The way they taught in 1912

Races of Mankind
The economic importance of race, as of religion, is very great, For instance, in the case of the Negro, climatic influences – acting direct and through the typical food – lead to the early closing of the 'seams' between the bones of the skull; and thus the development of the brain is arrested, and the adult is essentially unintellectual. On the other hand he is naturally 'acclimatized' against numerous diseases and other conditions of life and work which are very adverse to the White man. He is, therefore, of great use as a manual labourer in a 'steamy' climate e.g. on a cane-sugar plantation.
School Economic Atlas by J.G.Bartholomew, pub. Clarendon Press 1912

Its pretty handy when you have a colony to run and you can comfortably assign the native population to do the spadework as they are "essentially unintellectual". If young Brits were digesting this stuff at school, it is no wonder when they grew up and fanned out to do the work of the Empire they took a sense of innate superiority with them.