A belief in social Darwinism and a desire for national survival was justified by the idea of "Survival of the fittest" from Charles Darwin. Some Social Darwinism followers thought that applied to society, white Anglo-Saxon were a superior race.
Social Darwinism and a desire for national survival motivate countries to dominate other countries or regions justified imperialism by taking over under-developed colonies that were not modern with the purpose to help them with decisions and actions to modernize its society.
Imperialism is a consequence of considering the idea of "Survival of the fittest". A strong nation wants to take control of another weak to impose its dominion