Two great difficulties have of late stood in the way of such experimental investigation. One of them is of a theoretical, the, other of a practical nature. One is the general belief in the supposed slowness of the process, the other is the choice of adequate material for experimental purposes. Darwin's hypothesis of natural selection as the means by which new types arise, is now being generally interpreted as stating the slow transformation of ordinary fluctuating divergencies from the average type into specific differences. But in doing so it is overlooked that Quetelet's law of fluctuating variability was not yet discovered at the time, when Darwin propounded his theory. So there [464] is no real and intimate connection between these two great conceptions. Darwin frequently pointed out that a long period of time might be needed for slow improvements, and was also a condition for the occurrence of rare sports. In any case those writers have been in error, according to my opinion, who have refrained from experimental work on the origin of species, on account of this narrow interpretation of Darwin's views. The choice of the material is quite another question, and obviously all depends upon this choice. Promising instances must be sought for, but as a rule the best way is to test as many plants as possible. Many of them may show nothing of interest, but some might lead to the desired end. For to-day's lecture I have chosen an instance, in which the grounds upon which the choice was based are very evident. It is the origin of the peloric toad-flax (_Linaria vulgaris peloria_). The ground for this choice lies simply in the fact that the peloric toad-flax is known to have originated from the ordinary type at different times and in different countries, under more or less divergent conditions. It had arisen from time to time, and hence I presumed that there was a chance to see it arise again. If this should happen under experimental circumstances [465] the desired evidence might easily be gathered. Or, to put it in other words, we must try to arrange things so as to be present at the time when nature produces another of these rare changes.