Darwin returned home in 1836 and developed a reputation as one of England's leading biologists. Darwin concluded that species evolved over long periods due to what he called natural selection. There may be differences in the offspring of an organism. These differences are called mutations. We generally think of mutations as harmful, such as birth defects, but a mutation is simply an alteration of the genetic information of an organism.
Some mutations are good.
A child might be taller or stronger than his or her parents. Darwin's theory suggested that organisms evolved to fit their environment based on natural selection. If a mutation was good, the organism lived and contributed the mutation to the gene pool of its offspring. If the mutation was bad, the organism was more likely to die before contributing to the gene pool. These mutations slowly produced the world we know today over billions of years.