DEBATE about immigration is steeped in metaphor, ranging from
Enoch Powell's rivers of blood to the more usual waves of
foreigners flooding the nation and swamping public services. The
imagery is usually liquid, carrying with it the idea of a force
of nature, rushing out of governments' control.  That notion
suits the Conservatives nicely. Michael Howard, the Tory leader,
wants immigration to be one of the big subjects for the election
expected in May. That's not surprising: it is the only important
policy area in which voters consistently give the Tories higher
marks than the government. The government is equally keen to
dispel the idea, which is why this week it announced plans to
restrict entry further.  Good migrants and bad migrants  The
government's determination to show that it has got things under
control is understandable, but is based on two mistakes. The
first is the idea that immigration is out of control. Three
years ago it was, partly because the bureaucracy was in a mess
and partly because conf