Beside the comparatively well-known groups of Domestica varieties, there are in Europe, with an occasional representative in America, especially in herbaria, numerous other groups either a part of _Prunus domestica_ or possibly, in a few cases at least, hybrids between it and other species. European botanists place some of these in distinct species or sub-species; but few, however, even of the recent writers on the botany of the plum, agree at all closely as to the disposition of these edible and ornamental plums which may be doubtfully referred to _Prunus domestica_. With this disagreement between the best European authorities where these plums have long been known, where some of them have originated, and all may be found in orchards, botanic gardens and herbaria, it does not seem wise at this distance to attempt a discussion of such doubtful forms. It is certain, however, that Borkhausen’s _Prunus italica_ and _Prunus œconomica_, as given in the synonymy, are but parts of _Prunus domestica_, the first including the Reine Claude plums and the latter the various prunes. So, too, a wild form named by Borkhausen, _Prunus sylvestris_, is probably a part of _Prunus domestica_. Bechstein[4] gave specific names to a number of plums which Schneider[5] holds are all cultivated forms of _Prunus domestica_. These names are not infrequently found in botanical and pomological literature, to the great confusion of plum nomenclature. The following are Bechstein’s species:—_Prunus exigua_, _Prunus rubella_, _Prunus lutea_, _Prunus oxycarpa_, _Prunus subrotunda_ and _Prunus vinaria_. The plum in which the world is chiefly interested is the Old World _Prunus domestica_. The Domestica plums are not only the best known of the cultivated plums, having been cultivated longest and being most widely distributed, but they far surpass all other species, both in the quality of the product and in the characters which make a tree a desirable orchard plant. How much of this superiority is due to the greater efforts of man in domesticating the species cannot be said, for the natural history of this plum, whether wild or under cultivation, is but poorly known. It is not even certain that these plums constitute a distinct species, there being several hypotheses as to the origin of the Domestica varieties. Three of these suppositions must be considered.