The external influence of climate and of food act only as transitory factors. A rich food supply produces fat, lack of food leads to leanness, a warm summer makes a plant more aromatic, and its fruit sweeter; a cold year means less odor and sour fruit. Of two similar seeds the one sown in rich soil will produce a plant with many branches and abundance of flowers; the other, planted in sandy soil, will produce a plant without branches, with few flowers, and with small leaves. The seeds from these two plants will behave in exactly the same way; they have inherited none of the differences of their parents. Influences of this sort, even if extending over many generations, have no permanent effect. Alpine plants that have lived since the ice age under the same conditions, and have the characters of true high-mountain plants, lose these characters completely during the first summer, if transplanted to the plains. Moreover, it makes no difference whether the seed or the whole plant itself be transferred. In place of the dwarfed, unbranched growth, and the reduced number of organs, the plant when transferred to the plains shoots up in height, branches strongly, and produces numerous leaves and flowers. The plants retain their new characters as long as they live in the plain without any other new variation being observed in them. Other characteristics also, which arise from different kinds of external influences due to different localities, such as dampness and shade, a swampy region, or different geological substrata, last only so long as the external conditions last. These transient peculiarities make up the characters of local varieties. That they have no permanency is intelligible, since they exhibit no new characters, but the change consists mainly in the over- or under-development of those peculiarities that are dependent on external influences. The effect of these influences may be compared to an elastic rod, which, however much it may be distorted by external circumstances, returns again to its original form as soon as released. Besides these temporary changes, due to external influences, there are many cases known in which the same plant lives under very diverse conditions and yet remains exactly the same. For example, the species of _Rhododendron ferragineum_ lives on archæan mountains and especially where the soil is poor in calcium. Another species, _Rhododendron hirsutum_ is found especially on soil rich in calcium. The difference in the two species has been supposed to depend on differences in the soil, and if so, we would imagine that, if transplanted for a long time, the one should change in the direction of the other. Yet it is known that the rusty rhododendron may be found in all sorts of localities, even on dry, sunny, calcareous rocks of the Apennines and of the Jura, and despite its residence in these localities, since the glacial epoch, no change whatever has taken place.