The answer is: it is about evolution.
Darwin himself characterized his seminal book, On the origin of the species, as “one long argument”. There is still debate as to what exactly did he mean by that characterization, but it is agreed upon that he evidently was referencing his theory of evolution based on common ancestry between species and natural selection as the process of differentiation between them, which, all in all, tells a story that took millions of years, or, in more poetic terms, it summarizes a very long argument.