Part 48. The multiplication of varieties. Besides the effect which geographical range has exerted upon the spread and period of cultivation, the differences in the number of varieties that have arisen through human agency among cultivated plants may be attributed largely to the same important factor; since, as may be readily shown, the number of varieties in a given species is much influenced by the extent and duration of its culture. For, taken as a whole, the plants of ancient or prehistoric cultivation, as compared with those of modern or recent introduction, present a marked contrast in the greater number of different varieties which have come to be cultivated. Thus we have the common buckwheat, a "modern" plant, without any well-marked varieties, as against the "ancient" oats and rye, each with several varieties; and the "prehistoric" wheat, barley, rice, and maize, with scores or hundreds of varieties. If the comparison of the newer with the older be extended to nuts, vegetables, and fruits, a similar rule will be found to obtain; although it is true that more or less important exceptions will be encountered. These exceptions go to show that other elements besides time of culture would have to be taken into account in any attempt to explain fully why one cultivated species should have more or less varieties than another. But these other factors need not be here considered, since our present purpose is to point out that just as the area of use and the culture-period of a plant have been dependent largely upon the geographical relation of its native home to a primitive center of agriculture, so upon these factors, in their turn, have largely depended the number of varieties which have been artifically developed.