How does your parsley grow?

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Several years ago I bought a little curly leaf parsley plant and put it in my herb bed. It has flourished, spread and it reseeds itself prolifically, and rewards me with all the parsley I want for cooking and makes a beautiful lush green mound during the cool months of spring and fall. This morning I was out weeding and cleaning up and noticed that the cool weather has already prompted the fall crop of parsley, but I have 3 flat leaf parsley plants in among the curley leaf. I thought that was strange since I only planted curly leaf. Is curly leaf parsley a manipulated plant? Is it originally a flat leaf that has been altered?