"And doesn’t it nearly always turn out that the breed with the higher yield is accepted in the end? Aren’t cattle crossbred to get hardiness, or milk-yield, or meat?"

A Belgian Blue cow. The defect in the breed's myostatin gene is maintained through linebreeding and is responsible for its accelerated lean muscle growth
GNU Free Documentation LicenseA Belgian Blue cow. The defect in the breed's myostatin gene is maintained through linebreeding and is responsible for its accelerated lean muscle growth - Credit: Kersti Nebelsiek
The selective breeding of plants and animals to emphasize certain genetic traits, such as milk yield, quality of meat and hardiness, has been carried out for many centuries.