Records from 1850 to 1900 show that in a certain region, babies' birth weights each year varied with the success of the previous year' s crops: the more successful the crops, the higher the birth weights
This indicates that the health of a newborn depends to a large extent on the amount of food available to the mother during her pregnancy
inferring from the claim that two phenomena have fluctuated together that one of those phenomena must be the sole cause of the other