Charles Darwin's theory of evolution by means of natural selection lacked valid hypotheses on the nature and mechanism of inheritance as well as on the origin and preservation of biological variation. Gregor Mendel complemented Darwin's theory by presenting in his corpuscular theory of inheritance solutions to both problems. In addition to the discovery of the basic rules of the mechanism of heredity, he showed that the hereditary material is constituted of independent and discrete elements which recombine during the gamete formation and fertilization in sexual reproduction of organisms thus creating a huge amount of persistent genetic variation.