Speed is the bottom line of horse racing and racehorse breeding.
For more than 500 years, the Thoroughbred has been the world's fastest horse. Alexander Mackay-Smith devoted 10 years to researching the development of speed in the Thoroughbred. He identifies the three speed strains that founded the breed and traces their tail-female lines to the dominant horses of today. Speed, says Mackay-Smith is transmitted through female lines. He cites as proof the 15 tail-female lines that have dominated Thoroughbred breeding for the past 300 years and have survived unbroken.