The expected explorers -- Meriwether Lewis, Zebulon Pike, John Charles Fremont -- march through the pages of "Discovering America 1700-1875," but along with them go men who are generally snubbed by American historians: John Lawson, John and William Bartram, Alexander Wilson, John James Audobon, Thomas Nuttall and other members of that rare and wonderful company of American naturalists whose devotion to botany and zoology took them everywhere in the new land, often before anybody else got there. Henry Savage Jr. has a long-standing bias toward naturalists, and he threads their journeys all through his new book, the latest in the New American Nation series.

John Bartram, the botanist, walked or rode over as much of the colonies as any man of his time, digging up plants for his English patrons. His son William rambled through the south from the Altamaha River to the Mississippi, noting the ways of nature and the natives for his classic "Travels." Wilson went by foot, horse and rowboat from Maine to Louisiana, not just to see birds but to sell subscriptions to his pioneering "American Ornithology." Audobon carried his sketch books from Labrador to Texas, from the Florida Keys to the Upper Missouri. Nuttall fought fever, starvation and river pirates as he went from one side of the continent to the other, packing in flowers, birds, bugs and rocks. And George Catlin, the artist, whom Savage sensibly includes, collected Indians as a botanist collected plants while at the same time acquiring honorary names -- Ec-cha-zoo-kah-wa-kon and To-ho-pe-nee Wash-ee -- the way a modern achiever acquires honorary degrees.

The explorers all set down the images of a pristine land with the surprised freshness of things seen or reported for the first time. Captain Batts marveled at "the inifite quantity of Turkies, Deer, Elks and Buffaloes so gentle and undisturbed, that they had no fear at the appearance of men." Lawson told of the Congaree Indians, who kept cranes as domestic fowl, an image worthy of the Swiss Family Robinson. Nothing evokes the innocence of a new world more than Antoine du Prate's glimpse of wild turkeys sitting helpless in the trees as hunters shoot at them and "only look at the bird that drops, and set up a timorous cry, as he falls." Or the fierce Sioux Indians, who cried when Meriwether Lewis flogged a member of his party, for "they never whipped even their Children."

Discovery came in many guises. Clarence King saw Yosemite Falls as "a silver scraf, light, lace-like, graceful, luminous, swayed by wind." Fremont, pushing through mountain snows, saw, far below, a green prairie and lovely lake and climbed down to find saline, lifeless water and sere stretches of sagebrush. The discoverers confronted danger ingeniously. Thomas Say and two companions took winter shelter in a hollow sycamore trunk so big it accommodated all three and a fire to keep them from freezing. John Wesley Power, conqueror of the Colorado, clinging to a silver of rock on a canyon wall, was pulled up by a companion who took off his long underdrawers and lowered them to his leader.

Of course, explorers had other things besides nature or scenery on their minds, notably finding quick and practical ways to get to the Pacific. Savage reminds us of their miscalculations. Verrazano, sailing along the eastern coast, saw the Pacific a few miles to the west. John Lederer sighted a great sea just beyond the Appalachians. Fremont's father-in-law, Senator Thomas Benton, got ready to dig a convenient canal from St. Louis to the Columbia. The process of exploration was one of disillusion as well as discovery. The continent grew wider, the mountains higher, the western ocean more distant.

Savage, who has to cover almost two centuries and the entire U.S., goes at it thoroughly, efficiently and a little hurriedly. Occasionally he gives space to irrelevant or bread-and-butter detail when he could more happily let his subjects tell more of what they saw or heard or felt. Sometimes, in fact, he seems to begrudge their interruptions of his own serviceable prose and procession of dutiful facts. Not that Savage doesn't appreciate the way they wrote -- he has given them full expression in "Lost Heritage," his admirable book on American naturalists. And in "Discovering America" he does give credit and honor to the men who, out looking for plants and birds, explored America more patiently and perceptively than those who went simply looking for new places.