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.

Assuming, then, that the plums known in pomology as Domestica plums belong to one species, the original habitat of the species may be sought. In spite of the great number of varieties of plums now grown in Europe and western Asia, and the importance of the fruit both in the green and dried state, the history of the plums cannot be traced with much certainty beyond two thousand years. Though stones, without doubt those of the Insititia or Damson and the Spinosa or Blackthorn plums, are found in the remains of the lake dwellings in central Europe[7] the pits of Domestica plums have not yet come to light. In the summer of 1909 the writer, in visiting historic Pompeii, became interested in the illustrations of fruits in the frescoes of the ancient buildings, but neither in the houses of the ruined city nor in the frescoes in the museums in Naples could he find plums, though several other fruits, as apples, pears, figs and grapes were many times illustrated. An examination of the remains of plants preserved in the museum at Naples taken from under the ashes and pumice covering Pompeii gave the same results. No stone-fruits were to be found, though if widely used these should have been on sale in the markets of Pompeii at the time of the destruction of the city, which occurred late in August,—the very time of the year at which the examination was made and at which time plums were everywhere for sale in Rome. This observation is but another indication that plums were not well-known before the beginning of Christianity, since Pompeii was destroyed in 79 A. D. In Greek literature the references to plums are few before the Christian era and these are more likely to some form of Insititia, as the Damsons, rather than to the Domesticas.

Pliny gives the first clear account of Domestica plums and speaks of them as if they had been but recently introduced. His account is as follows:[8] “Next comes a vast number of varieties of the plum, the particolored, the black, the white, the barley plum, so-called because it is ripe at Barley harvest, and another of the same color as the last, but which ripens later, and is of a larger size, generally known as the ‘Asinina,’ from the little esteem in which it is held. There are the onychina, too, the cerina,—more esteemed, and the purple plum; the Armenian, also an exotic from foreign parts, the only one among the plums that recommends itself by its smell. The plum tree grafted on the nut exhibits what we may call a piece of impudence quite its own, for it produces a fruit that has all the appearance of the parent stock, together with the juice of the adopted fruit; in consequence of its being thus compounded of both, it is known by the name of ‘nuci-pruna.’ Nut-prunes, as well as the peach, the wild plum and the cerina, are often put in casks and so kept till the crop comes of the following year. All the other varieties ripen with the greatest rapidity and pass off just as quickly. More recently, in Baetica, they have begun to introduce what they call ‘malina,’ or the fruit of the plum engrafted on the apple tree, and ‘amygdalina,’ the fruit of the plum engrafted on the almond tree, the kernel found in the stone of these last being that of the almond. Indeed, there is no specimen in which two fruits have been more ingeniously combined in one. Among the foreign trees we have already spoken of the Damascene plum, so-called from Damascus, in Syria, but introduced long since into Italy, though the stone of this plum is larger than usual, and the flesh small in quantity. This plum will never dry so far as to wrinkle; to effect that, it needs the sun of its own native country. The myxa, too, may be mentioned as being the fellow countryman of the Damascene; it has of late been introduced into Rome and has been grown engrafted upon the sorb.”

While the records are somewhat vague it is probable that the Domestica plums came from the region about the Caucasus Mountains and the Caspian Sea and especially the section east of these mountains and the sea. What seems to be the wild form of this species has been found by several botanists in this great region.[9] Here the Huns, Turks, Mongols and Tartars, flowing back and forth in tides of war-like migration, maintained in times of peace a crude agriculture probably long before the Greeks and Romans tilled the soil. The plum was one of their fruits and the dried prune a staple product. Here, still, to the east, west and north toward central Asia, plums are among the common fruits and prunes are common articles of trade. Even in the fertile oases of the great central Asian desert, plums are cultivated, but whether domesticated here or brought from elsewhere cannot be told. Koch,[10] speaking of prunes in particular, gives the following account (translated) of their Asiatic origin:

“According to my investigation Turkestan and the southern Altai Mountains are the place of origin. When in the year 1844 I found myself in Baku on the west coast of the Caspian Sea, I had plenty of opportunity to draw accounts of the fruits of their native lands from the Turkestan and Bokharan merchants, and was astonished over the high cultivation of stone fruits in these places—at the same time I was able to taste dried the most choice because best flavored, the Ali-Bokhara, that is Bokhara prune. Some of these Bokharan prunes were transplanted a long time ago to Trans-Caucasia and were especially cultivated in the ancient city and residence of the Ruler of the modern Elizabethpol. Unfortunately the cultivation is less now than in earlier times. A further spread toward the west and toward Europe, I have not been able to follow. In Greece, the prunes are even to-day an unknown fruit.”

At about the time Pliny wrote, or somewhat before, communication had been opened between the Romans and the countries about the Caspian Sea, and a few centuries later the devastating hordes of Asiatics came westward and for several centuries continued to pour into eastern Europe. What more probable than that they should have carried dried prunes as an article of food in the invasions, and eventually, as they made settlements here and there, have introduced the trees in Europe. It is certain, at any rate, as we shall see, that several of the groups of cultivated plums trace back to the Balkan countries of Europe and the region eastward. There, now as then, the plum is a standard fruit and prune-making a great industry.

The plum when first known in Europe, as described by Pliny and other early writers, seems to have been a large and well-flavored fruit, indicating that it had been under cultivation for a long while. This, and the fact that the fruit was not known by the earliest writers on agriculture, indicate that the plum was not originally an inhabitant of southern Europe, as some suppose. It is likely that the tree has escaped from cultivation and become naturalized in the localities where it is now supposed to grow wild. Prunus domestica has not been found wild nor under cultivation in eastern Asia, so far as can be learned by the botanical and horticultural explorers of China and nearby regions, Prunus triflora being the domesticated plum of that part of the continent, though it may well be surmised that some of the Domestica plums are cultivated in western China, a region as yet but imperfectly explored for its plants.