Greg Priest

From his earliest student days through the writing of his last book, Charles Darwin drew diagrams. In developing his evolutionary ideas, his preferred form of diagram was the tree. An examination of several of Darwin’s trees—from sketches in a private notebook from the late 1830s through the diagram published in the Origin—opens a window onto the role of diagramming in Darwin’s scientific practice. In his diagrams, Darwin simultaneously represented both observable patterns in nature and conjectural narratives of evolutionary history. He then brought these natural patterns and narratives into dialogue, allowing him to explore whether the narratives could explain the patterns. But Darwin’s diagrams did not reveal their meaning directly to passive readers; they required readers to engage dynamically with them in order to understand the connections they disclosed between patterns and narratives. Moreover, the narratives Darwin depicted in his diagrams did not represent past sequences of events that he claimed had actually occurred; the narratives were conjectural, schematic, and probabilistic. Instead of depicting actual histories in all their particularity, Darwin depicted narratives in his diagrams in order to make general claims about how nature works. The conjunction of these features of Darwin’s diagrams is central to how they do their epistemic work.

About 540 million years ago, a rapid radiation of animal phyla radically changed the Earth’s biota in a geological eye-blink. What caused this “Cambrian explosion”? Over the years, paleontologists have pointed to a wide array of different physical mechanisms as the causal “trigger” for the explosion. More recently, some paleontologists have proposed complex causal pathways to which multiple physical mechanisms are said to have contributed. Despite their variety, these answers share an assumption that a single explanation can in principle be constructed that identifies some factor or confluence of factors as the cause of the Cambrian explosion. That assumption is unjustifiable. The Cambrian explosion had multiple causes, and different aspects of the event are best explained by different causes. These different causes cannot, even in principle, be integrated into a single causal explanation. We can learn much about the causes of the Cambrian explosiondor for that matter about any historical eventdbut only by attending more carefully to how we frame our causal questions about the past.

When we read On the Origin of Species, we cannot help but hear echoes of the Wealth of Nations. Darwin’s “economy of nature” features a “division of labour” that leads to complexity and productivity. We should not, however, analyze Darwin’s ethics through this lens. Darwin did not draw his economic ideas from Smith, nor did he base his ethics on an economic foundation. Darwin’s ethics rest on Smith’s notion—from the Theory of Moral Sentiments—of an innate human faculty of sympathy. Darwin gave this notion an evolutionary interpretation, concluding that any species of social animals that develops a high enough level of intelligence will evolve moral principles. But the specific moral principles evolved by any particular species will vary, depending on that species’ particular evolutionary history. Darwin’s theory of moral sentiments thus diverged profoundly from Smith’s, while remaining in the same intellectual lineage. Although my reading of Darwin’s ethics departs from the dominant trend, it is not without precedent. A century ago, William James propounded a similar interpretation of Darwin. James’s interpretation has been largely forgotten, but he had Darwin right. It seems that James then developed his own moral philosophy on what he saw as a Darwinian foundation. It may be that James read Darwin’s evolutionary ethics through a pragmatic lens and so refashioned it, in his turn, into something again profoundly different from what Darwin had conceived, but again remaining in the same intellectual lineage.