Before concluding this chapter, a few remarks on the botanical aspect of the Alpine problem might not be out of place. It will enable us to judge which of the views indicated is the more probable, and will add to the interest which may have been aroused by the perusal of this sketch of the fauna of the Alps. Very much the same train of argument was applied as to the course of events in the formation of the Alpine flora as in the case of the fauna. The plants were all supposed to have been killed or driven away by the arctic temperature of the Glacial period, and their place taken by new migrants from the north or east when the climate ameliorated. Professor Engler, one of the highest living authorities on the geographical distribution of plants, is of opinion (p. 102) that a large number of the indigenous Alpine species did not originate till after the close of the Glacial period, because so many of them are absent from the Sierra Nevada in Spain, where the condition for their well-being exists, while many have evidently spread from the Alps to the Carpathian Mountains and to the Pyrenees. He does not believe that a glacial flora could have existed in the plain between the Sierra Nevada and the Pyrenees during the Glacial period (p. 109). In speaking of the Caucasus, Professor Engler informs us (p. 117) that a good many species which do not occur in the Alps reached these mountains from Siberia. Apart from the northern glacial plants, the Caucasus has only few species in common with the Alps, more with the Balkan mountains and Northern Persia. Turning to Afghanistan, our author mentions (p. 121) a few Alpine plants as occurring in that country, and likewise in the Caucasus and the Himalayas. He considers it probable that the route of migration of some glacial plants from the east to the west, and _vice versâ_, lay across the Afghan mountains. Many of our Alpine plants occur in the Siberian mountains, but in the Altaï and Eastern Siberia generally a considerable number of these are by no means confined to the mountains (p. 125). They are also met with in the lower regions, and the rare Alpine Edelweiss (_Leontopodium alpinum_) frequently covers wide tracts in the plain, and is passed by almost unnoticed by the Siberian botanist.