Darwin’s theory of evolution is also known as the Theory of Natural Selection or Darwinism. Darwin explained that despite having enormous potential of fertility, the population size of any kind of organism remains within a limit. It is due to the struggle between members of the same species and different species for food, space and mate. This struggle eliminates the unfit individuals. In other words, the fit organisms possess some variations, which are favorable and can leave the progeny to continue the variations. This is called Natural Selection.