The Galapagos tortoise or Galapagos giant tortoise (Chelonoidis nigra) is the largest living species of tortoise and 10th-heaviest living reptile, reaching weights of over 400 kg (880 lb) and lengths of over 1.8 meters (5.9 ft). With life spans in the wild of over 100 years, it is one of the longest-lived vertebrates. A captive individual lived at least 170 years. The tortoise is native to seven of the Galapagos Islands, a volcanic archipelago about 1,000 km (620 mi) west of the Ecuadorian mainland. Spanish explorers who discovered the islands in the 16th century named them after the Spanish galapago, meaning tortoise. Shell size and shape vary between populations. On islands with humid highlands, the tortoises are larger, with domed shells and short necks. On islands with dry lowlands, the tortoises are smaller, with "saddleback" shells and long necks. These differences from one island to the next helped Charles Darwin to develop his theory of evolution.