There is no basic number of chromosomes common to all genera in the Onagraceae hence the family was probably of "hybrid" ancestry. One parent is presumed to be aquatic, with 4n chromosomes, the other parent terrestrial with 7n elements; the hybrid consequently has 11n chromosomes. The hybrid is considered capable of back-crossing with both parents, thus giving progeny with 15n and 18n chomo-somes respectively. Jussieua (8n) and perhaps Lud-wigia arose from the first parent; the other, in 3 branches, gave rise to all 7n-chromosome genera. One branch includes Boisduvalia-Godetia-Clarkia and Eu-chardiurn; the second Hartmannia (from which Anogra arose) - P eniophyllum - Kneiffia - Raimannia - Oenothera-Gauridium-Gaura- Heterogaura-Stenosiphon-Gongylocar-pus-Barragea; the third, which probably came from the same ancestor as the second group, includes Taraxia-Lavauxia-Pachylophus-Megapterium-Gaurella-Galpinsia-Meriolix-Eulobus-Sphaerostigma-Chylismia. The hybrid is presumed to include the present Circaea (11n), from which arose Lopezia (-Risenbachia, Diplandra, Semei-andra) and Fuchsia (-Skinnera, Encliandra). The back-crosses are Zauschneria (15n), in which genus Hauya might be included; and Epilobium (18n), to which genus Gayophytum is closely connected. Some genera concerning whose cytology nothing is known are omitted; genera considered synonyms are listed. A list of the species whose chromosomes have been counted by the author is appended.