What are the motifs in On the Origin of Species by Charles Darwin?

The main purpose of the book is to undermine the belief that the species were created exactly as they appear today by God and never changed. This belief, as is evident from the final chapter, was the most common belief among biologists at the time. Darwin's theory that the species descended from other species and were modified through natural selection is opposed to this, because it would mean that the species were not created by God and did change.