CARL SAGAN'S MISSION--to popularize contemporary science--is beyond reproach. Yet, like a hard-sell broker, his execution is chronically faulty. In this collection of 25 essays he spreads before the reader a whole gamut of the most tasty scientific ideas, concepts, and predictions, ranging from the prospect of extraterrestial life to the role of gestation in shaping world religion. But he skips frenetically from one to the next, milking only the most sensational aspects and indulging freely in irresponsible speculation. Our aggravation is heightened by his persistent editorial voice in the background, applauding the genius of this, the limitless potential of that.

Sagan has a history of this sort of dabbling. First swept into the public eye on the coat-tails of the Viking missions to Mars as one of the expedition's moving minds, his impudence, optimism, and imagination, won him national attention. Johnny Carson has toasted him. The New Yorker has profiled him, countless universities have ensnared him as their graduation speaker. He made movies with Francis Ford Coppola, chaired the National Book Awards, and rubbed elbows with celebrities of every ilk. He is, if you will, the shooting star of the astronomical profession.

But all the while there have been grumblings. His colleagues, principally, confront him with their contention that publicizing and practicing science are two irreconcilable aims. So Broca's Brain is an entirely representative work--on the one hand we're treated to Sagan's youthful enthusiasm, imagination, and charm; on the other we must contend with the superficiality and disjointedness which, many claim, have marred his entire career.

First, the charms. The flamboyance and imagination which raise Sagan to something of a '70s cult figure rescue a lot a Broca's Brain. Sagan recounts, for example, a colorful and enthusiastic history of his profession, emphasizing the incredibly rapid blossoming of American astronomy. In an equally lively essay, he describes the ludicrous procedure scientists use to name newly-discovered craters:

There have been objections that political philosophers are too controversial. I myself would be delighted to see two enormous, adjacent craters, called Adam Smith and Karl Marx.

Also entertaining are two short portraits, one of Albert Einstein, the other of Robert Goddard. Broca's Brain was published to commemorate the 100th anniversary of Einstein's birth, and the chapter Sagan devotes to him is reflective of the event, brimming with amusing anecdotes and quotes. The portrait of Goddard glows with Sagan's adulation of the great eccentric and pioneer. If there's any problem with these two portraits, in fact, it's that they're almost oo good--you wish you were reading a book by one of the two, instead of just a chapter about him.

Generally, you can't but leave the book with a rudimentary understanding of some of the contemporary beliefs and trends in astronomy. And Sagan conscientiously avoids littering the narrative with autobiography--though that will doubtless come in a couple of years. Then there's his imagination: Broca's Brain reaches it greatest heights when he unleashes it fully, describing, for instance, the prospect of a solar sailing regatta.

All these recommendations, though, can't compete with Sagan's and his editor's mistakes. The main problem lies in Broca's Brain's construction. Sagan strings loosely together 25 of his essays--published in everything from Physics Today to Holiday magazine--only one of which is more than 15 pages long. The book is hence painfully disjointed; he leaps from topic to topic at random. Redundancy creeps in--a theme introduced in one essay is often uselessly repeated in a second, and not infrequently beaten into the ground in a third. Most seriously, though, these essays are just too short to develop their subject. No sooner do you get caught up in one than Sagan discards it in favor of something fresh.

What might have happened here--what Sagan had undoubtedly hoped for--does not. No tone reflecting the enthusisam of one passionately interested in scientific inquiry permeates these pages. On the rare occasion when such spirit does break forth, the writing is enchanting:

We have caught our first glimpses of the ammonia clouds and great storm systems of Jupiter; the cold, salt-covered surface of the moon; and desolate crater-pocked, ancient and broiling Mercurian wasteland; and the wild and eerie landscape of our nearest planetary neighbor, Venus.

There should be more of this: instead we sense only a continual jolting as Sagan bumps us from essay to essay.

Insofar as a unifying thread in this collection of essays exists, it is Sagan's series of ill-disguised emotional crusades. His first mission, of course, is to dangle accessible science provacatively before the public. A second is his virulent attack on the theory-mongers of science, the "paradoxers"--those irresponsible practitioners who propose theories without ample evidence, make lots of noise in support of them, and then fall niftily by the wayside when someone with the facts comes along. Sagan debunks them in several delightful essays, taking to task, among others, the proponents of mathematically gifted horses and human levitation.

In line with this, Sagan's watchword is a citation from Bertrand Russell: "William James used to preach the will to doubt." This is, of course, a sound scientific viewpoint. What's awry in Broca's Brain is that Segan doesn't practice this, save for one chapter. His essay on Emmanuel Velikovsky takes a once popular but porous theory explaining a series of converging mythological catastrophes and subjects it to an exacting analysis. This piece, three times as long as any other, is the most interesting, the most developed, and certainly the most scientifically responsible in the book.

Aside from this, however, Sagan retreats into the misty land of speculation--on the future, on the possibility of extraterrestrial life, on the possibility of intergalactic communication. For example, he draws a hyperbolic and fatuous parallel between the Big Bang theory of the birth of the universe, and the human birth experience. He proposes a seemingly infinite number of theories in these chapters and substantiates each less well than its predecessor, abandoning totally the close scrutiny he has just advocated so strongly.

The possibility of extraterrestrial life is the cause to which Sagan has dedicated much of his life. His earlier book The Cosmic Connection, treats this exclusively. When scientists examining the samples brought back to earth by Apollo found no signs of life, Sagan proclaimed to their collective infuriation that the moon was "dull." This polemic grates in the course of Broca's Brain. It pops up in almost every chapter, tied tortuously to whichever theme is central at the time. Sagan ought to have called his first book "Why I Think There's Life on Other Planets" and been done with it. Instead, he has embellished his thesis a bit, disguised the central theme, and called it Broca's Brain.

Since Sagan clearly wrote the book for a general public, he should have trodden gingerly when he encountered political and religious issues. His consistent bumbling in these spheres is the unintentional leit-motif of Broca's Brain. When in doubt. Sagan shies away from the secular implications of his lofty ideas. In the course of declaring, for example, that we will one day have robots for garbagemen (at current prices, the human version are "expendable"). Sagan mentions hastily that "the effective re-employment of those human beings must, of course be arranged; but...that should not be too difficult." Such is his political sagacity.

On the religious front, he fares little better. Debates on the existence of God, he writes cavalierly, are "good fun." And at the end of the book he discusses the need to apply his rigid scientific criteria to all schools of belief, reviving the Victorian controversy. If inaccuracies are found, he says, the culprit must at once be discarded--thus the Bible must go.

The future of science in general, and of astronomy in particular, has not tickled the public fancy quite the way Sagan and others thought it would. Broca's Brain constitutes an effort to revive interest. But in blundering as he does. Sagan suffocates his own cause. "Science," he writes, "is not a body of knowledge, but a way of looking at the world." Ironically, while Sagan touches dramatically on this body of knowledge, he never approaches, save in the lonely chapter on Velikovsky, that elusive point of view