The Republic by Plato

Then clearly the next thing will be to make matrimony sacred in the highest degree, and what is most beneficial will be deemed sacred?

Exactly.

And how can marriages be made most beneficial?—that is a question which I put to you, because I see in your house dogs for hunting, and of the nobler sort of birds not a few. Now, I beseech you, do tell me, have you ever attended to their pairing and breeding?

In what particulars?

Why, in the first place, although they are all of a good sort, are not some better than others?

True.

And do you breed from them all indifferently, or do you take care to breed from the best only?

From the best.

And do you take the oldest or the youngest, or only those of ripe age?

I choose only those of ripe age.

And if care was not taken in the breeding, your dogs and birds would greatly deteriorate?

Certainly.

And the same of horses and animals in general?

Undoubtedly.

Good heavens! my dear friend, I said, what consummate skill will our rulers need if the same principle holds of the human species!