A second theory is that _Prunus domestica_ is derived from _Prunus spinosa_ and that _Prunus insititia_ is an intermediate between the two.[6] This hypothesis is based upon the supposition that when Domestica plums run wild they revert to the Insititia or Spinosa form. It is not difficult to test this theory. A study of the origin of the several hundred Domestica and Insititia plums discussed in Chapters III and IV of _The Plums of New York_ does not show for any one of them a tendency to reversion or evolution to other species; nor do the descriptions indicate that there are many, if any, transitional forms. During the two thousand years they have been cultivated in Europe the Old World plums have been constant to type. Domestica seedlings vary somewhat but they do not depart greatly from a well marked type. Such very few striking departures as there seem to be are more likely to have arisen through crossing with other species than through reversion or evolution. This Station has grown many pure seedlings or crosses of varieties of Domestica within the species and has had opportunity of examining many more from other parts of the State, and none of these show reversion to the other two Old World species. Nor, as we shall see, is there much in what is known of the history of these three species to lead to the belief that the Domestica, Insititia and Spinosa plums constitute but one wild species or have arisen from one. It has been remarked that there are few, if any, transitional forms between the Domestica and other European plums. It is a significant fact that _Prunus domestica_ can be hybridized with other species of plums only with comparative difficulty, species of plums as a rule hybridizing very freely. This is as true with the Insititia and Spinosa as of other plums, there being few recorded hybrids of either of these species with the one under discussion. Quite to the contrary the varieties of the several pomological groups of Domestica plums hybridize very freely. If all were of one species we should expect many hybrids between the Domestica, Insititia and Spinosa plums. We are now left with the third hypothesis, which is, as we have indicated in a preceding paragraph, that the varieties of Domestica plums belong to one species; or if they have come from more than one species the wild forms have not been distinguished and must have grown under much more nearly similar conditions than is the case with _Prunus domestica_ and any other species. Without knowledge of more than one wild form, and in view of the intercrossing of the varieties of these plums it seems best to consider all as parts of one species, leaving to the pomologist the division of the species into horticultural groups founded on the characters which make the fruit valuable for cultivation.