By Charles Darwin
Late this summer, I mentioned to a colleague that I had just finished Adam Smith's The Wealth of Nations and was working on Karl Marx's Capital. He replied: "Now you just have to read Darwin's The Origin of Species." Of course he was right: Darwin's book had a huge impact on everything, but especially on economics via Marx and Engels. So, as is my practice with classic books, I downloaded the cheapest Kindle version I could find.
The Origin of Species didn't provide me with any fundamentally new ideas, of course—but that's because it's one of those basic texts whose ideas have been worked into most subsequent works. The fundamental shift was in understanding the world in terms of "modification and coadaptation," as Darwin put it, rather than as static systems. Darwin applied this viewpoint to the biological world (and, through Lyell's work, the geological world); Marx and Engels made their breakthroughs by applying it to the economic and social world as well. Engels is clearest about this borrowing in Dialectics of Nature, in which he makes clear that biological evolution is just one example of dialectics.
Darwin's approach to the argument was interesting to me as well. He brings the reader along quickly, starting with a basic discussion of coadaptation with which no one could disagree, then using that discussion as a basis for challenging the notion that each species was created in place, as is. As he builds the argument, he uses Lyell's work in geography to give us an idea of the vast spans of time in which the coadaptation process could work. Later, he answers the question of why we don't see continual adaptation in the fossil record—the record is incomplete. (Modern evolutionary theory suggests that this answer is only partial and that coadaptation typically happens in spurts rather than gradually.)
Overall, I was glad I read this book, especially in terms of my current project, understanding how it provided the basis for Marx's and Engels' work.