At the beginning of this century the mutation theory of evolution, based by HUGO DE VRIES on many years of culture experiments mainly with Oenothera Lamarckiana, took the world by storm when published in 1901. As early as 1889 DE VRIES had shown that various plant "monstrosities" are inherited. His breeding experi­ ments with other plants involved the rediscovery of MENDEL'S laws and the resuscitation of MENDEL'S breeding methods, which had been in abeyance in the long period from 1865 to 1900. MENDEL began his crossing experiments in 1854, DE VRIES began his Oeno­ thera cultures in 1886. The genus Oenothera was thus the first after Pisum to be the subject of extensive genetic experiments. Thus was introduced the method now so widely pursued. It may be remembered also that DE VRIES drew the distinction between fluctuations and mutations, but he regarded all fluctuations or continuous variations as non-heritable. Both can be equated with the continuous and discontinuous variations of BATESON. DE VRIES' conception of mutations corresponds with the "single variations" of DARWIN, except that he regarded each mutation as producing a new elementary species, now generally called microspe­ cies. He said in the introduction to his Mutationstheorie, that the characters of organisms are built up from units which are as sharply distinguished from each other as the molecules of chemistry. These units can be united into groups, and in related species of plants and animals the same units and groups are repeated.