Right now, everybody’s talking about the Summer of Love, a phrase that first appeared in a 1967 press release. The San Francisco Travel Association is using the slogan to help fill hotel rooms, online merchants are using it to sell everything from refrigerator magnets to T-shirts, and the de Young Museum has embraced these three words in the hopes of producing an art-world blockbuster. The phrase also rears its hairy head in a new exhibition at the California Historical Society. Titled “On the Road to the Summer of Love” and curated by Dennis McNally and Alisa Leslie, the exhibition (on view through September 10) answers a couple of essential questions about that now mythical time: Where did this revered cultural watershed come from, and when did it end? As the author of “Desolate Angel,” his 1979 biography of Jack Kerouac and entrée to Jerry Garcia and the Grateful Dead, Dennis McNally has a unique perspective on the Summer of Love’s roots, even if he was not in San Francisco at the time to experience it firsthand. For McNally, the seeds of the Summer of Love that flowered in San Francisco’s Haight-Ashbury neighborhood in 1967 were actually planted in the coffee houses and book stores of North Beach, circa 1955. To be clear, it’s not like the links between the Beats and the Hippies are unknown or unexplored—the “Cowboy Neal” at the wheel of the bus to never, ever land in “That’s It For The Other One” is, of course, Neal Cassady, who was the model for the Dean Moriarty character in Jack Kerouac’s 1957 novel, “On the Road.” But McNally and Leslie do a terrific job of connecting a great many dots, mostly via black-and-white photographs, itself a counterintuitive way to present an era better known for psychedelic hues. Thus, we see a Harry Redl photo of poet Michael McClure sitting with his contemporaries in the basement of Lawrence Ferlinghetti’s City Lights bookstore, as well as Jerry Burchard photos of artists Jay Defeo, Wally Hendrick, and Bruce Connor. The point is not that McClure directly influenced, say, the lyrics to “Cream Puff War,” or that one can draw a straight line from Jay Defeo’s “The Rose” to Alton Kelley’s earliest efforts for the Family Dog. Rather, the Beats presaged a new consciousness, as McNally puts it, that was amplified—literally and figuratively—in the mid-1960s. But the curators go further than the Beats, finding numerous wellsprings for the spirit of the 1960s. First and foremost, there were the protests, whether they were against a visit to San Francisco City Hall by the House Un-American Activities Committee in 1960 or the racist hiring practices of Van Ness Avenue car dealers in 1964. And, of course, there was the founding of the Free Speech Movement in Berkeley in 1964. Again, the links between these events and the so-called “flower power” of 1967 are not mathematical or formulaic. It’s more about the contagious spirit of freedom and the breaking down of archaic barriers that were in the air. McNally and Leslie capture that. The exhibition also includes a section on the critical impact of new theater and performance forms on the Hippie scene, and not just the fact that Bill Graham was once the manager of the San Francisco Mime Troupe. McNally and Leslie devote a fair amount of wall space to the Tape Music Center, a political-comedy group called The Committee, and The Actor’s Workshop. A crucible of a completely different sort was the Red Dog Saloon in Virginia City, Nevada, where the Charlatans performed for six acid-soaked weeks in the summer of 1965. By the time viewers find their way to the 1966 and ’67 sections of the exhibition, the show will start to feel more familiar, more “of the era.” There are images of acid tests, a copy of Mari Tepper’s “Hallelujah, the Pill” poster, and photograph after photograph of head shops in the Haight, poolside parties at Olompali, the Diggers handing out free food in the Panhandle, and numerous bands playing up on Mt. Tam and down in Monterey. Throughout the exhibition, we also meet the pilgrims, the Cosmic Charlies and Charlenes, if you will, who flocked to the Haight thanks to saccharin nursery rhymes such as “San Francisco (Be Sure to Wear Flowers in Your Hair),” written by John Phillips of The Mamas & the Papas. In October of 1967, though, the scene was officially declared dead. The first catalytic event occurred on October 2, 1967, when the police made six pot busts in the Haight, including one at the Grateful Dead’s house at 710 Ashbury. A few days later, on October 6, a coffin was carried down Haight Street by members of the Diggers to mark the Death of Hippie. Today, most people forget this important nail in the coffin of the Summer of Love, which means they miss the opportunity to learn what the whole enterprise might have really meant, as well as what it wasn’t—the Summer of Love wasn’t exactly the cultural utopia that marketing interests, then and now, would wish it to be. By giving the Summer of Love a proper beginning, middle, and end, “On the Road to the Summer of Love” does not make those same mistakes.