Can you imagine a planet without wolves? Without frogs? Without pollinating insects? In this excerpt from his book “Rare: Portraits of America’s Endangered Species,” photographer Joel Sartore uses haunting, exquisite images to document the ongoing struggle to save wildlife from extinction. This exclusive sampling of Sartore’s photographs of some of North America’s most endangered species gives voice to creatures great and small that are teetering on the brink of extinction, and presents us with an urgent call to action.