The theory of evolution by natural selection, first formulatedin Darwin's book "On the Origin of Species" in 1859, is the processby which organisms change over time as a result of changes inheritable physical or behavioral traits. Changes that allow anorganism to better adapt to its environment will help it surviveand have more offspring.The theory is sometimes described as "survival of the fittest,"but that can be misleading. Here, "fitness" refers not to anorganism's strength or athletic ability, but rather the ability tosurvive and reproduce more offspring.Given enough time and enough accumulated changes, naturalselection can create entirely new species, known as"macroevolution." It can turn dinosaurs into birds, amphibiousmammals into whales and the ancestors of apes into humans.But Darwin wasn't the first or only scientist to develop atheory of evolution. The French biologistJean-Baptiste Lamarck came up with the idea that an organismcould pass on traits to its offspring, though he was wrong aboutsome of the details. And around the same time as Darwin, Britishbiologist Alfred Russell Wallace independently came up with thetheory of evolution by natural selection.