"Cerion is the land snail of maximal diversity in form throughout the entire world. There are 600 described species of this single genus. In fact, they're not really species, they all interbreed, but the names exist to express a real phenomenon which is this incredible morphological diversity. Some are shaped like golf balls, some are shaped like pencils.…Now my main subject is the evolution of form, and the problem of how it is that you can get this diversity amid so little genetic difference, so far as we can tell, is a very interesting one. And if we could solve this we'd learn something general about the evolution of form."