'Here the term "races" is used as an alternative for "varieties" and does not carry the modern connotation of human races—the first use in the book refers to "the several races, for instance, of the cabbage" and proceeds to a discussion of "the hereditary varieties or races of our domestic animals and plants"'

http://en.wikipedia.org/wiki/On_the_Origin_of_Species - on Darwin's use of the word 'race'

Right. So why is the category 'human races' different to 'cabbage races'? What is it that makes a race within a species? The human population's differences are different to the differences within all other types of organism. Cabbages and finches can have races, but, as luck would have it, homo sapiens is one species whose genetic homogeneity is so great that race has no real meaning. Besides, there are no clear boundaries between races.  In any case, we have a choice - we don't have to take race into account. How fortunate we are that science gives us a warm, fuzzy feeling!