This chart creates a contrast of the populations of mules and tractors over a period of time.
Horses and mules once numbered in the mid four thousands in the year 1910, when they suddenly experienced a sharp decline. Their numbers slid from about four thousand, five hundred in the year 1910 to about zero in 1960.
Tracctors, on the other hand, experienced an upward rise which coincided with the decline of horses and mules. Tractor populations numbered at zero in 1910 and rose to twenty-five thousand by 1960. The rise in tractor levels was steady and sure, just as the descent in mules and horses was.