Back in the lab at Princeton in 1991, the Grants begin a sabbatical year in which they will analyze data collected since their first year in the Galápagos. Due to the enormous amount of information they’ve amassed, this analysis must be done with the aid of computers. Rosemary searches her files to identify an individual bird by its serial number, and in a matter of moments she retrieves data detailing its breeding habits, its mates, its offspring, and the entirety of its family tree. She has similarly thorough numbers on the island’s annual seed supply and extensive recordings of the finches’ mating songs. In the manner of devoted religious scholars, the Grants have documented an extensive account of their flocks: “four or five generations counting by life-spans, more than twenty generations counting by begats: a Book of the Chronicles, or a Book of the Kings, all devoted to Darwin’s finches” (119).

With the benefit of distance and statistical analysis, the Grants see things about the birds that were invisible while they were in the Galápagos. They detect increased fitness among hybrid birds, offspring of finches that mated across species lines. Darwin discusses hybrids in Origin but assumes that most are either sterile or more generally unfit and thus of no consequence to evolution of species.