Write to discover
Both Brand and Darwin are writing to discover. But they're doing it in different ways. Darwin only decided to write the Origin after he'd already understood the principle of evolution by natural selection. That is, he had not only his question, but also confidence in his answer, building on decades of prior work and creative output. And so Darwin is sharpening and developing an idea he already believes; he also, en route, discovers many surprising implications and applications. By contrast, Brand seems to have begun with a question, and discovered the basic shape of his answer through the writing process.