From introduction to "Patterns in Nature" by Peter S Stevens.

"When we see how the branching of trees resembles the branching of arteries and the branching of rivers, how crystal grains look like soap bubbles and the plates of a tortoise's shell, how the fiddleheads of ferns, stellar galaxies, and water emptying from the bathtub spiral in a similar manner, then we cannot but help wonder why nature uses only a few kindred forms in so many contexts...It turns out that those patterns and forms are peculiarly restricted, that the immense variety that nature creates emerges from the working and reworking of only a few formal themes."