It will be seen how little of the original vegetation of England is left to us for purposes of study--less than one-fifth, almost the whole of which has been influenced to some degree by human operations; while in Scotland and Ireland a much larger area is more or less in its primitive condition. The Scottish mountain-sides and Irish moorlands still to a great extent retain a natural flora, save that the greater number of grazing animals which they now support, as compared with the times when wolves and other enemies roamed unchecked, leaves its impress upon the vegetation. Viewing the plant world as a whole, its primary divisions, from the point of view of ecology, are governed by the factor of rainfall. It is true that the plants of the Tropics differ profoundly from those of the Temperate regions, and those again from the plants of the Arctic. But this is a difference in the _species_ and families which constitute the vegetation, rather than a difference in the _types of vegetation_ or plant formations which occur. A certain area in Siberia may not have one species in common with a certain area in India, but in both we may find the three great vegetation types of forest, grassland, and desert. A rainfall gradient, on the other hand, will cause a progressive change in vegetation type, as may be seen in crossing North America from east to west, where the forests of the New England States give way as precipitation diminishes to the prairies of the middle States, and these again to the deserts which stretch far over the west. It is only in the extreme north that temperature, apart from precipitation, becomes the dominant influence in determining the presence or absence of vegetation, or its character.