Again, the same writer points out that the battle-ground between the French and German peoples round the Rhine is the region where the growth of the sweet chestnut as a planted tree reaches its eastward limit. Such facts must not, of course, be over-emphasised. Both must indicate a climatic change, but it can hardly be supposed that this change of climate is sufficient to affect man directly. It seems at least justifiable to point out that every human group which reaches any degree of civilisation and stability must depend for its permanence in the early stages on some special skill in the growing of certain cultivated plants, and the rearing of certain domesticated animals. We have much reason to believe that this skill is often difficult to acquire by other groups. The great difficulties which have been experienced in introducing _e. g._ Smyrna figs and dates into the United States; the fact that Europeans seem to find it impossible to manage camels without native help, and that they have been hitherto unable, despite most elaborate and costly experiments, to tame the African elephant, seem to be minor illustrations of this fact. Given, then, an evolving group spreading over the surface of the globe, and taking with it its characteristic plants and animals, it is probable enough that such a change of climate, even a minor change, as may be sufficient to render it impossible to cultivate these plants, or to rear these animals, may give a definite and more or less permanent check to the spread of the race. There is at least some evidence to this effect, and it gives an additional interest to the study of plant geography. We have limited ourselves in this chapter practically to a consideration of the European area, because the existing cultivated plants and domesticated animals of North America are almost all derived from Europe, with the exceptions already indicated, and a few others not of great importance, and their distribution in America is determined by the same conditions as in Europe. CHAPTER VIII THE RACES OF EUROPE AND THEIR ORIGIN We have spoken in the previous chapter of the three chief races of Europe, but before proceeding to discuss them in detail it is necessary to clear the ground of certain misconceptions and difficulties.