The DNA code, which forms our genes, was Darwin's missing key to understanding how things evolved. DNA does not stay the same, it can be changed by mutations. Mutations are necessary to generate variations. Without mutations, things would remain the same generation after generation. Pieces of DNA called switches can turn certain genes on or off. Genetic switches help create mutations that are sometimes responsible for breeding an entirely new species from another. This is how a snake can evolve from a four-legged animal, and how a whale's front flippers contain bones that look like