"No-one can point at the position on the bar where black changed to white, it is just merging greys all the way along."
And that brings up another question. (Sorry I have a lot of them)
What causes the differences between races?
Is that based on environment as well?
Maybe I should save this for a different thread if I'm going to far off base.
I assume that you are talking about the races of man. Environmental differences cause different selection pressures, micro-adaptions like skin-colours that are suitable for climate. It is a real bad idea to be a very dark-skinned inuit for example. Darwin correctly concluded in "The descent of Man" that despite superficial differences such as skin colour all the races of man are the same species.
Incidentally, for the avoidance of all doubt, in case there are any terminally stupid people around, my anology about a coloured bar was nothing to do with skin colour.