IN a handsome neo-Classical building near the tourist hubbub of Piccadilly Circus in London, Gina Douglas, a librarian with reading glasses and a knot of keys on lanyards around her neck, leads the way down a grand marble staircase, past oil portraits of great men. (“There's old Chop-Chop,” she says affectionately of one.) In the basement of the Linnean Society, the locks on a bank vault slide back and the door swings open.

“You're going back in time,” Ms. Douglas says as she steps across the threshold. “You're now in the library of an 18th-century scientist.”

In fact, the leather-bound volumes and everything else in this intimate little vault belonged to the Swedish botanist Carolus Linnaeus, one of the most influential figures in science, and also one of the most colorful. He invented the system of classification by which every living species now gets its two-part scientific name, by genus and species — for instance, Homo sapiens, a name Linnaeus coined. In a sense, he invented the modern science of living things, largely with materials gathered in this room.

Walking around the room, Ms. Douglas pulls open mahogany specimen drawers containing pinned-out dragonflies that last took wing 250 years ago, shells in small, handmade tin boxes, and a mullet pressed flat as a leaf on a sheet. In one drawer, the specimens are cradled like candies in heavy wrapping paper. Some early naturalist shipped his trophies to Linnaeus “wrapped in little screws of paper,” she says, “and they're still in little screws of paper.”

She takes down a 30-inch-high double-folio volume, the author's own copy of Systema Naturae from 1735, the first published statement of the Linnaean system, and lays it open to a page of species organized into neat boxes.

“And this is where it all begins,” she says.

FOR modern travelers, it is almost impossible to comprehend how vast and confusing the natural world must have seemed at the beginning of the 18th century. Australia and Antarctica were still largely blank spots on the map. But explorers in Africa, Asia and the Americas were already reporting, and sometimes sending home, a bewildering assortment of strange new species, from the iguana and the opossum to the chambered nautilus.

How did these creatures live? Where did they fit in the scheme of Creation? How did that change ideas about our own species?

Linnaeus, whose tricentennial is being celebrated this year, provided some of the most far-reaching answers to such questions and helped spark the passion for natural history that made the 18th century a great age of discovery. He began one of the greatest intellectual quests in human history: to understand life on earth in all its permutations. After 300 years, it seemed like a good time for me, as a writer about the natural world, to make a sort of pilgrimage to see where this journey got its start.

Most of the places and artifacts associated with Linnaeus are in Uppsala, Sweden, a university town outside Stockholm. (The main exception is his library, purchased by British naturalists from his widow and now under the care of Ms. Douglas in London.)

Then, as now, Uppsala was a college town, with lots of pink-, and cream-, and ochre-colored buildings arranged around a pretty little river, the Fyrisan. The garden where Linnaeus worked as a botanist and a professor at Uppsala University occupies much of a city block in the middle of town, and it has been largely restored to how it was in his lifetime: perennials occupy meticulously organized beds on one side, annuals on the other, all enclosed in hedges of multiple species planted to see which ones might serve best in the Swedish countryside.

I stayed last summer at the First Hotel Linne, which forms one border of the garden, and I found I could step out the back door before breakfast, slip through a gap in the hedge, and savor a cup of coffee alone on a bench amid the plants Linnaeus knew, listening to the same cathedral bells that rang for him, with the same bird species trilling and squabbling in the trees. Linnaeus is a national hero in Sweden. His face appears on the 100 krona note. But the connection to him still seems remarkably informal, even intimate.

“Come and stand here,” said Eva Bjorn, in the lecture room on the second floor of the house across the garden where Linnaeus lived with his wife, five children, several monkeys, parrots and a pet raccoon. The house and garden have both been restored to what they were in his lifetime.

“Do you feel the way the floor is worn away under your feet?” said Ms. Bjorn, who runs the Linnaeus Museum there.

Linnaeus was, by all accounts, a charismatic teacher, and the words he spoke in that room inspired 19 of his students to undertake voyages of exploration to the remotest corners of the earth. From there, his ideas also rapidly spread throughout the scientific world.

The Linnaeus Museum is open to the public from May through September, which is naturally the best time to visit the Linnaeus Garden, particularly in late July and early August, when the annuals bloom. The staff maintains the orderly beds just as Linnaeus designed them, with one sentimental exception.

Linnaeus coveted a flower from China that he heard had a long chain of heart-shaped red flowers. One of his correspondents sent him seeds from Siberia, but the flowers that finally emerged were yellow, and Linnaeus was supposedly so disappointed he flung the pot out the window. The plant he wanted did not actually arrive in Europe till long after Linnaeus's death, but it grows in Uppsala now.

“This is the one we shouldn't have,” said a staff member, indicating the red-flowered Fumaria spectabilis, or bleeding heart.

From Uppsala, Linnaeus led regular excursions into the countryside, with up to 300 students at a time. They wore loose-fitting clothes for collecting, armed themselves with butterfly nets and carried their trophies home pinned to their hats. Kettle drums and hunting horns would announce their jubilant return, along with cries of “Long Live Linnaeus.”

Linnaeus mapped out eight walking routes for these trips, and it's still possible to walk the same routes and see the landscapes he knew. Sweden protects its open spaces, so the countryside around Uppsala is a mix of fields and forests, with the view back to Uppsala dominated by the pink, turreted castle and the brick spires of the cathedral (where Linnaeus is buried).

