The writer seems to suggest that the development of mechanized transportation was due to society's desire to avoid manure. Sounds like manure to me. I find it much easier to believe that advances brought on during the "industrial age" including metallurgy, steam power, electricity, and internal combustion engines is more likely the cause of the shift away from  "horsepower".  We cannot utilize technology we don't have. Solar and wind energy are contributors to the answer but not the entire answer, as demand far outweighs supply. Right now we still need coal power, but when the technology becomes available, coal may go the way of the horse. Key phrase- when technology becomes available.