What theory did fascists use to support their own ideology?

The theory is Social Darwinism. It is a theory that people, gatherings, and people groups are liable to indistinguishable Darwinian laws of regular determination from plants and creatures. Presently to a great extent ruined, social Darwinism was pushed by Herbert Spencer and others in the late nineteenth and mid-twentieth hundreds of years and was utilized to legitimize political conservatism, dominion, and bigotry and to dishearten mediation and change.

Social Darwinism (Apex)