Ordinarily the silver fox is a canny, elusive creature, quick to bite when cornered, and well nigh impossible to tame.

This is hardly surprising, considering that foxes over the eons have been hunted for sport, trapped for fur, and generally maligned by human beings, especially those with hen houses.

But because of selective pressure imposed by experimenters over a 40-year period, a new and once undreamed of breed of fox has evolved -- one that whines for attention from human beings without any prior conditioning, licks its masters' faces, and has even begun to look something like a domestic dog.

It is as if by an act of will human beings had transformed an innately hostile creature into a friend.

In a long-term experiment at a Siberian fur farm, geneticists have created this new version of Vulpes vulpes, the silver fox, by allowing only the friendliest animals from each generation to breed. Having selected only the most "tamable" of some 45,000 foxes over 35 generations, the scientists have compressed into a mere 40 years an evolutionary process that took thousands of years to transform ancestral wolves into domestic dogs.

The original purpose of the breeding was to create a friendly breed less likely than wild animals to fight when put to death. But in time, geneticists saw that far-reaching changes they observed in the foxes' physical and neurological makeup merited scientific study. The scientists apparently underwent some changes, too. Close bonds developed between the tame foxes and their human wardens, and the staff at the fur farm is trying to find ways of saving the animals from slaughter.

To keep environmental influences to a minimum, none of the foxes in the Siberian experiment received any training, and their contacts with human beings were limited to brief behavioral tests.

The director of the experiment is Dr. Lyudmila N. Trut of the Institute of Cytology and Genetics of the Siberian Department of the Russian Academy of Sciences at Novosibirsk. She reported in a paper that selective breeding to create genetically docile animals had resulted in a breed of ultra-tame foxes that make good house pets "as devoted as dogs but as independent as cats, capable of forming deep-rooted pair bonds with human beings."

Selective breeding of farm animals to enhance commercially useful traits is as old as human history. But a striking consequence of the Russian program was that breeding foxes to strengthen a single behavioral trait, "tamability," brought about a wide variety of seemingly unrelated physical changes.

A paper by Dr. Trut discussing these changes, translated from Russian, was published in the current issue of American Scientist, the magazine of Sigma Xi, a scientific research society. Dr. Trut wrote that by breeding the tamest animals from each of about 35 successive generations, the final offspring were not only tame from early puppyhood but also looked different from their wild forebears.

The normal pattern of coat color that evolved in wild foxes as camouflage changed markedly in the genetically tamed fox population, with irregular piebald splotches of white fur appearing in some animals. The tame foxes sometimes developed floppy ears in place of the straight ones of wild foxes. The domesticated foxes generally had shorter legs and tails than ordinary foxes, and often had curly tails instead of straight, horizontal tails.

Moreover, the faces of adult tame foxes came to look more juvenile than the faces of wild adults, and many of the experimental animals developed dog-like features, Dr. Trut reported. Although no selective pressures relating to size or shape were used in breeding the animals, the skulls of tamable foxes tended to be narrower with shorter snouts than those of wild foxes.

Even reproductive cycles changed in foxes bred to be docile; wild vixens are receptive to sex only once a year, but some of the docile females become receptive more often.

Domestication also apparently affects genes controlling the timing of physiological development. For example, the genetically tamable fox puppies open their eyes sooner after birth than do ordinary silver foxes, and they show a fear response to unfamiliar stimuli about three weeks later than their wild counterparts.

Over successive generations and increasing congenital tameness, Dr. Trut wrote, the Russian team measured a steady decline in the hormone-producing ability of the foxes' adrenal glands. Adrenal hormones prepare an animal for fight or flight.

Even the brain chemistry of the tame animals differed from that of their wild ancestors; after 12 generations of selective breeding, Dr. Trut reported, test animals' brains contained significantly higher levels of serotonin than did their forbears. Serotonin, a neurotransmitter, is intimately involved in an animal's psychological state.

Dr. Darcy F. Morey, an anthropologist at the University of Kansas who has studied the physical changes that ancestral wolves underwent as they evolved into domesticated dogs, said that Dr. Trut's report underscores the complexity of seemingly simple genetic traits.

"Clearly," he said in an interview, "the link between modifying the neurology of an animal and its biochemical balance is all intertwined with other aspects of physiology."

Domestication, with all the physiological changes it brings, has not been limited to the the dog family, he said. "Human beings more or less domesticated themselves," Dr. Morey said. "Of course, we're not as docile as we'd like to think."

The Russian breeding experiment began in 1959 under the direction of a geneticist, Dr. Dmitry K. Belyaev, who started with a population of 30 male foxes and 100 vixens from a commercial fur farm in Estonia. From the offspring of these animals and from each successive generation, Dr. Belyaev's team would winnow out about 5 percent of the tamest animals for further breeding. Dr. Belyaev died 14 years ago, but his experiment has continued.

In nature, the process of domestication is long and complex.

"Behavioral responses," Dr. Trut wrote, "are regulated by a fine balance between neurotransmitters and hormones at the level of the whole organism. Even slight alterations in those regulatory genes can give rise to a wide network of changes in the developmental processes they govern." Changing a complex system of genes (or a "polygene") by selectively breeding for some trait like docility is not necessarily beneficial, she said. Even a slight alteration of a polygene like that presumed to lead to docility "might upset the genetic balance in some animals, causing them to show unusual new traits, most of them harmful to the fox," Dr. Trut wrote.

Through selective breeding to enhance the docility of 45,000 foxes over the last 40 years, Dr. Trut said, "we have compressed into a few decades an ancient process that originally unfolded over thousands of years."

"Before our eyes, the 'Beast' has turned into 'Beauty,' as the aggressive behavior of our herd's wild progenitors entirely disappeared," she said.

Dr. Trut concluded her paper with a prediction that the experiment in tamability breeding would probably have to end soon; last year there was no money in Russia's budget to pay the scientists or even feed the foxes, most of which were destroyed.

"Recently we have sold some of our foxes to Scandinavian fur breeders who have been pressured by animal-rights groups to develop animals that do not suffer stress in captivity," she wrote. "We also plan to market pups as house pets, a commercial venture that should lead to some interesting, if informal, experiments in its own right."