As you report, Tim Coulson of Imperial College in London and his colleagues have been puzzled by the shrinking size of wild Soay sheep that live on the remote island of Hirta beyond the Outer Hebrides in Scotland (“Survival of the Less Fit,” July 4, 2009). This primitive breed has not changed much since the Bronze Age. The shrinking was first noted some twenty years ago. The researchers concluded that this is the result of shorter and warmer winters caused by climate change. First, the sheep do not need to have such large fat reserves to live off through milder winters. Second, more sheep survive the winter, and thus the lambs face more competition with larger animals for food, which is why they grow less fast. This shows that environmental change can change animal populations surprisingly fast. Would that climate change had such a swift impact on the human population!