The refugee tide so sorely testing the continent's institutions and conscience is unquestionably a harrowing emergency, with flows of desperate people risking their lives to reach Europe on a scale unseen since World War II uprooted tens of millions. Over time, the perpetual motion has blended people, their ideas and technologies and helped to build Europe into the resilient, deeply textured, economically and culturally rich continent now so appealing to those seeking new lives. Unlike Romans, Huns, Vikings, Moors, Normans, Nazis and other aggressors whose invasions left indelible marks, today's newcomers aren't coming to make war. [...] Europe's travelers have over centuries covered the broadest possible range, from intrepid medieval merchants like Marco Polo to today's twentysomethings who grew up as the continent legislated away internal borders. In the heart of Europe, Viennese cooked on open fires before Italian Renaissance architects introduced them to chimneys, spawning a whole new industry — chimney-sweeping — which Italians and Italian-speaking Swiss built up, migrating with their families to what became the capital of the Austro-Hungarian Empire, said Annemarie Steidl, a University of Vienna expert on migration. Like waves of settlers before them, they will have to contend with fears that they'll take away jobs, housing and resources from people already established. "Each time there was an intense resistance and a notion that 'You don't belong here, you're invading us,'" says Columbia University sociologist Saskia Sassen, author of "Guests and Aliens," which analyzes the history of migration and refugees. Race riots in the 1950s in England, mobs chasing African immigrants through streets of southern Spain in 2000 and immigrants' children torching cars in a three-week orgy of rioting in France in 2005 showed that the process can be spectacularly fraught and violent. Having moved from Punjab state in northern India to England in 1975, his grandfather started the fabric store, piled from floor to ceiling with hundreds of rolls of cloth, where Sunny works with his mother, Malkit. France's Barcelona-born prime minister, Manuel Valls, and Paris Mayor Anne Hidalgo, who spoke not a word of French when she moved with her parents from Spain at age 2, also stand out as examples of how quick integration can be, and of the energy migrants can bring with their desire to succeed. Yet the footsteps left by migration show that Europeans' lives and their continent would be very different, certainly poorer, had people stayed rooted to the spot. Had their ancestors not left Ireland, the Beatles might not have met in Liverpool, one of the great European ports that for centuries have shipped people, ideas and other treasures in and out.