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 once reached, there is only an artificial barrier to prevent his sympathies extending to the men of all nations and races.
Source:(John Murray, ed. 1871), Volume I, Part I. On the Descent of Man, Chapter III. Comparison of the Mental Powers of Man and the Lower Animals, Page 100
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