There is a limit to the number of (and I'm going to use this word in an entirely neutral sense) aliens who can be brought into a nation, particularly as close-knit and concentrated a nation as Britain is, without breaking the bounds of that society and setting up intolerable frictions and stresses as damaging to one side as to the other. Now, this is a question of number. But the relationship between number and difference is clearly important, because the more different they are – and colour is a signal, an outward signal of differences (not significant in itself, but it signalizes other differences that one can't deny) – the greater the difference, the smaller the numbers that can at any one time be accepted without breaking, or being thought to break (which comes to the same thing if we're talking about psychology), the framework of a nation and a society. So it's numbers.