After mature reflection, some Atlantic salmon have decided to stop spending several years fattening up in salt water. Instead, they shall become smaller, fresh-water fish. This is fresh evidence that 100 years after Charles Darwin's death, the theory he launched -- the theory of evolution -- is thriving.

Most male salmon have usually gone down to the sea to feed, and have then struggled back up rivers to spawn. Now they are saying the hell with it. They are saying that sport and commercial fishermen are destroying us, so we'll stay in the rivers and breed while small.

Actually, some dumb, macho salmon are still going to sea and are being destroyed. But the intellectual, nonconformist salmon who never did go to sea are becoming dominant and are transmitting their genetically controlled nonconformity. The quickly changing gene pool of the salmon shows how plastic some species are.

Darwin was fascinated by the finches on the Galapagos Islands, because different birds have different bills, according to the different foods they eat. He assumed that "the survival of the fittest" through mutations occurred slowly, over millions of years.

But five small pupfish species in Nevada seem to have evolved in the few thousand years -- a blink -- since they were isolated in a waterhole by an accident of nature. Called "the Darwin finches of the desert," they confirm Darwin's great proposition that "species are not (it is like confessing a murder) immutable." The fact that five pupfish species may have evolved in just 50,000 years suggests that Darwin underestimated the speed with which gene mutations can cumulate and result in new species.

Excavating in Wyoming, paleontologists have found an inch-long fossil of a jaw of a mammal about the size of a chipmunk. It may have been the first successful plant-eater in the age of dinosaurs. It had a sliding jaw-joint and sort of serrated teeth, which enabled it to shred the large amount of plant fibers needed for the high metabolism rates of higher animals. If the evolution of species fits a postulated pattern, something like this creature should have existed in an evolutionary "gap" between earlier flesh-eating animals with primitive jaws and teeth, and later animals that have better grinding mechanisms.

Another Wyoming find is the oldest fossil of a primate's foot. It reveals a grasping big toe. This (with a few intervening steps) made modern man possible. It enabled primates to scamper quickly across small branches, evading predators and thus reducing the need for high birthrates. This, in turn, made possible an extended developmental period required for complex creatures, such as us.

In Kenya, scientists have found a jawbone of a "humanlike" creature. The bone, believed to be about 8 million years old, has both humanoid and ape characteristics. This find came just months after the discovery in Ethiopia of the oldest known human ancestor, a 4-foot, 6-inch creature that walked upright 4 million years ago.

Facile critics of Darwin used to say that the important point about "the missing link" was that it was missing. Actually, of course, there are uncountable links, and interesting ones are being found all the time. Recent findings suggest that the divergence between ape and man may have occurred just 6 million years ago -- only yesterday, paleontologically speaking.

Forty million years ago, a gnat died in a way that got it "mummified" -- preserved in amber (fossilized resin from coniferous trees). Most mummified tissues are just a few thousand years old. This gnat's DNA may illuminate the evolution of that substance.

In 100 years -- years that have dealt roughly with many theories -- Darwin's basic line of inquiry has been emphatically vindicated. So has this axiom: the universe is not only stranger than we suppose, it is stranger than we can suppose.

Recently, an otherwise normal baby was born with a two-inch tail. The tail had hair and nerves but no bone or cartilage. It was removed by routine surgery. Such occurrences are rare but hardly unknown. A similar instance in 1894 was "explained" by the fact that the mother, while pregnant, had held a baby pig by its tail.

Actually, all normal human embryos in their fourth week have tails longer than the embryos' developing legs. (All human embryos have gill slits, too, but let's deal with one discomposing fact at a time.) The tail regresses and is absorbed to form part of the base of the spine.

There is in our gene pool the stuff of our relationship with other vertebrates. The perpetual incompleteness, the eternal becoming that characterizes man and all of nature, moved a wise man to say that the universe was dictated, but not signed.