The facts to which I shall refer concerning _Oenothera_ are for the most part quoted on the authority of Dr. Ruggles Gates, and taken from his book _The Mutation Factor in Evolution_ (London, 1915). The occurrence of mutations in _Oenothera_ was first noticed by De Vries, the Dutch botanist, in the neighbourhood of Amsterdam in 1886. He found a large number of specimens of _Oenothera Lamarckiana_ growing in an abandoned potato-field at Hilversum, and these plants showed an unusual amount of variation. He transplanted nine young plants to the Botanic Garden of Amsterdam, and cultivated them and their descendants for seven generations in one experiment. Similar experiments have been made by himself and others. The large majority of the plants produced from the _Oe. Lamarckiana_ by self-fertilisation were of the same form with the same characters, but a certain percentage presented 'mutations'--that is, characters different from the parent form, and in some cases identical with those of plants occurring occasionally among those growing wild in the field where the observations began. Nine of these mutants have been recognised and defined, and distinguished by different names. The characters are precisely described and in many cases figured by Gates in the volume cited above. The first mutant to be recognised--in 1887--was one called _lata._ It must be explained that the young plant of _Oenothera_ has practically no stem, but a number of leaves radiating in all directions from the growing point which is near the surface of the soil. The plant is normally biennial, and in the first season the internodes are not developed. This first stage is called the 'rosette.' From the reduced stem are afterwards developed one or more long stems with elongated internodes, bearing leaves and flowers. In the mutation _lata_ the rosette leaves are shorter and more crinkled than those of _Lamarckiana,_ and the tips of the leaves are very broad and rounded. The stems of the mature plant are short and usually more or less decumbent with irregular branches. The flower-buds are peculiarly stout and barrel-shaped, with a protrusion on one side. The seed-capsules are short and thick, containing relatively few seeds, and the pollen is wholly or almost wholly sterile.