the Hays Spring amphipod was officially proposed for addition to the endangered species list. Though the eastern cougar is listed when most wildlife biologists doubt its existence, some people questioned whether hayi should be listed, since no one could find it.

If the Hays Spring amphipod were going to be saved, join the Appalachian monkeyface pearly mussel and the Socorro isopod on the U.S. list of endangered species. Holsinger was going to have to establish its continued presence in the wild.

His expedition in Rock Creek Park did just that, and the way seemed clear for an amphipod to join the elite list.

Holsinger and others had suggested several species of amphipods for protection, but the Hays Spring one was alone in finding acceptance at the Fish and Wildlife Service.

Steve Chambers of the Office of Endangered Species in the Fish and Wildlife Service works on proposals to protect crustaceans. He has to sort through all reports on the suggested species that come to his office, collect all the scientific information, all the economic facts relevant and put together and environmental and economic assessment for a proposal. Then he takes the possible species into an office panel discussion where peers consider the prospects for success.

One particularly relevant item in favor of the Hays Spring amphipod was its habitat on land already protected by the federal government.

The environmental and economic impact of listing the amphipod would be about nil, but the hayi still needed protection. It was vulnerable to floods, pollution and proposed construction nearby. Its only known predator was the interested scientist who might want to collect a few samples and, once hayi was listed, a permit would be needed to dip one rock out of the spring.

The proposal was moving along with all due speed when the rules were changed.

In the furor over the Tellico Dam and the endangered snail darter, opponents of the endangered species act attempted to scuttle the whole thing. They didn't succeed, but they managed to change the authority and the requirements for adding new species to the list. The Department of Interior withdrew all the pending proposals in December 1979, and that of the Hays Spring amphipod wasn't revised and resubmitted until last July.

Holsinger is delighted that his amphipod is going to be protected. "You never know down the road what something like this could be useful for."

If any fame resides in the field of amphipods, Holsinger may harvest it. A hundred-odd species of amphipods have been identified and named in the United States and Holsinger is in the process of adding another hundred to the described total. That will leave, he estimates, another hundred yet to be found.

The Washington area is likely to be the home of at least a few.

While the city has few, if any, caves, it has a lot of ground water amphipods, says Holsinger. "It's rare to have so many species in one place." But Washington is in a mixing zone for ground water, where two geological types, piedmont and costal plain, come together.

Six different white, blind amphipods are found here. All but one are rare and one is more elusive than the Hays Spring amphipod.

The only known samples exist on a shelf in the Smithsonian, in a small bottle labled "well water, Alexandria." The bottle was filled sometime during the 1920s by an unknown hand.

The rediscoverer of hayi is very interested in old, hand-dug wells that might still survive in this area. He's ready to go hunting again with his camel's-hair brush and the 70 percent solution. CAPTION:

Illustation, no caption, By Allen Carroll