Monkey Business .

Konrad Lorenz was a 20th century Austrian naturalist who was awarded a Nobel Prize in 1973. His best known published works are 'King Solomon's Ring' and 'On Aggression. Lorenz studied the animal kingdom and developed two theses: first, that Aggressiveness is a natural quality, essential to species survival, and second that all vertebrates from fish upwards to our simian cousins possess strong Territorial instincts. Lorenz held that the Territorial instinct is a primal drive, no less powerful than Hunger or Sexuality - indeed perhaps more fundamental, since the establishment and defense of a Territory makes possible the acquisition of food and of mates.
Animals do not normally fight others of their own pack over food. They do fight over mates but do not kill each other, the fight ending in a symbolic surrender by the loser who bares his throat to the teeth of the winner. But higher animals will fight and kill even those of their own species who intrude into their Territory. 

In a famous experiment, 350 rhesus monkeys, gathered at random from different parts of India, were released onto a small island near Puerto Rico. Here, they encountered what primates must regard as Monkey Utopia... no threat from leopards at night or pythons by day…food in abundance, distributed twice a day by the caretakers. Yet, within a year, the monkey community had divided itself into social groups, each holding and defending a permanent Territory and exhibiting violent hostility towards other groups.

Now we do not know by what means monkeys determine group differentiation, one from the other. We humans have invented a variety of devices (race, creed, sect, caste, tribe, nation, ethnicity and so on) by which to distinguish one bunch of people from another. Serious problems arise when human packs that regard themselves as different from one another are required by circumstance to live within the same geographic space. 
Violation of the Territorial instinct leads to eruptions of violence. Recall, in our own country, the horrors of the Partition massacres, of the killings during the East Pakistan civil war. How ferocious and uncompromising was the instinct, once a particular self-consciousness is achieved, to purify each Territory from people perceived as 'others'!