Note 10 — Chapter XX · HEREDITY
Reading edition
This unit is known by the uncouth and not easily justified name of “Probable Error,” which I suppose is intended to express the fact that the number of deviations or “Errors” in the two outer fourths of the series is the same as those in the two middle fourths; and therefore the probability is equal that an unknown error will fall into either of these two great halves, the outer or the inner.