Britain's late Prime Minister Margaret Thatcher said it all when she wrote that the world has "never ceased to be dangerous," but the West has "ceased to be vigilant."

Nothing better illustrates her point than the fact that the West has imported vast numbers of people who hate our guts and would love to slit our throats. Political correctness has replaced self-preservation. The Boston Marathon killer who set a bomb down right next to an eight-year-old child is only the latest in an on-going series of such people.

Senator Patrick Leahy has warned us not to use the Boston Marathon terrorists as an argument against the immigration legislation he advocates. But if we are not to base our laws on facts about realities, what are we to base them on? Fashionable theories and pious rhetoric?

While we cannot condemn all members of any group for what other members of their group have done, that does not mean that we must ignore the fact that the costs and dangers created by some groups are much greater than those created by other groups.

Most members of most groups may be basically decent people. But if 85 percent of group A are decent and 95 percent of group B are decent, this means that there is three times as large a proportion of undesirable people in group A as in group B. Should we willfully ignore that when considering immigration laws?

It is already known that a significant percentage of the immigrants from some countries go on welfare, while practically none from some other countries do. Some children from some countries are eager students in school and, even when they come here knowing little or no English, they go on to master the language better than many native-born Americans.

But other children from other countries drag down educational standards and create many other problems in school, as well as forming gangs that ruin whole neighborhoods with their vandalism and violence, and cost many lives.

Are we to shut our eyes to such differences and just lump all immigrants together, as if we are talking about abstract people in an abstract world?

Perhaps the most important fact about the immigration bill introduced in the Senate is that its advocates are trying to rush it through to passage before there is time for serious questions to be explored and debated, so as to get serious answers.

Anyone who suggests that we should compare welfare rates, crime rates, high school dropout rates and drunk driving arrest rates among immigrants from different countries, before we set immigration quotas, is likely to be stigmatized as a bad person.