One day this month an immigrant will arrive or, more likely, a baby will be born who will make the United States a nation of 300 million. This demographic milestone has prompted hand-ringing among environmentalists on the left and immigration opponents on the right, all of whom are misguided. Passing the 300 million mark should be cause for celebration: Never in the history of mankind have so many people lived such free and prosperous lives in one country. Anti-immigration activists blame newcomers for driving up the population, when in fact most growth is natural. Since 2000, births have averaged 4.05...