Darwin, Natural History, and Explanatory Breadth

As Charles Darwin moved on from the drafting of the Origin after its publication in 1859, he sought to bolster the case for his nascent theory of evolution by natural selection. One way in which he did so was to start to marshal a host of other data that we might recognize as natural-historical – detailed treatments of the fertilization of orchids, the workings of insect-eating plants and of climbing plants, and the structures of flowers, to name only a few. In this talk, I want to consider why he would have taken these to be the kinds of data needed to support his view of natural selection. I’ll use the example of Darwin as an opportunity to explore a concept that I will call “explanatory breadth.†This notion best stands out against two other, more common features of scientific explanations which have frequently appeared in recent literature. On the one hand, explanatory depth (commonly discussed in the literature on causal mechanisms, for instance) doesn’t at all seem to capture the kind of extension of selection that Darwin was after – there is nothing here about the “more fundamental†ways in which natural selection might be taken to act. On the other hand, explanatory scope (often discussed in the literature surrounding the physical sciences) normally involves working on the construction or formulation of the theory itself so as to render its claims more general, applicable to a larger range of target objects. Darwin’s pursuit seems to be neither of these – it is a precisely targeted effort to elucidate the way in which natural selection works in particular cases that he thought would cement the strength of his explanation for diversity and adaptation in the living world. This pursuit of explanatory breadth can offer us a lens for understanding the role of natural history both for Darwin and more broadly, including, potentially, current debates over natural history and museum-collection-based biological study.