"Unassimilability" was a criticism made of the Jews and thus its use as a criterion was seen as a pretext for excluding Jewish migrants.
The United Party government led by Prime Minister Hertzog introduced the Bill in order to restrict and regulate immigration and exercise control over resident aliens.
[1] The National Party's Daniel Malan, leader of the opposition, unsuccessfully attempted to amend the bill to prohibit Jewish immigration explicitly, to end the further naturalization of Jewish permanent residents of South Africa and to close certain professions from Jews and "other non-assimilable races".
[1] The Aliens Act became law in February 1937 and put into place an Immigrants Selection Board which was given the power to issue or refuse to grant permits to any prospective immigrant who was not a citizen by birth or descent of the British Empire or Irish Free State.
[1] Jewish immigration from Germany fell as a result of the Act to a few thousand with approval for entry normally being given only to wives and young children of Jews already resident in South Africa or to their elderly parents or grandparents.