This also still needs a special treatise on its stimulus, variation, and limitations.

But the classical economists treated it as absolute and unvarying.

The 'economic man,' who had no more concern than a lone wolf with the rest of the human species, was treated as possessing a perfect and permanent solidarity of feeling with his 'family.' The family was apparently assumed as consisting of those persons for whose support a man in Western Europe is legally responsible, and no attempt was made to estimate whether the instinct extended in any degree to cousins or great uncles.
A treatise on political impulses which aimed at completeness would further include at least the fighting instinct (with the part which it plays, together with affection and loyalty, in the formation of parties), and the instincts of suspicion, curiosity, and the desire to excel.
All these primary impulses are greatly increased in immediate effectiveness when they are 'pure,' that is to say, unaccompanied by competing or opposing impulses; and this is the main reason why art, which aims at producing one emotion at a time, acts on most men so much more easily than does the more varied appeal of real life.

I once sat in a suburban theatre among a number of colonial troopers who had come over from South Africa for the King's Coronation.

The play was 'Our Boys,' and between the acts my next neighbour gave me, without any sign of emotion, a hideous account of the scene at Tweefontein after De Wet had rushed the British camp on the Christmas morning of 1901--the militiamen slaughtered while drunk, and the Kaffir drivers tied to the blazing waggons.