What creatures did Darwin study that made him believe variation arose constantly?
Darwin's barnacle studies convinced him that variation arose constantly and not just in response to changed circumstances. In 1854, he completed the last part of his Beagle-related writing and began working full-time on evolution. His thinking changed from the view that species formed in isolated populations only, as on islands, to an emphasis on speciation without isolation; that is, he saw increasing specialisation within large stable populations as continuously exploiting new ecological niches. He conducted empirical research focusing on difficulties with his theory. He studied the developmental and anatomical differences between different breeds of many domestic animals, became actively involved in fancy pigeon breeding, and experimented (with the help of his son Francis) on ways that plant seeds and animals might disperse across oceans to colonise distant islands. By 1856, his theory was much more sophisticated, with a mass of supporting evidence.
The Eagle and Child and the Lamb and Flag, Oxford, were regular meeting places of the Inklings, a writers' group which included J. R. R. Tolkien and C. S. Lewis. The Eagle in Cambridge is where Francis Crick interrupted patrons' lunchtime on 28 February 1953 to announce that he and James Watson had "discovered the secret of life" after they had come up with their proposal for the structure of DNA. The anecdote is related in Watson's book The Double Helix. and commemorated with a blue plaque on the outside wall.