How did europeans use the idea of eugenics to justify colonization and imperialism

Scientific racism (sometimes race biology or racial biology or pseudoscientific racism) is the pseudoscientific study of techniques and hypotheses attempts to reveal the biological differences between races which can be used to support or justify belief in racism, racial inferiority, or racial superiority; alternatively, it is the practice of classifying individuals of different phenotypes or genotype into discrete races. Historically it received credence in the scientific community, but is no longer considered scientific.