the practices resorted to by aliens in the control of distribution , as already pointed out above , their secret manipulations of stocks of commodities and prices , their utter disregard of the welfare of their customers and of the ultimate happiness of the people of the nation of which they are mere guests , which practices , manipulations and disregard do not attend the exercise of the trade by the nationals , show the existence of real and actual , positive and fundamental differences between an alien and a national which fully justify the legislative classification adopted in the retail trade measure .