…I am become most deeply interested in the way facts fall into groups. I am like Crœsus overwhelmed with my riches in facts.—Charles Darwin to his cousin W. D. Fox, 8 February 
It’s not just the absence of a natural narrative arc that poses a challenge when writing an essay collection loosely themed around Charles Darwin.
The almost limitless potential subject matter for my Darwin book, and Darwin’s continuing appeal and relevance, create another problem: all manner of interesting new stories continue to appear with reckless abandon on my Darwin news radar. It’s wonderful, really. The temptation to head off down some fascinating new rabbit-hole is constant and immense. It takes considerable will-power not to be distracted. I often fail.
But the real killer is when new facts emerge about some topic I’ve already ‘finished’ writing about. You wouldn’t believe how often, for example, a brand new press release appears in my RSS feed-reader concerning the evolutionary history of domestic dogs… I’ve done that chapter. Stop finding out interesting new stuff, bloody scientists!
Sometimes, new material can appear on the most esoteric subjects. Just weeks after I completed a chapter on how Darwin made species classification make sense, by identifying the natural way to group them is by genealogical descent, up pops a brand new academic paper about Darwin’s disagreement with his friend Thomas Henry Huxley on that very subject! I’ve earmarked that one for consideration when working on my second draft. But if I were less thick-skinned, I might start to take this sort of thing personally.
Over the years, I’ve slowly come to embrace the idea that factual writing in the essay format—my preferred genre—can never be definitive. Indeed, I see its incomplete, provisional nature as a large part of its appeal. Interesting new fact keep emerging. If it were possible to write a definitive book on the wide variety of Darwin-related topics that interest me, it would already have been written, and I would have to find something less interesting to write about—and to read.
The challenge to me as a writer is not to write the final word on the topics that interest me, but to try to convey some of that interest to my readers. Whether or not I succeed is another matter—but it is at least something I can get my teeth into, and try to do something about.