He was on to something with this: "We can allow satellites, planets, suns, universe, nay whole systems of universe, to be governed by laws, but the smallest insect, we wish to be created at once by special act." [Notebook N (1838), as quoted in Darwin's Religious Odyssey (2002) by William E. Phipps, p. 32]
This is a perfect heuristic by which to begin pondering the sorts of laws that govern the smallest insects, of course, and that's fine. But this also touches on something crucial to human thinking. We do tend to think of 'law' as applying on the larger scales; and for there to be something exceptional, or magical-divine, about the smaller. We do so because the smaller scale is the one we inhabit. We want the heavens to follow immutable necessity, but at the same time we want to reserve for ourselves the comforting belief 'ah, but I have free will.' This is the same rationale that informs beliefs like 'taxation is good, because I want the state to provide me with roads and hospitals, schools and police; people must pay their taxes ... but I'm the exception.' Or: 'global warming is a threat and people must reduce their carbon footprint ... although of course I shall still expect to fly about the world with impunity.'
What interests me particularly is the way physics falls into this big/small distinction ... that the physics of the very large talks of the inevitable operation of physical laws, where the physics of the very small talks about quantum uncertainties.