... but mules are sterile, so horses and donkeys are distinct species.... of cross-breeding a pegasus, so horses and hummingbirds are distinct...zoologists trying to breed a horse with a hummingbird to prove...color coat, with or without wings—and then use the interbreeding...a stallion to prove that horses aren't birds; but if...animals stretching from hummingbird to horse. Neighboring animals could differ so...