As man advances in civilisation, and small tribes are united into larger communities, the simplest reason would tell each individual that he ought to extend his social instincts and sympathies to all the members of the same nation, though personally unknown to him. This point being reached, there is only an artificial barrier to prevent his sympathies extending to the men of all nations and races.

Charles Darwin, The Descent of Man, and Selection in Relation to Sex, London, 1871, pp. 100-101