Part 50. Artificial selection. Besides these principal ways in which cultivated varieties arise, there are some others the consideration of which must be deferred to a later chapter. What at present concerns us is the general truth that to a very large extent human or artificial selection exerts a controlling influence either upon the development or the perpetuation of varieties, and frequently upon both. Since the longer and more widespread the cultivation of a given plant has been, the more extensive and more varied must have been this influence, we should expect in general that the number of varieties of a cultivated plant would be proportional to the time and area of its cultivation; and this expectation we find to be justified by the facts.