Are Beavers really fish?

Before Darwin came along there was a belief in fixity of species. 

However, Medieval thought could be very flexible in getting round it's own strictures; a good example is the Monks classifying Beavers as fish (the excuse being they had a scaly tail); done in order that meat could be eaten on the numerous "fish only" Saints' days.

Another example is the long held superstition that Wheat can degenerate into Rye. This all began with Pliny 2,000 years ago and a philological muddle and mix-up when interpreting his texts. Subsequent writers compounded the muddle and propagated this absurd belief.

This had curious repercussions. In the 13thC, Saint Thomas Aquinas, in his decision of which bread might be used for the Host at Communion, ruled that Wheat bread should be used. But a concession he made was that Rye bread might also be used because Wheat degenerates into Rye and therefore Rye is the same species as Wheat.

Not to be outdone, there began a general belief that other crops can degenerate into weedy species. Even as late as the 1920's in Cambridgeshire it was believed Oats changed into weedy Brome and in the 1930's some still believed that Barley can transmute into Oats.

Above information is mostly gleaned from Agnes Arber's book 'The Gramineae' published in 1934.