twenty one horses - nice photo
This source says that the horse population in the U.S. peaked at over 26.5 million in 1915 and had declined to 7.6 million by 1950.
http://www.americanequestrian.com/pdf/US-Equine-Demographics.pdf
Wow Mark, people sure had to deal with a whole lot more "horse crap" in 1915 then in 1950.
Jay,
That's why automobiles were initially considered a godsend to the environment. People (particularly in the big cities) were up to their ankles in road apples! Cars don't leave road apples.