What played a more important role in distinguishing rulers from their colonial subjects in the imperialism of the nineteenth century than in earlier instances of imperialism?

Race.

Explanation:

In the nineteenth century, Europeans were influenced by the belief in their racial superiority justified their imperialist policies by conquering distant lands of Asia and Africa (which were considered land inhabited by inferior peoples). They believed that they were on a civilizing mission to promote the modern European culture to the barbarian colonial population.