I believed, à priori, that a great number of the species cultivated for more than four thousand years would have altered from their original condition to such a degree that they could no longer be recognized among wild plants. It appears, on the contrary, that the forms anterior to cultivation have commonly remained side by side with those which cultivators employed and propagated from century to century. This may be explained in two ways: 1. The period of four thousand years is short compared to the duration of most of the specific forms in phanerogamous plants. 2. The cultivated species receive, outside of cultivated ground, continual reinforcements from the seeds which man, birds, and different natural agents disperse and transport in a thousand ways. Naturalizations produced in this manner often confound the wild plants with the cultivated ones, and the more easily that they fertilize each other since they belong to the same species. This fact is clearly demonstrated in the case of a plant of the old world cultivated in America, in gardens, and which, later, becomes naturalized on a large scale in the open country or the woods, like the cardoon at Buenos Ayres, and the oranges in several American countries. Cultivation widens areas, and supplements the deficits which the natural reproduction of the species may present. There are, however, a few exceptions, which are worth mentioning in a separate article.
Article IV.—Cultivated Plants which are Extinct, or becoming Extinct in a Wild State.
These species to which I allude present three remarkable characters:—
1. They have not been found wild, or only once or twice, and often doubtfully, although the regions whence they come have been visited by several botanists.
2. They have not the faculty of sowing themselves, and propagating indefinitely outside cultivated ground. In other terms, in such cases they do not pass out of the condition of adventitious plants.
3. It cannot be supposed that they are derived within historic times from certain allied species.
These three characters are found united in the following species:—Bean (Faba vulgaris), chick-pea (Cicer arietinum), ervilla (Ervum Ervilia), lentil (Ervum lens), tobacco (Nicotiana tabacum), wheat (Triticum vulgare), maize (Zea mays). The sweet potato (Convolvulus batatas) should be added if the kindred species were better known to be distinct, and the carthamine (Carthamus tinctorius) if the interior of Arabia had been explored, and we had not found a mention of the plant in an Arabian author.
All these species, and probably others of little-known countries or genera, appear to be extinct or on their way to become so. Supposing they ceased to be cultivated, they would disappear, whereas the majority of cultivated plants have become somewhere naturalized, and would persist in a wild state.
The seven species mentioned just now, excepting tobacco, have seeds full of fecula, which are the food of birds, rodents, and different insects, and have not the power of passing entire through their alimentary canal. This is probably the sole or principal cause of their inferiority in the struggle for existence.
Thus my researches into cultivated plants show that certain species are extinct or becoming extinct since the historical epoch, and that not in small islands but on vast continents without any great modifications of climate. This is an important result for the history of all organic beings in all epochs.