"".it seems to me then that "preservation of favoured races" would include man ""

There you go again, misinterpreting what Darwin actually said. Evolution, according to Darwin's Theory, is a matter of individuals (not species or races)displaying advantageous traits which increased their chances of survival.

This might have been a colour variation making individuals less visible to predators, or a physical difference which allowed certain individuals to take advantage of a hitherto unfilled niche in the food chain.

The point is that when these variations bred true in later generations, they led to species change.

As an example, a white butterfly on a dark tree gets eaten, while a darker one lives to breed and eventually, if the variation breeds true, only dark butterflies of that species will exist in that area of dark coloured trees. In a different location where silver birch are in the majority, dark individuals get eaten and the result is that in time only white butterflies exist in that location.

I cannot understand why you find that a difficult concept.

Don T.