… is from page 441 of the 3rd edition (1972) of what is regarded by many (including me) to be the greatest economics textbook ever written, University Economics  by Armen Alchian  and William Allen:
If a uniform or minimum-wage law is effective in raising wages, discrimination will be transferred from wage rates to employee types. Fewer “inferior” people will get jobs. Therefore, pressure mounts for “fair-employment laws,” prohibiting employers from choosing employees on the basis of any criterion ruled unethical – usually race, creed, age, and sex. These laws probably reduce the extent of observable discrimination among employers. But they are incredibly difficult to enforce. How can one tell whether an employer is hiring as many workers as his customers consider “inferior” as he would if he really didn’t think them “inferior”? Furthermore, the employer will be even more reluctant to observe the spirit of the law when he knows it will be more difficult to fire those whose services are unsatisfactory.