While Darwin couldn’t say what causes the variations that lead to evolution, he believed that it would be discovered in the future, imagining a kind of code, “a swarm of letters streaming through the blood” (214), effectively anticipating the discovery of DNA. In the basement of Princeton’s Natural History Museum, where the Grants keep an extensive collection of blood samples from the finches, Peter Boag extracts DNA molecules from the blood and searches for clues. Work with genes and DNA seems like a foreign language to Peter Grant, but for Boag and other evolutionists who came of age after advances in molecular biology, the work is revelatory. Weiner explains DNA’s basic structure: four chemical compounds (guanine, adenine, thymine, and cytosine) that “make the treads in every spiral staircase of DNA” (216) and are symbolized with the letters G, A, T, C, an oddly exact fulfillment Darwin’s vision.

Noting that finches, like human beings, have roughly 100,000 genes, Weiner discusses Boag’s intention to learn more about how and when the finch species may have diverged by studying their DNA. Though DNA is often a shorthand for describing one’s unchanging essence, the molecules in DNA change constantly, both across generations and within the life of an individual organism.