These walls leave their mark deep in the minds of men. They set up a principle of "divide and rule" in our mental outlook, which begets in us a habit of securing all our conquests by fortifying them and separating them from one another. We divide nation and nation, knowledge and knowledge, man and nature. It breeds in us a strong suspicion of whatever is beyond the barriers we have built, and everything has to fight hard for its entrance into our recognition. When the first Aryan invaders appeared in India it was a vast land of forests, and the new-comers rapidly took advantage of them. These forests afforded them shelter from the fierce heat of the sun and the ravages of tropical storms, pastures for cattle, fuel for sacrificial fire, and materials for building cottages. And the different Aryan clans with their patriarchal heads settled in the different forest tracts which had some special advantage of natural protection, and food and water in plenty. Thus in India it was in the forests that our civilisation had its birth, and it took a distinct character from this origin and environment. It was surrounded by the vast life of nature, was fed and clothed by her, and had the closest and most constant intercourse with her varying aspects.