Source: Darwin, C. (1876). The Effect of Cross- and Self-Fertilization in the Vegetable Kingdom, 2nd edition, London: John Murray.

These data are from Charles Darwin's study of cross- and self-fertilization. Pairs of seedlings of the same age, one produced by cross-fertilization and the other by self-fertilization, were grown together so that the members of each pair were reared under nearly identical conditions.
The aim was to demonstrate the greater vigour of the cross-fertilized plants.
The data are the final heights of each plant after a fixed period of time. Darwin consulted Galton about the analysis of these data, and they were discussed further in Fisher's Design of Experiments.