'IMAGINE a world without pizza, Swiss chocolates, or French fries! Even harder, imagine Italy without the tomato or the [cowboy] without his horse," says Herman J. Viola, the father of the massive new show "Seeds of Change" at the Smithsonian's National Museum of Natural History.Mr. Viola, curator of the 400-object show, notes that before Columbus encountered the Americas none of those fixtures of modern life had been discovered. The impetus for the show was "Seeds of Change," the l986 book by Britain's Henry Hobhouse on the five plants he believed changed the development of humanity: quinine, tea, cotton, sugar, and the potato. He talked to Viola about doing a lecture on it for the museum at just the time a 1492 exhibition was in the wind. The ideas merged and produced an expanded concept for the present exhibition "Seeds of Change" that would focus on five seeds chosen from a list of nearly l00. As Viola says, this exhibitio n focuses on "an exchange of peoples, animals, plants, and diseases between Europe, Africa, and the Americas" over 500 years that began when the New and Old Worlds met. This show's seeds are: sugar, corn, the potato, disease, and the horse, selected says Viola "because of the human dimension to their story." And from the exhibition has also come another book called "Seeds of Change," edited by Viola and and Carolyn Margolis, assistant director of the museum's quincentenary programs. Piece of advice: Read the book first, if you can, before seeing the exhibition, because it expands the theme of the five seeds in a vivid way that will bring a rich background and needed facts to your tour. The book is already a Smithsonian bestseller at 140,000 copies. In comparison with the book, the show is somewhat shallow. For those who have already sampled the excellent seven-part PBS series "Columbus and the Age of Discovery" and the huge treasure troves of the 600-object "Circa 1492: Art in the Age of Exploration" at the National Gallery, there is some overlap here. The presentation seems a bit lightweight and sometimes less than lively in comparison. It may also have been handicapped by lack of space for background information, planted as it is in part of the lowest level in the museum. It requires acres to accommodate both displays and helpful information for huge shows like this. You enter on the main floor of the museum through a vast Spanish-Moorish facade made entirely of 14,000 yellow, red, black, and grey-blue ears of corn, at the top of which a globe is embedded, with "Seeds of Change" underneath. A l0-minute introductory video is beyond that, then an escalator, which takes you down to an awesome subject: the Aztec Empire, one of the largest and most advanced in America, the peak of 3,000 years of the Mesoamerican civilization, and the "People of the Fifth Sun." You learn of Aztec sacrifices, slaughtered because the Aztecs believed the universe had been created and destroyed four times by the Sun god. The People of the Fifth Sun believed he would be appeased and they would not die if the god were nourished by human blood. Ritual "flower wars" provided the captives for the sacrifice. Ironically, the next display on "Encounter and Conquest" deals with Cortez's conquering the Aztecs in as violent a way. A beautiful but gruesome 14-foot painted screen in panels from 16th-century Spain documents that. "Treasures of the New World" includes art reminiscent of that in the "Circa 1492" show, like an Inca silver lama and Disquis bird god in gold, flanked by crocodile heads, and silver processional crosses made in Spain from silver mined in conquered Mexico. You'll find this quote on the wall in the "Enduring Seeds" section: "For it is well known today the value of the world's annual yield of potatoes far exceeds the value of all the gold and silver that was savaged from [South America] during the century of conquest," as Donovan Stewart Correll wrote in "The Potato and Its Wild Relatives." You learn from this section where potatoes began: in the highland mountains of the Andes, apparently about 850 BC, long before the conquerors carried them to the Old Worl d, changing the planting and eating habits of Ireland and Europe. High in caloric yield like corn, they helped fuel the people who worked in the Industrial Revolution later. And the corn or maize first grown in Mexico about 3,500 BC, flourished in the Americas, so that when the conquerors came, they took it back to Spain. It was corn, which spread throughout Europe and Africa, that increased the caloric intake of African peoples, resulting in more and bigger crops of slaves for the conquerors to take to work in the Americas. In "The Corn and Potato Show," a short Claymation movie, those vegetables sing accompanied by the banjo. They are surrounded by displays of old- fashioned, boxed corn and potato products. In treating disease as a seed, of change, the exhibition emphasizes that diseases the explorers brought took a great toll on native Americans, who had no previous exposure to them nor resistance to this unexpected enemy. A huge electric map traces the route of their devastation. Experts say the diseases went mainly from the Old World to the New. In what might have been called "hoofbeats of change" there is a section on horses poetically called "And the Grass Remembered." Although there had been horses in the Americas l0,000 years ago, the last Ice Age made them extinct. Columbus and Cortez brought fresh horses when they arrived, the Andalusian horses and others that flourished here, sweeping the plains. The horses brought new mobility to the native Americans, enabling them to hunt and wage war in a widespread way that changed their lives. They also created a race of animals that later gave us the Morgan, the mustang, the Tennessee Walking Horse, and the American quarter horse among other breeds. The horse section gives you a glimpse of Crow women, the only Indian women who rode horseback; a McClellan saddle; and the Argentine gaucho on horseback. Stop there, take a brisk left and don't miss one of the best parts of the show, as some viewers do, thinking it's the end. Beyond it is the story of "Sugar: the bittersweet legacy." As background information suggests: "In the Americas, sugar meant slavery. Brought from the Old World on Columbus's several voyages, sugar flourished in the tropical climate of Brazil and the Caribbean." The exhibit focuses on a handsome display of life on Galways, a sugar plantation on the English-owned island of Montserrat in the West Indies. "When the native Americans died of disease and overwork, and the attempt to recruit European labor failed, African slaves became the backbone of the sugar industry," exhibit notes explain. Over a period of 350 years, l0 million Africans were enslaved in the New World. Other displays deal with the challenge of change: a merry wall sculpture called "Spaghetti Meets Tomato" by Roark Gourley; a section on "Seeds of Our Future," and finally a quote from anthropologist Margaret Mead: "Never doubt that a small group of thoughtful, committed citizens can change the world.... It is the only thing that ever has."