It is not difficult to account for the small size of these plants; for in the first place we cannot expect a large or luxuriant growth where the air is cold, the soil scanty, and the light of the sun often obscured by clouds, and where the changes of temperature are rapid,--which is very unfavourable to most plants. Again, in the close struggle for existence which takes place on the plains and low tree-clad hills, the smaller forms of plant life are often overrun by trees, trailing plants, bushes, and vigorous herbs; but where these cannot find a home, owing to the severity of the winter and other causes, the little Alpine plants, covered up by snow in the winter, can thrive abundantly. And lastly, like the older and conquered races of men who have been driven to the hills (see chap. i., p. 28) and find shelter there, so there are both plants and animals living in the mountains which man will not suffer to live in the plain where he grows his crops, pastures his cattle, or builds his cities. We would also venture to suggest that possibly some plants have been ousted from plains by newer and more aggressive types, which came and took their place. If so, vegetable life would afford an illustration of a process which has so often taken place in human history. This is only a speculation, but still it might be worth following up. If Alpine plants, or any considerable number of them, could be shown to belong to more ancient types, such as flourished in the later geological periods, that would afford some evidence in favour of the idea. Whether this is so or not, plant life on the mountains is almost entirely protected from the destroying hands of men with their ploughs and scythes, as well as from many grazing animals. As Mr. Ruskin quaintly says: "The flowers which on the arable plain fell before the plough now find out for themselves unapproachable places, where year by year they gather into happier fellowship and fear no evil." It is clear that the climate of a mountainous region determines the character of the vegetation. Now, the climate will be different in different parts of a mountain-range, and will depend upon the height above the sea and other causes.[16] Some writers upon this subject have attached too much importance to absolute height above the sea, as though this were the only cause at work. It is a very important cause, no doubt, but there are others which also have a great influence, such as the position of each locality with respect to the great mountain masses, the local conditions of exposure to the sun and protection from cold winds, or the reverse. However, in spite of local irregularities there are in the Alps certain broad zones or belts of vegetation which may be briefly described as follows:-- [16] The following remarks are largely taken from the Introduction to Ball's well-known "Alpine Guide."