Four years ago I adapted the first four chapters of Darwin’s The Origin of Species for my year 6 classes (see here). We used the adapted text following careful teaching of Natural Selection. They already understood the ideas before we introduced Darwin’s text.
I have now updated the teaching activities for using the text and created a packet: Adapted Origin of Species Packet
I think it would be useful for any learner who had already studied Natural Selection. My dream is that some more learners progress to reading the full text.