”If I keep making this face,” asks the little boy in Jim Carrey’s smash hit Liar Liar, which arrives Sept. 30 at a video emporium near you, ”will it get stuck that way?” The character that Carrey plays in the film is named Fletcher Reede, but it’s really the postmodernly reflexive Carrey who replies, ”Uh-uh. In fact, some people make a good living that way.”

Popular opinion has it that Carrey’s physiognomy is almost infinitely malleable, that he can distort his features to an unprecedented degree. But if you study the actor’s seemingly vulcanized visage in his 13 films, it’s clear the mugging has a definite direction: For no reason known to science, the Canadian-born comedian is recapitulating a line of primate evolution. Herewith, the six eons of Jim.

1. Tree Shrew Jim: Take a look at 1989’s Earth Girls Are Easy, which predates Carrey’s rise to superstardom by a good five years. As the alien Wiploc, he clearly hadn’t yet completely diverged from his apelike ancestors; although he’s bipedal and capable of both speaking and working with tools, his body is covered from head to foot with fur. Nor is his performance any less primitive than his appearance, consisting as it does largely of wide-eyed stares and herky-jerk movements. A long road lay ahead.

2. Lemur Jim: Eldredge and Gould’s controversial theory of punctuated equilibrium finds support in the rapid alteration of Carrey’s features in the half decade between Earth Girls and his first hit, Ace Ventura: Pet Detective. The helmetlike crest seen previously has been modified into a bizarre tidal wave of hair, which presumably protected Carrey from barbs hurled by critics hostile to his outrageous and often infantile antics. The distortion of the mouth and lips seems beyond biology — could Carrey have filmed parts in a wind tunnel?

3. Tasmanian Devil Jim: Evolution is sometimes mistaken for a goal-oriented process in which the species is somehow striving to ”improve” itself. The truth is, some paths are simply dead ends. In The Mask, released in 1994, six months after Ace Ventura, Carrey undergoes a macro mutation at the hands of animators, complete with green skin, a radically deformed cranium, and occasional elephantiasis of the eyes, tongue, and teeth. The survival rate of such misshapen creatures is abysmally low…and, sure enough, there’s still no sign of the long-planned Mask sequel. Nonetheless, his acting shows a new and almost startling subtlety that signals the shape of things to come.

4. Orangutan Jim: A year later, in Batman Forever, Carrey’s genetic journey is back on course. The distortion of the mouth (seen as well in Dumb and Dumber) is considerably less severe. His other features, while unquestionably twisted in a manner likely to frighten and delight small children, are now recognizably humanoid. However, this facial ”softening” has no demonstrable behavioral effect — fortuitously for the viewer, since only Carrey’s over-the-top histrionics as the Riddler prevent mass narcosis.

5. Gorilla Jim: As the star of last year’s The Cable Guy, Carrey is as Homo sapient as the next guy — or as Jay Leno, given the jutting jaw of an extra-determined stalker. A slight overbite and accompanying lisp are vestigial tokens of goofiness. His movements too are far less frenetic, an acclimation to the black-comic environment.

6. Attorney Jim: In Liar Liar, the transformation is complete; Carrey’s appearance is so impeccably ordinary that he looks every bit the mendacious attorney that he plays in the film. Oh, sure, every now and then he regresses. (Given the public’s comparatively indifferent reception to the intriguing but uneven Cable Guy, it’s hard to blame him.) When Fletcher’s son makes a birthday wish that his dad be unable to lie for 24 hours, and the wish comes true, Fletcher finds himself in a rather tight spot, and the stress of playing such a high-strung character takes its toll on Carrey’s anatomy.

In the film’s quieter, more contemplative scenes, however, as Fletcher recognizes his inadequacy as a father and resolves to improve, Carrey looks…well, normal. True, the result is more often treacly than touching, and Liar Liar is most entertaining when Carrey is least in control of his facial muscles. Every once in a while, though, you get a brief and promising glimpse of what might be the next phase in Carrey’s evolution: dramatic actor. Next up for him is Peter Weir’s The Truman Show; it ain’t Gary Sinise as Truman himself, maybe, but it’s another small phylogenetic step. B-

Jim Carrey’s demonstration of Darwinism: A+