As to book length 'arguments' on this postion, Francis Galton's 'Hereditary Genius' is a good one if only for the fact that Nietzsche himself was familiar with it.
I will quote from a couple of paragraphs;
"The ablest race of whom history bears record is unquestionably the ancient Greek, partly because their master-pieces in the principal departments of intellectual activity are still unsurpassed, and in many respects unequalled, and partly because the population that gave birth to the creators of those master-pieces was very small. Of the various Greek sub-races, that of Attica was the ablest ..." [...] "... (and) by a system of partly unconscious selection, she [Attica] built up a magnificent breed of human animals ..."
[Galton ib.]

The idea then, that a race is a pro-ject of what Nietzsche calls 'Discipline and Breeding', suggests that the ancient Greeks 'bred themselves up' - no doubt using the sublime ideal racial types seen in their statuary as a goal to aim for.
Once the cirstances change, however, and the self-imposed eugenic rules are relaxed, then the type is liable to be spoiled and even bred out altogether. Modern day Greece offers an example of this decline of a type; and nothing is more detrimental to the breeding of a higher type than "compassion for all men" !