 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.Charles Darwin's observations of these differences on the second voyage of the "Beagle" in 1835, contributed to the development of his theory of evolution.Tortoise numbers declined from over 250,000 in the 16th century to a low of around 3,000 in the 1970s.