I found On the Origin of Species tiring to read. Its central conclusion was already familiar, and Darwin spends page after page establishing points I was ready to accept. What kept earning my respect was the scale and patience of the work behind them.
Take the domestic pigeons in Chapter I. Darwin kept the breeds he could obtain, received specimens from abroad, and joined two London pigeon clubs. He compares the beaks and skulls of carriers and tumblers. Even this early example rests on sustained attention to a particular group of animals. A summary can tell you that breeders change animals through selection; the book makes you spend time with the differences they produced.
That attention extends to the objections. In Chapter VI, Darwin takes up the difficulty of explaining how something as complex as an eye could arise through small changes. He asks whether simpler forms can still be useful and whether variations can be inherited, then examines gradations in the eyes of living animals. He also warns that these animals are not a line of ancestors laid out for inspection. The comparisons help establish the possibility of intermediate forms; they do not supply the missing history.
This is where the book became demanding for me. Understanding the general idea did not make me eager for every further example. The qualifications and repeated returns to difficulties slowed the reading down. I could respect why a passage was there and still find it a slog.
I would recommend it to someone curious about how Darwin built his case, with that warning about the pace. Knowing the conclusion leaves plenty to discover in the work that supports it. It also means sitting through the pigeons after you think you have understood the point.
