G.J. Wenham

Why Were Some Animals Unclean In Leviticus?

"The animal world is divided into three spheres: those that fly in the air, those that walk on the land, and those that swim in the seas (cf. Gen 1:20-30). Each sphere has a particular mode of motion associated with it. Birds have two wings with which to fly, and two feet for walking; fish have fins and scales with which to swim; land animals have hoofs to run with. The clean animals are those that conform to these standard pure types. 

Those creatures which in some way transgress the boundaries are unclean. Thus fish without fins and scales are unclean (Lev. 11:10; Deut 14:10). Insects which fly but which have many legs are unclean, whereas locusts which have wings and only two hopping legs are clean (Lev. 11:20-23). Animals with an indeterminate form of motion, i.e., which "swarm," are unclean (Lev. 11:41-44). "Holiness requires that individuals shall conform to the class to which they belong." (see M. Douglas, Purity and Danger, p. 53). Insofar as some animals do not conform (to what is normal and appropriate to their class), they are unclean." (The object was to teach order within Gods camp. His people were divided into an orderly nation of various "classes," priests, levites, elders, leaders, common people, tribes, and so on)."