There are creatures living in almost every part of the world--from the hot, rainy tropics to the icy polar caps, and from the dry desert to the depths of the ocean. No matter where they live on earth, all living things seem to be perfectly suited to their environment. Scientists say that these adaptations did not happen overnight. They are the result of a long process of gradual changes over many generations. Well-known science writers Alvin and Virginia Silverstein and Laura Silverstein Nunn explain Darwin's theory of evolution, unique adaptations of humans and other animals, and what the latest research on adaptation reveals.