Immigration Then and Now...What's Changed?

Excellent reminder from Prof Mankiw to those of us who may have forgotten or never learned their history. For my part, knowing how my Irish immigrant forebears were viewed upon coming to this country, I find it very hard to countenance the anti-Mexican tinge in much of today's immigration debate. That said, I am not sure the Irish were so presumptous in their demands. Either way, what evidence is there to suggest that amnesty or continued immigration is economically detrimental?