Such varieties however, do not exist at present. The ordinary twisted races on the other hand, are found in the wild state and have only to be isolated and cultivated to yield large numbers [447] of twisted individuals. In nature they are able to maintain themselves during long centuries, quite as well as normal species and varieties. But they owe this quality entirely to their dimorphous character. A twisted race of teasels might consist of successive generations of tall atavistic individuals, and produce yearly some twisted specimens, which might be destroyed every time before ripening their seeds. Reasoning from the evidence available, and from analogous cases, the variety would, even under such extreme circumstances, be able to last as long as any other good variety or elementary species. And it seems to me that this explanation makes clear how it is possible that varieties, which are potentially rich in their peculiar monstrosity, are discovered from time to time among plants when tested by experimental methods. Granting these conclusions, monstrosities on the one side, and dimorphous wild species on the other, constitute the most striking examples of the inheritance of latent characters. The bearing of the phenomena of dimorphism upon the principles of evolution formulated by Lamarck, and modified by his followers to constitute Neo-Lamarckianism, remains to be considered. Lamarck assumed that the external conditions directly affected the organisms in [448] such a way as to make them better adapted to life, under prevailing circumstances. Nageli gave to this conception the name "Theory of direct causation" (Theorie der directen Bewirkung), and it has received the approval of Von Wettstein, Strasburger and other German investigators. According to this conception a plant, when migrating from lowlands into the mountains would slowly be changed and gradually assume alpine habits. Once acquired this habit would become fixed and attain the rank of specific characters. In testing this theory by field-observations and culture-experiments, the defenders of the Nagelian principle could easily produce evidence upon the first point. The change of lowland-plants into alpine varieties can be brought about in numerous cases, and corresponding changes under the influence of soil, or climate, or life-conditions are on record for the most various characters and qualities.