Physical environment, moreover, determines in some measure the relations of the various races with their neighbors. Men will be influenced by neighboring groups of men in so far as physical features make intercourse easy or difficult. A race of men who live on a distant island, or in any other shut-off region, will not be so easily influenced by neighboring races as if they dwelt in the midst of a fertile plain. To some extent, as has been said, man is able to overcome the difficulties which physical environment sets in the face of progress. If there is inadequate rainfall, he may devise a system of irrigation and carry on certain forms of agriculture as successfully as though rainfall were abundant. By means of railroads, steamships, and electric or radio communication he can be in constant contact with other men who are separated from him by physical obstructions. But however much the conditions of nature may be controlled, they still exert a great influence upon human progress. [Sidenote: (_b_) Social.] The social environment is quite a different thing. By it we mean the conditions altogether apart from geographic or natural features, which influence the daily life of mankind. We include within social environment such things as family life, the schools, the churches, the organization and methods of industry, the form of government—everything that society develops in the way of institutions. Many of these, as has been pointed out, are natural growths, but the mind of man has also had a large part in shaping their course. [Sidenote: How customs and laws create a social environment.] Most of the things we do, whether as a body of people or as individuals, are merely the result of custom or general habit. Why do men have their hair cut short while women let their hair grow long? Why do people wear black when they are in mourning? In some countries they wear white. The answer is merely that every nation, through long-continued habit, develops its own ways of doing things and keeps on doing things in that way regardless of any present reason. Orientals, when they eat their meals, squat on the floor; Europeans and Americans seat themselves at the table. Aryans shake hands when they meet; the Esquimos hold their hands high above their heads as a token of greeting. The gentleman of today, when he greets a lady on the street, raises his hat. This is not a particularly graceful custom, nor is it in rainy weather an altogether hygienic one; but it has been in vogue among the people of western Europe for many centuries. It goes back to the days of chivalry when the armored knight raised his visor to show his countenance and disclose his identity.