Immigration restrictionists often claim that there are no "jobs Americans won't do," if only US borders would be secured against economic migrants who are willing to work hard for low pay and few benefits. If the fruit-and-vegetable industry couldn't rely on seasonal farmhands from Mexico and Central America, for example, growers would perforce offer the higher wages necessary to attract American citizens to pick the country's fresh produce. What alternative would they have? Let crops rot in the field? Field workers pick strawberries in Oxnard, Calif. In reality, agriculture is no more of a zero-sum industry than any other, and...