How difficult it is to trace the relations between cause and effect is frequently illustrated by everyday occurences. A chemical factory started in England in the vicinity of a market garden where leeks were a staple crop. The same season the leeks failed disastrously, and of course the factory was associated in everybody's mind with the fact, and a suit against the factory resulted. Fortunately in this case, science saved a rank injustice from being perpetrated. It was shown that the leeks were suffering from the attacks of a small fungus- the Uredo alliorum or onion rust, - with which the factory had nothing whatever to do.