I transplanted some cabbage about a month ago (after starting it from seed in a flat). Now I see that something is eating the cabbage leaves:

I also see that one pair of seedlings is doing very poorly:

What is eating these leaves? the lettuce growing next to the cabbage appears unmolested. I have seen a rabbit or two in the neighborhood, but I don't think a rabbit would cause this kind of damage, would it?

And what (organic) options do I have to protect my cabbage?