Latch on early to a growing market. Relentlessly drive down costs. Research and innovate. Take large but calculated risks. Wait for your competition to weaken, then gobble them up. Over 25 years, that formula took Express Scripts from a five-person startup in 1986 to a company with $45 billion in revenue and 14,000 employees. Three times, in 1998, 1999 and 2009, the pharmacy benefits company bought competitors twice or nearly twice its size in bet-the-company gambles that paid off. On Thursday, it made its biggest bet yet: the $29.1 billion purchase of Medco, its biggest rival. If antitrust regulators approve, and executives can successfully knit the two firms together, it will become the banner corporate success story of the new century in St. Louis. The bold acquisition further provides a lesson for St. Louis in corporate Darwinism, analysts say.