Almost 100 years elapsed between Linnaeus’ naming the then lone species ofArachis (A. hypogaea L.) known to Europeans, and the first taxonomic treatment of the genus by Bentham in 1841. During the next 100 years five to ten additional species descriptions appeared, assigning different species to the same names, and different names to the same species. By mid-20th Century, it was impossible to examine any herbarium collection of Arachis and assign any epithet with any assurance to any specimen (which was not a type collection) except to A. hypogaea, A. guaranitica, A. tuberosa and A. villosulicarpa. In our treatment, the literature of this botanical chaos in Arachis is reviewed in detail and an assessment is made of the foundations for its occurrence. It is shown that the bases for the confusion lay in the combination of the esoteric nature of the differentiating morphological features of Arachis, the fragmentary early collections, and the representation of species by seedling specimens. Also, it is related how, in 1959, we decided to re-explore the type locality of each species then known, collect therein complete plant specimens and thereby resolve the problem. Thirty-five years, two generations of plant collectors and around 2000 collections later, we present here 69 species descriptions of Arachis, species distributed in South America east of the Andes, south of the Amazon, north of La Plata and from NW Argentina to NE Brazil. We soon discovered that the most significant characters ofArachis lay in their underground structures, including their fruits, rhizomatous stems, root systems and hypocotyls. We showed that these defining characters tended to cluster the collections into groups which were associated with generally different geographic areas and ecological features. We drew a sample of 100 collections representing these clusters, areas and features, and arranged them in a hybridization diallel and showed, in crosses between collections representing different clusters of characters, areas and features, a remarkable number of complete failures to cross-fertilize and, in those hybrids that were recovered, a high degree of F1 hybrid infertility. When these cross-incompatibilities and pollen infertilities were combined with the data on character clustering, the nine distinct sections of the genus presented here then crystallized. Figures imposed upon maps of South America illustrate the geographic distributions of these sections. The collections were then assigned to the different sections on the bases of cross-incompatibility and exo-morphologic character clustering. When these groups were made, the esoteric characteristics referred to above, so confounding when applied across sectional lines, became highly pertinent when applied to the problem of species differentiation between collections within sections. These characteristics, applied in conjunction with chromosome cytology, chromatographic and antigenic reactions, variations in intra-sectional hybrid fertility and adaptations of plant form, and annual and perennial habit, allowed us to assemble the following taxa of the genus Arachis: Section I. TRIERECTOIDES nov.: 1. A. guaranitica, 2. A. tuberosa.Section II. ERECTOIDES nov.: 3. A. Martii, 4. A. brevipetiolata nov., 5. A. Oteroi nov., 6. A. Hatschbachii nov., 7. A. cryptopotamica nov., 8. A. major nov., 9. A. Benthamii, 10. A. douradiana nov., 11. A. gracilis nov., 12. A. Hermannii nov., 13. A. Archeri nov., 14. A. stenophylla nov., 15a. A. paraguariensis subsp. paraguariensis, 15b. A. paraguariensis subsp. capibarensis nov. Section III. EXTRANERVOSAE nov.: 16. A. setinervosa nov., 17. A. Macedoi nov., 18. A. marginata, 19. A. prostrata, 20. A. lutescens, 21. A retusa nov., 22. A. Burchellii nov., 23. A. Pietrarellii nov., 24. A. villosulicarpa. Section IV. TRISEMINATAE nov.: 25. A. triseminata nov. Section V.HETERANTHAE nov.: 26. A. Giacomettii nov., 27. A. sylvestris, 28. A. pusilla, 29. A. Dardani nov. Section VI. CAULORRHIZAE nov.: 30. A. repens, 31. A. Pintoi nov. Section VII. PROCUMBENTES nov.: 32. A. lignosa nov. comb., 33. A. Kretschmeri nov., 34. A. Rigonii, 35. A. chiquitana nov., 36. A. matiensis nov., 37. A. appressipila nov., 38. A. Vallsii nov., 39. A. subcoriacea nov. Section VIII. RHIZOMATOSAE nov., Series PRORHIZOMATOSAE nov.: 40. A. Burkartii. Series RHIZOMATOSAE nov.: 41. A. pseudovillosa nov. comb., 42a. A. glabrata var. glabrata, 42b. A. glabrata var.Hagenbeckii. Section IX. ARACHIS: 43. A. glandulifera, 44. A. cruziana nov., 45. A. monticola, 46. A. magna nov., 47. A. ipaënsis nov., 48. A. valida nov., 49. A. Williamsii nov., 50. A. Batizocoi, 51. A. duranensis nov., 52. A. Hoehnei nov., 53. A. stenosperma nov., 54. A. praecox nov., 55. A. palustris nov., 56. A. benensis nov., 57. A. trinitensis nov., 58. A. decora nov., 59. A. Herzogii nov., 60. A. microsperma nov., 61. A. villosa, 62. A. helodes, 63. A. correntina nov. comb., 64. A. Simpsonii nov., 65. A. Cardenasii nov., 66. A. Kempff-Mercadoi nov., 67. A. Diogoi, 68. A. Kuhlmanii nov., 69a. A. hypogaea subsp. hypogaea var. 1. hypogaea, var. 2.hirsuta, 69b. A. hypogaea subsp. fastigiata var. 1. fastigiata, var. 2. peruviana nov., var. 3. aequatoriana nov., var. 4. vulgaris. The autogamous reproductive systems, agametic reproduction, underground fruiting habit and the limited means of seed dispersal are shown to be logically tied to the drift in chromosomal organization which gives rise to noticeable increases in infertility in crosses between different collections of the same species, to a variably higher infertility in crosses between species within sections, to a near total infertility in crosses between species from different sections. The evolutionary and phylogenetic relationships between the different sections are discussed and are further shown in a sequence of diagrams illustrating the ideas presented. It is evident that the genetic distances separating the sections are far from being of the same magnitude. The presumably older sections (Triseminatae, Trierectoides, Erectoides, Extranervosae and Heteranthae), except for section Erectoides, are much more isolated from the remaining sections and from each other than those taken to be of more recent origin (Procumbentes, Caulorrhizae, Rhizomatosae and Arachis). Section Arachis is by far the largest, containing about 40% of the species described. Species of this section appear to be spreading into new territory and to be invading areas occupied by species of other sections. They grow intermixed with populations of Extranervosae in the upper Paraguay basin and occupy common ground with section Procumbentes in the Gran Pantanal. They have reached the shores of La Plata and the southeastern coast of Brazil and grow from Yala in NW Argentina to the Tocantins in NE Brazil. They include the worldwide cultivar, A. hypogaea. Essentially every published work on the botanical history and taxonomy of Arachis is presented here in the individual specimen references and in the general bibliography. The history of A. hypogaeafrom the early 16th Century to more recent times, along with the common names in several native American languages, provide a perspective on the antiquity of this cultivar and the level of civilization required for its creation. Six appendices provide supporting data and matters of record. Diagnostic keys to the sections and to the species within each section select the more sharply distinguishing characteristics of the sections and species. Nineteen line drawings cap

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