NATURE’S OWN MORPHING Nature draws no sharp boundaries between races, which blend gradually one into the next around the Earth. But the human urge to classify causes people to socially define themselves in racial categories, which become catchall labels for physical features, social identity, and nationality.

© John-Francis Bourke/zefa/Corbis; © Owen Franken/Corbis; © Richard T. Nowitz/Corbis; © Haruyoshi Yamaguchi/Corbis; © David Turnley/Corbis; © Dave Bartruff/Corbis; © Rob Howard/Corbis; © Barbara Bannister/Gallo Images/Corbis.