For the 300th anniversary of Linnaeus's birth, the city has restored these walking routes and trained guides to lead tours. (Bring your own drums and horns.)

One route leads to Hammarby, his modest country house, painted the traditional Swedish boat-bottom red. The walls of his study are still papered with 18th-century botanical engravings and hung with portraits of the family, including the pet raccoon.

A story locals like to tell is that Linnaeus walked with his dog Pompe every Sunday to attend the nearby church at Danmark. When Linnaeus was too ill to attend one Sunday, Pompe went alone and took his usual place in the pew.

Linnaeus sometimes lectured to larger audiences from the well of the anatomical theater at the top of what is now the Museum Gustavianum, on the Uppsala University campus. The light raked down from the cupola, and a listener later recalled, according to a sign there, “To see a rose from the Cape, an Amaryllis from Asia, monkeys and snakes from Africa, and parrots from America amounted to miracles, that no one but Linnaeus could demonstrate.”

Linnaeus certainly had the ego to attempt miracles. God had guided him, he once wrote, to “see more of his created works than any mortal man before him. ... Nobody has been a greater botanist or zoologist.”

He considered himself the new Adam, born to bring order to the chaos of creation. He was equally obsessed not just with order, but with sex, and these two forces shaped his grand scheme to classify all living plants according to their reproductive parts. While this approach was simplistic (and other botanists soon resorted to using a broader range of traits), it had the advantages of simplicity. Any lay person could look into a flower, count the stamens and pistils, and have a key for placing a plant in a general scheme of the natural world.

This system of classification spread to other countries with astonishing speed, especially given the slow and highly unpredictable means of communication then. As early as 1737, botanists were already eagerly applying the new system on the American frontier in New York.

Linnaeus shrewdly helped popularize his system by spicing up his lectures with asides about the flower as a bridal bed, where a member of one sex could often be found disporting with multiple members of the opposite sex.

Despite his ego, Linnaeus also managed to charm. The Gustavianum includes a handsome Rococo-style collecting case that belonged to Queen Lovisa. Linnaeus once spent a summer helping her organize her collections at the Drottningholm Palace outside Stockholm, and she wrote about the experience to her mother:

“He is a most entertaining person, who has the esprit of the high society without carrying its manners, and who for these two reasons amuses me beyond words. ... There is not one day that passes without him putting everyone in a good mood.”

Portraits of Linnaeus often convey a quality of puckishness around the eyes and the corners of the mouth, a hint of delight and the urge to say mischievous things. But it was certainly the ego and single-mindedness that made Linnaeus such a powerful force in the history of science.

“When Linnaeus started, natural history was a mess, and people needed guidelines,” said Thierry Hoquet, an associate professor in the philosophy of science at the University of Paris X-Nanterre. “Do you know in Greek myth the story of how Ariadne fell in love with Theseus, and gave him a ball of string to help him find his way out of the Minotaur's Labyrinth? Linnaeus gave us the thread.”

The Linnaean system was simple. It was also certainly flawed. But people grasped desperately for this thread because the natural world was so confusing without it.

Linnaeus went on relentlessly refining his idea. He also continued to pursue converts among those who resisted him. In 10 years, he advised one skeptic, “you will be defending the very ideas that make you vomit now.” And he was right.

A result is that all modern scientists are de facto Linnaeans. All names in botany today go back no further than his “Species Plantarum,” published in 1753, and all names in zoology begin with the 10th edition of Systema Naturae, published in 1758.

VISITOR INFORMATION

In Philadelphia, the American Swedish Historical Museum (1900 Pattison Avenue; 215-389-1776; www.americanswedish.org) is marking the Linneaus tricentennial with an exhibition, “Come Into a New World: Linnaeus and America,” through July 1. General admission is $6; closed Mondays.

In Britain, lectures, exhibitions, garden tours and other events marking the tricentennial are listed by the Linnean Society at www.linnean.org. Its library and collections at Burlington House, Picadilly, London, are open to researchers and the public by appointment.

A list of tricentennial events worldwide can be found at www.linnaeus2007.se. In Sweden, Uppsala will celebrate with a Linne Gala Event and Festival of Love, May 20 to 27.

The First Hotel Linne in Uppsala (Skolgatan 45; 46-18-102-000; www.firsthotels.com) borders the garden where Linnaeus lived and worked. It has 116 rooms, with doubles from 1,029 kronor, or $151 at 6.8 kronorto the dollar.

Linnaeus garden, at Svartbcksgatan 27 (46-18-471-2838; www.linnaeus.uu.se), is open 11 a.m. to 8 p.m. The house, the greenhouse and the Linnaeus Museum (46-18-136-540; www.linnaeus.se) are open daily 11 a.m. to 5 p.m. from May through September (except for June 22 and 23). Admission is 50 kronor.

Linnaeus's country house and garden, Hammarby, nine miles southeast of Uppsala, is open daily from May to September (46-18-471-2838; www.hammarby.uu.se); 50 kronor.

The Museum Gustavianum at Uppsala University (46-18-471-7571; www.gustavianum.uu.se) displays Linnaean artifacts and an anatomical theater where Linnaeus lectured. Open Tuesday through Sunday, 11 a.m. to 4 p.m.; 40 kronor. Tours are offered on Saturday and Sunday at 1 p.m.

Uppsala University also maintains Linn online, about Linnaeus and his research, at www.linnaeus.uu.se/online.