Devolution.

DINOSAURS, it seems, have changed more in the past twenty years than in the previous 200 million. First of all they are not, as you may have heard, extinct. Through a neat act of clerical redefinition, birds, once thought to be descended from dinosaurs, have now become dinosaurs. Then the reptile formerly known as Brontosaurus, or Thundering Lizard, has been renamed Apatosaurus, that is, Deceiving Lizard, a point upon which paleontologists insist with great solemnity, concealing the fact that the change is mostly one of labeling. Everything from the size to the speed and posture of these big chaps has been called into question, and the truculent certitude of yore has yielded to pandemic doubt.

The center of such revisionism is the American Museum of Natural History, which, amid much fanfare, has just reopened its Hall of Dinosaurs after many years of renovation. Given the $12 million that went into the restoration, one almost feels that the curators insisted on these changes largely in order to have something to show for the vast expense of time and money. The new section comprises two main halls, one for the Saurischians and the other for the Ornithischians, arranged according to the principles of cladistics. This classification stresses shared characteristics such as grasping hands or backward-pointing pubis bone, rather than chronology as in the past.

Unfortunately, it will be very difficult for anyone to see the present exhibition with real pleasure. The museum demonstrates the same questionable judgment that I remarked on in these pages several months ago apropos of the Smithsonian's new Museum of the American Indian: the spaces are cramped labyrinths abounding in verbiage, written and recorded, which thoroughly swamps the objects on display. There are more ramps and slopes and alcoves in the cluttered rooms than you will find on a miniature golf course.

But the real problem here is children. With all the fervor of Muslims at Ramadan pressing upon the altar of the Kaaba, they come by the busloads from their schools in the winter and their camps in the summer to look at the dinosaurs and the elephants and the big blue whale. And the museum is ready for them. Cutesy dinosaur footprints lead from everywhere in the museum to the new fourth-floor displays. Once you get there, you find yourself at the Dinostore, which purveys all the lemon- and grape-colored stuffed dinosaurs you will ever really need, not to mention socks, baseball caps, ties (for Dad), pencil sharpeners, games, and coloring books. Once you reach the actual display, you will find video games throughout posing as information centers -- imparting the message that science is fun.

Granted, the American Museum of Natural History has not yet sunk to the depths of the Franklin Institute in Philadelphia, which has become a cross between a theme park and a day-care center. But it is on its way. This is evidenced in the newly opened Hall of Human Life, whose candy-colored array of flashing lights has the appearance of a pinball machine at tilt, as well as a recent exhibit on spiders, which, with its big mockups of spiders and its goofy captions, could not have held the attention of anyone over 8 years old.

It was not always so. Once upon a time the American Museum of Natural History, like London's Natural History Museum, the Smithsonian, and the Franklin Institute, was an augustly Victorian structure whose wares were displayed in glass cases with very few distractions. Now this is no longer possible, the tendency in museology being to show ever fewer objects with ever more commentary.

One had expected more from this renovation, especially since the Hall of Mammal Evolution, which was completed a year ago, was so sober and successful and, despite its glinty postmodernity, perfectly matched to the Victorian architecture. The transformation of the corner turret into a silo of living light opening onto Central Park remains perhaps the most brilliant architectural coup de the`tre in many years. Unfortunately there is nothing of the sort about the new exhibitions, which have created an entangled chaos where formerly there was order.

On seeing them, one cannot help longing for what has been lost, for the museum of Teddy Roosevelt and Holden Caulfield, rather than the faintly vulgar spectacle that is gradually taking its place. As the American Museum of Natural History, now celebrating its 125th anniversary, prepares a massive expansion in the next millennium, it must take care to preserve, and where necessary to recreate, that older sobriety. It should have more temporary exhibitions, as art museums do, and it should hold them to as high a standard as does the Metropolitan Museum. Above all, it must resist the creeping contagion of infantilism that has turned the Franklin Institute, once a venerable temple of science, into a pitiable, paltry waste of time.