Since April share prices for some of America’s best-known retailers, including Macy’s and Gap, have plunged by more than 25%. Some of their problems are fleeting, but one is not. Rather than driving to a big box, many Americans are shopping online instead. American e-commerce accounted for 10.4% of retail sales last year, up from 9.3% in 2014, according to Morgan Stanley, a bank. Amazon is the force behind this, with sales in North America rising by nearly 30% in 2015. The choice for bricks-and-mortar retailers is clear: evolve or decline.