In Pursuit of the Gene

34 ¨ REVERSION TO THE MEAN
pleted a study of inheritance in peas, in his case the common garden pea,
but Gregor Mendel’s remarkable account of this study would remain unread
on the shelves of both the Royal Society and the Linnaean Society for
another twenty-six years.
Mendel had chosen to analyze the inheritance of single contrasting
characters, such as yellow or green peas, or smooth or wrinkled ones. But
Galton was interested in a different kind of inheritance, the inheritance of
continuously varying traits. According to his cousin Charles’s theory, these
traits were the raw material on which natural selection acted. The particular
trait that Galton settled on was seed size. Specifically, he wanted to see if
plants that produced exceptionally large or small peas would give rise, in
turn, to plants that also produced above- or below-average-size peas. Darwin
was fascinated by botany and natural history in general, but Galton’s
interest was purely theoretical. Years later he wrote, “It was anthropological
evidence that I desired, caring only for the seeds as means of throwing
light on heredity in Man.” 30
Galton’s first step was to weigh thousands of peas and sort them into
seven different weight groups, evenly spread out from lightest to heaviest.
After a failed attempt to grow the plants in the summer of 1874, Galton enlisted
the support of Darwin and eight other friends who lived throughout
England. 31 In the spring of 1875 he sent out nine sets of seeds for cultivation.
Each set consisted of seven packets, one for each weight class, made up of
ten identical peas of each weight class. Along with the seeds, he included
detailed instructions for their planting: Seven beds were to be prepared, one
for each size class, and seeds were to be planted in ten evenly spaced holes
of 1-inch depth. After they had born fruit, the plants were to be harvested
and returned to Galton for analysis. 32
Seven of the nine crops were successful, which provided an ample supply
of progeny seeds from each of the seven parental size classes. In a working
notebook Galton made a graph of parental pea size versus average
daughter pea size. The seven parental pea sizes were arranged in increasing
order from left to right along the base, and the vertical scale gave the
daughter pea sizes. Above each of the seven parental pea sizes, he placed a
mark at the height corresponding to the size of the average daughter pea