Jamieson, Fausset and Brown on Leviticus 11:47

47. make a difference between the unclean and the clean--that is, between animals used and not used for food. It is probable that the laws contained in this chapter were not entirely new, but only gave the sanction of divine enactment to ancient usages. Some of the prohibited animals have, on physiological grounds, been everywhere rejected by the general sense or experience of mankind; while others may have been declared unclean from their unwholesomeness in warm countries or from some reasons, which are now imperfectly known, connected with contemporary idolatry.