Gates, R. R. An Analytical Key to some of the Segregates of Oenothera. _Twentieth Annual Report of the Missouri Botanical Garden_, 1909. Studies on the Variability and Heritability of Pigmentation in _Oenothera_. _Ztsch. f. Abstammungslehre_, 1911, IV, p. 337. Honing, J. A. Die Doppelnatur der _Oenothera Lamarckiana_. _Ztsch. f. Abstammungslehre_, 1911, IV, p. 227. Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K. Small). Mutants and Hybrids of the _Oenotheras_. _Carnegie Institution's Publication_, No. 24, 1905. Macdougal, D. T., Vail, A. M., Shull, J. H. Mutations, Variations and Relationships of the _Oenotheras_. _Carnegie Institution's Publication_, No. 81, 1907. de Vries, H. On Atavistic Variation in _Oenothera cruciata_. _Bull. Torrey Club_, 1903, Vol. 30, p. 75. On Twin Hybrids, _Bot. Gaz._, Vol. 44, 1907, p. 401. Ueber die Zwillingsbastarde von _Oenothera nanella_. _Ber. Deut. Bot. Ges._, 1908, XXVI, _a_, p. 667. Bastarde von _Oenothera gigas_. _Ibid._, p. 754. On Triple Hybrids. _Bot. Gaz._, 1909, Vol. 47, p. 1. Ueb. doppeltreziproke Bastarde von _Oenothera biennis_ L. und _Oenothera muricata_ L. _Biol. Cbltt._, 1911, XXXI, p. 97. Zeijlstra, H. H. _Oenothera nanella_ de Vries, eine krankhafte Pflanzenart. _Biol. Cbltt._, 1911, XXXI, p. 129. NOTE. Since this chapter was written two contributions of special importance have been made to the study of the _Oenothera_ problems. The first is that of Heribert-Nilsson.[9] The author begins by giving a critical account of the evidence for de Vries's interpretation of the nature of the mutants. In general this criticism pursues lines similar to those sketched in the foregoing chapter, concluding, as I have done, that the chief reason why factorial analysis has been declared to be inapplicable to the _Oenothera_ mutants is because no one has hitherto set about this analysis in the right way. He has also himself made a valuable beginning of such an analysis and gives good evidential reasons for the belief that at least the red veining depends on a definite factor which also influences the size of certain parts of the plant. He argues further that many of the distinctions between the mutants are quantitative in nature. With great plausibility he suggests that the system of cumulative factors which Nilsson-Ehle discovered in the case of wheat (subsequently traced by East in regard to maize) may be operating also in these _Oenotheras_. According to this system several factors having similar powers may coexist in the same individual, and together produce a cumulative effect. Scope would thus be given for the production of the curious and seemingly irregular numbers so often recorded in the "mutating" families.