 According to Rosenberg, their evolution resulted from the mixing of pre-existing races rather than natural selection.The theory of semiticization was typically associated with other longstanding racist fears about the dilution of racial differences through miscegenation, which were manifested in negative images of mulattos and other mixed groups.Modern European antisemitism has its origins in 19th century theories—now mostly considered as pseudo-scientific, but then accepted as credible—that said that the Semitic peoples, including the Jews, are entirely different from the Aryan, or Indo-European, populations, and that they would not be able to assimilate.