What is the theme in On the Origin of Species by Charles Darwin?

Another theme to consider is natural selection. Natural selection is the process by which organisms in the wild adapt to their environment. Natural selection depends first upon the fact that plants and animals occasionally vary in some way from their parents. For example, an animal might be born with slightly longer legs or sharper teeth than either of its parents. While many of these variations will not be helpful to the organism, and in fact may actually be harmful, in some rare cases the variation would be helpful and give the organism a greater chance of survival.