Man, this guy didn't know anything.
I don't mean that as an insult. Darwin, as he admits, knew almost nothing about inheritance, about how variation is produced, or about the origins and history of domesticated plants and animals. You'd think that would be a handicap in using domestication as an analogy for evolution.
And yet, in chapter 1 of the Origin, 'Variation Under Domestication', Darwin uses what little knowledge he has so deftly that nowhere do you feel his conclusions are outstripping his data. This, believe me, is quite a skill, both in a scientist and a writer. What, he asks, is the minimum we can infer from what we know?
Well, here's what his readers can be sure of at the end of the chapter:
Domestic animals and plants vary – most obviously between different breeds of the same species.
Domestic animals and plants have changed through time.
This change results from humans breeding selectively from individuals that carry a valuable trait, because offspring tend to look like their parents. And that's about it — hard to argue with any of that, then or now. Darwin does speculate about mechanisms: most variation, he suggests, comes from within the organism, by “the conditions of life … [acting] on the reproductive system”. You could see this as analogous to mutation, although I think that would be stretching inference beyond knowledge. There's also a shorter discussion of inheritance, the laws governing which are “quite unknown”.
But none of this speculation is load-bearing — the only essentials are right before our eyes.