Observing the Facts, Observing the Fast

“…in the Middle Ages it was thought that barnacles formed from fruits and dropped into the water from trees along the riverbanks.  Once submerged, they were believed to develop into the geese that arrived on the continent each spring.  No one had ever actually seen these geese lay eggs and rear young, but only adults migrating across Europe on their way to breeding grounds in the Arctic.  People did, however, notice a resemblance between the shapes of barnacles and goose necks – and therefore concluded that gooselike barnacles developed into barnacle like geese.

Christian scholars debated whether these geese should be classified as fish or fowl and, if the latter, whether they could be eaten on Friday or during Lent, when meat was forbidden but fish allowed.  Jewish philosophers debated whether this goose was a shellfish, hence food forbidden to Jews, and if it was not, whether it had to be butchered according to ritual practice.  Some saw a convenient compromise:  Since both barnacle and goose fell from a tree, they were neither fish nor fowl, but fruit.”  (Theories for Everything, pp 245-247)