Why were the Israelites not allowed to wear clothes mixed with both linen and wool?
Theologically, in the Garden of Eden, vegatable matter was the original sole food source, supposedly for all living things. "Take ye and eat of any tree…", etc. However, after the fall of man, Adam and Eve were clothed with animal skins, and we suppose now given permission to eat animals for meat. (This can be seen as a broader reading of the "emnity" pronouncement from God as being emnity between man and all of creation.) The animal skins also represent the first animal "sacrifice" as a "covering" for man’s nakedness (representing man’s sin). (We have to assume God or Adam killed the animals to obtain their skins, not that they were shed voluntarily or created without an owning animal.)
Therefore, it was necessary to maintain a separation between items which were the result of an animal being killed for the item, and items which were not produced from killing animals. Otherwise, the Jews would be "dilluting" the entire concept of animal sacrifice and animal covering. Obstensibly, one could argue the purpose was to prevent the Jews from questioning the need for or the "specialness" of animal sacrifice. It’s very subtle, but it’s consistent with the concepts of purity and blood sacrifice. (Note that the Jewish culture wasn’t the only culture which held animal products as being "special" or separate from vegetable products. For example, even the American Indians glorified animal items as still having the "spirit" of the animal attached to it.)
BTW, for those who want to argue for a "theist" form of evolution, if you read between the lines, the Genesis story implies that all animals were herbavores before the fall of man (at least if you accept the larger theological implications of the story). So, it wouldn’t be possible for evolution to be the "tool" that God used to create biological entities, unless carnivores all were herbavores which suddenly switched overnight to eating meat. So, either you have to accept Genesis as a allegorical story which casts a theological light on the origins of man (without being literal) – (which I believe is the stance of most modern Jewish scholars), OR if you are determined to take a literal reading, you have to completely reject the concept of evolution.
Leviticus 19:19 says “…Nor shall a garment of mixed linen and wool come upon you.” The idea is that the distinctions that God ordained in Creation are to be preserved. Not mixing wool and linen was perhaps an effort to reduce adulteration of a good product. Pure cloth is the best for overall wearability. Even today, the most expensive clothing is pure. God wants us to have the very best.
