Over the centuries there has been a degree of sibling rivalry between Franciscans and Dominicans. But one of the ways this rivalry has been kept in check is for Dominicans to preach to Franciscans on the feast of St Francis and for Franciscans to preach to Dominicans on the feast of St Dominic. Such a tradition helps to remind these two orders that their shared mission to spread the Gospel and serve the Church unites them more deeply than any rivalry could divide them. As a Dominican friar, I was very happy to celebrate the Franciscan Order on St Francis’s feast day, 4 October. But as someone with a scientific background, I’m also very happy to celebrate the Franciscan Order’s historical contribution to science.