AN Anglo-Saxon,” Mr. Dooley remarked almost a century .ago, “is a German that’s forgot who was his parents.” Some 65 years later another Irishman, Daniel Patrick Moynihan, observed with Nathan Glazer in Beyond the Melting Pot that “except where color is involved ... the specifically national aspect of most ethnic groups rarely survives the third generation.” Today, one suspects that, to the extent they are aware of their heritage at all, most Americans of German (or Irish) descent would regard themselves almost as Anglo Saxons, part of the old ethnic stock of this country.