MODEST at first, but now a rising wave splashing on every continent, "ecotourism" is loosely defined as travel by millions of people who want to see and save the natural habitats remaining on earth.Charles Darwin may have started it. But in 1835 it wasn't called "ecotourism" when he stopped at the Galapagos Islands 600 miles off the coast of Ecuador. The archipelago was then, and still is, one of the rare places on earth where a remarkably pristine environment flourishes. But last year, 45,000 ecotourists visited the islands. Environmentalists say, "Enough!" The Ecuadorian government says, "Maybe." And tour operators say, "Let's talk." Today the Monitor begins a four-part series by staff writer David Holmstrom and staff photographer Melanie Stetson Freeman. The series will explore ecotourism and why it is challenging the future of these fabled islands. SEE PAGE 10.