For tens of thousands of years bands of relatively homogeneous peoples roamed the earth. They either stayed clear of the other, or they fought with them. The victor in these clashes often slaughtered the vanquished or enslaved them. Sometimes if the differences were slight they intermarried them away.
If, however, significant internal difference were allowed to persist, allowing extremely different peoples to become part of a society, as with the later years of the Roman Empire, the social order eventually collapsed. When the old-line Southern segregationists said that birds of a feather flock together as a way to defend an American form of apartheid, they may have been hateful creeps, but they, alas, weren’t wrong.
Tolerance can be said to be the demarcation line between the sophisticated and the unsophisticated. Indeed, a minimum of worldliness (intellect) is require of one in order for one to rise above his or her hardwired fear and hatred of the other. With the advent of Fox News and the modern Republican Party, and finally the arrival of the Donald at the top of the heap, a bright line has materialized once again in this country between those supporting diversity and those dead set against it.
As the world’s population ever expands, driving dissimilar peoples evermore into each other’s spaces, hostility toward the other can only be expected to increase. Unfortunately for the segregationist minded, walls won’t solve their problem. Folks are either going to have to become generally more sophisticated and tolerant, or like the old Roman Empire, American democracy will perish from this earth and nature ain’t with the good guys on this one.