The theory of natural selection is based on the following factors: (i) Rapid multiplication and limited food and space which leads to struggle for existence. (ii) Struggle for existence and variations which leads to natural selection or survival of the fittest. (iii) Natural selection and inheritance of useful variation over many generation which leads to formation of new species. Darwin in his "Natural Selection Theory" did not believe in the role of discontinuous variation in natural selection. Darwin always believed in the universal occurrence of variation. In his opinion, variation is continuous in nature. Darwin did not understand the cause of variation and assumed it was one of the innate properties of living things. Now it is known that variation is due to mutation and thus it may be discontinuous.