Question

Why is On the Origin of Species described as “one long argument”?

Answer 1

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.

Answer 2

Most likely because it presents an opinion supported by reasons. On the Origin of Species is the paper in which Darwin presents his theory of macroevolution and the reasons he has for postulating it.