One of his best know ideas is that simple physical deformations of complex systems can give rise to whole families of apparently unrealted natural forms. The image shown below is page 744 of the first edition (you can find the whole book in electronic form at the Internet Archive = http://www.archive.org/details/ongrowthform1917thom). It shows how a complex shape representing the 2D shape of a crabs carapace can after simple geometric transformations give rise to a family of different carapace shapes.
