The traditional focuses of sociology include social stratification, social class, social mobility, religion, secularization, law, sexuality and deviance.
These were called calceus mulleus, and the name may have been shortened to mules by English speakers. Hence the primary difference between mules and clogs in early days was that clogs were likely to be worn by working people, and mules were fancier and favored by the aristocracy.
A horse and donkey are not the same species and the mule is born sterile. Another example is a lion and tiger, they are not the same species either, but they can produce a liger. A cat and rabbit can mate but it is most often done artificially (by humans), and their offspring will be born sterile.