He is, or was, a professor at Unseen University who made the professional error of trying to introduce the study of genetics to Discworld science. He began with the observation that even given the same soil, the same watering, the same fertiliser and the same exposure to sunlight, some plants of the same species grew taller and faster than their brethren seedlings in the same plot.
However, the standard prosaic Ankh-Morporkian attitude to observed facts came into play here, and the general response to Catbury's discovery was "Yeah, well, so what, they do, don't they?" It is clear Gregor Mendel would have been doomed had he lived on the Disc: his Abbot is likely to have told him to stop wasting time and get down to some serious prayer and contemplation right now.
Catbury experimented with such well-known fast-breeders as fruitflies and beanplants. But as he was still working it all out, the only tangible result of his experiment was to breed a green thing with wings that buzzed sadly until it was eaten by a passing vegetarian spider.
Catbury's vegetarian spider, the only almost-exclusively vegetarian arachnid kniwn to Roundworld science, may be found here.