Zoos have been around for centuries -- and they've changed a lot over the years. In the Middle Ages, wealthy people kept animals in their gardens. Public animal parks appeared in European cities in the late 1700s and early 1800s. The Philadelphia Zoo, the first in the United States, opened in 1874. Until a few decades   ago, most zoos were organized by creatures -- monkeys in one area, cats in another, birds somewhere else, just like museum collections. In recent years, zoos have instead begun grouping animals that would normally interact   in the wild. Moreover, instead of closing animals behind bars, designers are creating landscapes like the environments in which these creatures would naturally be found. Nearby signs provide information about the animals and their habitats in parts of the world where they normally live. The Denver Zoo's new Predator Ridge exhibit, for example, aims to teach visitors about Africa. Eight acres of land provide homes for 14 animal species, including lions, porcupines, cranes, and wild dogs. Plants from _ grow alongside African-like landform. Ten-foot-tall mounds   give lions a place from which to survey their surroundings, just as they would do in the wild. Landscape design makes visitors to the Denver Zoo's Predator Ridge exhibit feel like they're really in Africa. Animals in Predator Ridge can't actually be mixed with one another, for safety reasons. But hidden deep channels and other smart features allow visitors to see all the animals at once. Different species can see each other too. What is the passage mainly about?
Protection of zoo animals. Changes of zoos over the time. The safety problem of zoos. The living habits of zoo animals.