I read an interesting tweet the other day regarding this, and racially homogeneous societies/worlds in fantasy being justified by historical reference. This little ramble will look at how things were historically, and how/whether this should affect fantasy writing in an ancient/medieval setting.
First off, a disclaimer on my own approach. My serious fantasy all occurs within the same world within which every book has multiple races (mostly of human), skin colours etc. The comedy of Sir Edric doesn’t ever actually refer to human skin colour (he’s white on the cover because he has to be a colour, though I can’t recall if I actually specified that to my artist). There’s also elves, Ursk (tall, red-skinned carnivores), feathery chaps, gnomes, dwarves etc.
In a given farming village, in England in the medieval world, travelling would happen. Every so often the nearest market town (walking distance) would be visited to either sell or buy. More infrequent travel to a larger town or city (NB ‘city’ in this context might just be a settlement with a few thousand people) would happen to attend court or for other serious business. The population of the village itself would not change substantially in terms of people leaving or newcomers arriving. The largest churn would be marrying people from a nearby village.
Jumping from that to the opposite end of the spectrum (sticking with England, for now), London was the largest city by a mile. Smaller than several continental capitals, it still drew in people from the surrounding area (especially true after the Black Death which also threw the feudal servitude system up in the air). Not only that, but merchants from continental Europe and further afield were constantly coming and going. On a more permanent basis, there were embassies from foreign powers, and establishments set up by prosperous foreign merchants.
Leaping perhaps a few centuries back, and a few thousand miles away, we have Byzantium. Or Constantinople. Or Mikligard, if you’re feeling Viking. Or, as it’s currently known, Istanbul. Now, you might think a capital city at the heart of the Eastern Roman Empire wouldn’t have many Englishmen. And you’d be wrong. Basil II, who was a great military leader but whose treatment of prisoners is about as far away from the Geneva Convention as you can get, is emperor. He establishes the Varangian Guard, a bodyguard for emperors made up of non-Byzantines. Initially, it’s largely composed of Anglo-Saxons, irked at the pesky Normans who have conquered England. Later, it gains a more Viking flavour as Scandinavians prefer getting paid a small fortune for guard duty to raiding.
Under Basil II (and his co-emperors/predecessors Nicephorus “White Death of the Saracens” Phocas and John Tzimisces), the city has been enjoyed continual military triumphs. The city is bustling with merchants from the rising Italian commercial powers of Genoa and Venice. Soldiers are largely drawn from Anatolia, modern day Turkey.
In short, scale and geography determine to a substantial degree how homogeneous or diverse a settlement (or story) is. Before mass transit and easy travel, getting to the Shetlands was quite a slog. If you set a story there you could, depending on the period, credibly feature Picts, Scots, Scandinavians. But if you put Saracens and Byzantines there it would feel a bit odd.
Similarly, if you wrote about the Eastern Roman Empire it would be odd to paint Constantinople as a city of one people only. It was effectively the global (or at least continental) capital, at the crossroads of Europe and Asia, and drew in a correspondingly cosmopolitan population.
However, we should be wary both of imposing our own norms on the past, or of neglecting/misunderstanding those of history. It’s easy to overlook the prevalence of religion in medieval history (and they were addicted to philosophical religious debates in Byzantium). Similarly, the terror of disease or urban fire (arson in ancient Rome was reckoned a crime second only to parricide). There were, particularly in large continental cities, substantial populations of minorities (often same race, different country) but not on the scale that we see today.
What about a pure fantasy land? An island could legitimately be mono-cultural. And, of course, you can gerrymander the rules of science and nature as you please. Personally, I think that could feel quite odd, particularly if your story happens in major cities on a large continent. More importantly, different cultures and races also present opportunities for conflict which can help drive stories.
Although a game rather than book, I was not impressed when some people bleated about The Witcher 3 having no black people in it (it has white humans, elves, dwarves and halflings. And the odd troll). Does it come across as unrealistic because of this? Absolutely not. The world has a great backstory and lore. Just as it seems ridiculous to me that some criticised Idris Elba being in Thor (did amuse me some people were happy with a magic rainbow bridge that threw almost invincible demi-gods across the universe, but thought a black guy being in charge of it was unrealistic), it’s not right to condemn a fantastic game because it’s deemed to have committed the sin of being ‘too white’.
Ultimately, it’s down to the author’s own whim. I don’t think other people should be trying to constrain creative freedom and dictate that their own personal perspective is The Only Way To Do Things.