If such an array of repeated organs be chosen from the proper region of the body, within proper limits of time, in each of a large series of individuals belonging to a race, and if all the **arrays** so chosen be added together, a series will be formed from which the racial variability can be determined.

Thus a series of **arrays** of beech leaves, gathered, subject to the precautions indicated, from each of loo beech trees in Buckinghamshire by Professor Pearson, gave 16.1 as the mean number of veins per leaf, the standard deviation of the veins in the series being 1.735.