Book excerpt: This book of photography represents National Geographic's Photo Ark, a major cross-platform initiative and lifelong project by photographer Joel Sartore to make portraits of the world's animals -- especially those that are endangered. His message: to know these animals is to save them. Sartore intends to photograph every animal in captivity in the world. He is circling the globe, visiting zoos and wildlife rescue centers to create studio portraits of 12,000 species, with an emphasis on those facing extinction. He has photographed more than 6,000 already and now, thanks to a multi-year partnership with National Geographic, he may reach his goal. This book showcases his animal portraits: from tiny to mammoth, from the Florida grasshopper sparrow to the greater one-horned rhinoceros. Paired with the prose of veteran wildlife writer Douglas Chadwick, this book presents an argument for saving all the species of our planet.